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Matthias
2013-Jun-18, 08:39 PM
This isn't the IAU, but I'd like to propose a stricter definition of what a "moon" is.

There are nineteen known planetary satellites with sufficient mass to achieve hydrostatic equilibrium. According to the IAU definition of a planet, if these bodies were in primary orbit of the sun, they would all be suitable candidates for dwarf planet status.

There are dozens more planetary satellites which are too small to achieve hydrostatic equilbrium (due to gravitic collapse as opposed to chance). It is common enough for new ones to be discovered around the outer planets, many of whom are likely to have been captured asteroids, that published catalogues of planetary satellites quickly become outdated. That no minimum mass definition exists for an object to be considered a "moon" means that virtually any solid object not created by man which falls into a stable orbit around a planet becomes a moon, even those as absurdly small as a pebble or grain of sand.

My proposal therefore is to define a moon as a celestial body that: (1) is in orbit around a planet [as defined by the IAU], (2) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape.

Objects which meet requirement 1 but not requirement 2 would be defined as "minor satellites".

There should also be some kind of definition of what would constitute a "double planet" which would not depend on the radius of a planet, and which could apply to gas-giant double planets, which would have no solid surface. (Even though the configuration of Pluto and Charon has the common center of mass outside the surfaces of both planets, it is merely a convenience to have both bodies possessing solid surfaces.)

Jens
2013-Jun-18, 10:31 PM
I'm not really sure why this is in ATM. Astronomy seems fine. But in any case, I can add my opinion: first Pluto, now Phobos!

Jeff Root
2013-Jun-19, 02:20 AM
I have no idea what the IAU says about the meaning of the
word "moon", but as far as I'm concerned, a moon is an object
in gravitational orbit around another, larger object which may
be as large as a planet but not as large as a star. The smaller
object must be large enough to be visible, so an individual
atom or molecule wouldn't count.

So, for example, I consider Dactyl to be a moon of Ida, and
Vanguard 1 to be a moon of Earth.

I consider the terms "moon" and "satellite" to be synonyms.

-- Jeff, in Minneapolis

Superluminal
2013-Jun-19, 06:18 AM
I've always thought of moon as natural, and satellite as man made. What of ring particles? How big does a ring particle have to be before it is considered a moon?

Jeff Root
2013-Jun-19, 09:25 AM
Super,

As I said, I consider anything big enough to be visible to
qualify for the synonymous terms "moon" and "satellite".
I'll add that by "visible" I meant "identifiable", in that it
can be picked out as an individual, at least for a time.

-- Jeff, in Minneapolis

Ara Pacis
2013-Jun-19, 02:34 PM
Been there, done that. (http://cosmoquest.org/forum/showthread.php?42786-The-quot-What-is-a-moon-quot-definition-thread&highlight=)

NEOWatcher
2013-Jun-19, 04:48 PM
Been there, done that. (http://cosmoquest.org/forum/showthread.php?42786-The-quot-What-is-a-moon-quot-definition-thread&highlight=)
In the interest of my own laziness, and since it's your thread. What was the outcome?

mapguy
2013-Jun-19, 04:52 PM
...I'd like to propose a stricter definition of what a "moon" is. My proposal... is to define a moon as a celestial body that: (1) is in orbit around a planet [as defined by the IAU], (2) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape. Objects which meet requirement 1 but not requirement 2 would be defined as "minor satellites".

Well, let's try your proposal on for size in our solar system. Here's a pie chart from Wikipedia illustrating the relative masses of the Jovian moons:
http://upload.wikimedia.org/wikipedia/commons/0/01/Masses_of_Jovian_moons.png
Other than the Galileans, nothing else even shows up... which I think helps to reinforce your point that not all "moons" are created equal. So we could say, "Jupiter has 4 moons; 63 known minor satellites (46 of which are currently named); and countless moonlets in its ring system."

Here's a similar chart for Saturn's moons:
http://upload.wikimedia.org/wikipedia/commons/thumb/a/a0/Masses_of_Saturnian_moons.png/544px-Masses_of_Saturnian_moons.png
Here, we would say "Saturn has 7 moons; 55 known minor satellites (46 of which are currently named); and countless moonlets." (caveat: I didn't research whether or not all 7 are considered to be in hydrostatic equilibrium)

Here's Uranus, whose "moon" count would be 5, with 22 known minor satellites (same caveat as above):
http://upload.wikimedia.org/wikipedia/commons/thumb/5/59/Masses_of_Uranian_moons-en.svg/500px-Masses_of_Uranian_moons-en.svg.png

Neptune would have only one "moon", and 12 known minor satellites. (Adding the Earth's moon, my total comes to 18, so if 19 is the correct number, then I missed one.)

Anyway: after seeing it played out like that, I can say I like the proposal. I agreed with the IAU's decision to more narrowly define the word "planet", and I think it makes sense to work towards a similar defining of the word "moon" as well. The idea that both Saturn and Uranus have more "moons" than Jupiter is a little hard to swallow at first, but it helps to reinforce the fact the Galileans are Jupiter's only "serious" moons, whereas two other planets have a greater number of natural satellites that are in hydrostatic equilibrium.

Margarita
2013-Jun-19, 05:16 PM
Has there never been a discussion about the need for a definition of the terms 'moon' and 'satellite' at the IAU? I'm really surprised that there is no official definition, and the one suggested here seems eminently sensible.
But - how does it get proposed to the IAU?!

Margarita

DonM435
2013-Jun-19, 05:42 PM
I seem to recall that it used to be considered bad form to use "moon" as a generic term for a satellite. That word was reserved for Earth's natural satellite.

That rule/tradition must have changed in the past couple of decades.

Matthias
2013-Jun-19, 07:20 PM
Before I address Jeff's response I want to address concerns over terminology.

First, I've always understood the term "satellite" to be a commonly accepted identifier for anything that is in an orbit around something else. There are both artificial and natural satellites; the Earth is a natural satellite of the Sun, the Moon is a natural satellite of the Earth, and the ISS is an artificial satellite of the Earth.

I personally have no difficulty with using the term "moon" to describe any "Moon-like" satellite of a planet. In my estimation, there is negligible risk of confusing what is normally called "the Moon" (with an upper-case 'M' and always preceded by the definite article 'the') with "a moon" (lower-case 'm') of any other solar planet. Other languages may lack such a distinction but I am not informed about the other languages in which the IAU conducts its business.

Basically, every moon is a satellite, but not every satellite a moon.




I have no idea what the IAU says about the meaning of the
word "moon", but as far as I'm concerned, a moon is an object
in gravitational orbit around another, larger object which may
be as large as a planet but not as large as a star. The smaller
object must be large enough to be visible, so an individual
atom or molecule wouldn't count.

So, for example, I consider Dactyl to be a moon of Ida, and
Vanguard 1 to be a moon of Earth.

I consider the terms "moon" and "satellite" to be synonyms.

-- Jeff, in Minneapolis

This definition of what is a "moon" would be dependent on characteristics of the observer; for example the observer's distance from the object and the power of the instrument the observer is using to detect to the object (if any). A good scientific definition of what is a "moon" should be as objective as possible.

Part of my motivation for proposing a tighter definition of what a "moon" is so that (as mapguy has well shown) we can identify unambiguously how many "moons" a given planet has, if it has any at all. Astronomical instruments are good enough that we have pretty well found all the really big objects that orbit the eight IAU planets, and that as technology continues to improve, we will be able to resolve smaller and smaller orbiting objects. As it is, the only thing that distinguishes the smallest known "classical moons" of the outer planets and any of innumerable asteroids elsewhere in the solar system of similar size, is the fact that they have been captured by a planet. There is no theoretical minimum mass for a planetary satellite. As far as nature is concerned, it's all just clumps of atoms of different amounts and compositions. But should we not draw a line somewhere for the sake of convenience?

Jeff Root
2013-Jun-19, 08:10 PM
Matthias,

My opinion is that my definition is more objective -- due
largely to its simplicity -- than any other definition proposed
here or in the thread linked to by Ara Pacis. In what way
do you think it would be dependant on characteristics of the
observer? How does an observer's distance from the object
or the power of any instrument used have any relevance?

If you are just referring to my requirement that the object be
visible, then forget that requirement. I introduced it ad hoc
in order to eliminate objects that cannot be in orbit anyway.
As I said in the other thread, individual atoms, molecules,
and small dust particles are blown out of orbit by solar wind,
while ions are trapped in magnetic and electrostatic fields.
They do not orbit. So just drop the visibility requirement.

Elsewhere I do impose a visibility requirement in order
to distinguish between asteroids and meteoroids. See this
still-not-quite-finished web page:

http://www.freemars.org/jeff/meteor/

The definitions are close to the bottom of the page, but they
are the reason for the existence of the page. Immediately
below that section is a parallel definition which distinguishes
comets from asteroids. In all cases, the definitions follow
the historical as well as current uses of the terms, and are
simpler, clearer, and far easier to apply than alternatives
which have been suggested.

-- Jeff, in Minneapolis

Matthias
2013-Jun-19, 09:00 PM
Naming and classifying things fills a human (non-scientific) need to break information down into digestible and comprehensible pieces. For example, the electromagnetic spectrum is broad and continuous, and nature doesn't care about the limits of human perception of light. but for the sake of human understanding we have broken down the EM spectrum in different ranges such as radio, visible light, ultraviolet light, gamma rays, and so on. It is for human convenience that I suggest a definition of what a moon is. I do not believe it to be convenient to maintain a list of a dozen or more moons (or several dozen) for each of the outer planets. Children of every modern generation have been coached to memorize all the planets in order. They can do this because an eight-item list is convenient. It would not be convenient to memorize the names of more than sixty moons of Jupiter, sixty-plus moons of Saturn, 25+ moons of Uranus, and a dozen-or-so moons of Neptune, each in their order (and not all actually have names). It will be more convenient for children to memorize the names of only 18 or 19 moons spread among five planets, and their order. Convenience, and giving names to things, helps us grasp the universe from our limited human perspective. This need for convenience better summarizes the original point I was trying to reach by my expression of distate for the ever-expanding catalog of identifiable natural satellites that orbit the outer planets. Planets with sixty, one hundred, or one thousand moons are not convenient.

Now if I understand your last reply correctly, your definition reads as follows:

a moon is an object in gravitational orbit around another, larger object which may be as large as a planet but not as large as a star.

I disagree with this. Taking your definition to its logical conclusion will result in an unmanageably large, inconvenient list of "moons" for each planet in the solar system, whether they have the mass of Titan or the mass of a grain of rice. I don't expect you would want to name them all, or even to try numbering them. You might enjoy having such a broad definition of "moon" as a convenient shorthand for saying "this object is in orbit around a planet", but the term "planetary satellite" or "minor satellite" could perform that job just as well.

From a linguistic point of view, the word "moon" has more gravitas than the word "satellite". I would think that for something to be deemed a "moon", it would need to be a special class of planetary satellite. Given that the Moon is round and exhibits phases, other objects which share its characteristics (orbiting a planet, exists in hydrostatic equilibrium, able to exhibit phases from the POV of its orbited body) can also be called "moons" with the Moon being the archetypal member of that class of object.

I am not overly concerned with traditional or historical definitions of astronomical terms as you seem to be. Language should be consistent, but we should not let history or tradition dictate the usage of language--or else words could never change or modify their meaning, and we would have to invent a new word each time we understood a certain concept a little more clearly! As our understanding of things improves, we often find language incongruent with, or even contradictory to science. For example, the term "planet" has referred to a collection of celestial bodies which at various times included the Sun, the Moon, several asteroids, and two objects now currently defined as dwarf planets, and was a name for things which by the Greeks' understanding was nothing remotely similar to our understanding of what a planet is today. The Greeks' original definition of a planet, or any definition of a planet that would include asteroids and dwarf planets may be a historical or traditional definition, but it is one that no serious scientist would want to rely on today.

Jeff Root
2013-Jun-19, 09:48 PM
Part of my motivation for proposing a tighter definition of
what a "moon" is so that (as mapguy has well shown) we
can identify unambiguously how many "moons" a given
planet has, if it has any at all.
I see.

I have long considered that desire to be counterproductive.
The number of moons a planet has is pure trivia, having
nothing at all to do with understanding of the planet.
Giving people the idea that the number of moons orbiting
a planet is an important fact that they should memorize
is something I very much want to avoid, to the point of
taking proactive steps to prevent it.

I did, however, include counts of moons on my web pages
about the planets in our Solar System. linked from this
base page:

http://www.freemars.org/jeff/planets/planets5.htm

Saturn is a good example of how I expressed the sizes:



Moons

One large moon, four medium-size moons, and more
than two dozen small moons.
I made a simple chart comparable to the Wikipedia pie
charts posted by mapguy. Its purpose was to show
my rationale for grouping the moons into three classes by
size -- large, medium and small. It shows the 28 largest
moons, plus the two largest asteroids and two smallest
planets for comparison.

http://www.freemars.org/jeff2/moons1a.htm



Astronomical instruments are good enough that we have
pretty well found all the really big objects that orbit the
eight IAU planets, and that as technology continues to
improve, we will be able to resolve smaller and smaller
orbiting objects. As it is, the only thing that distinguishes
the smallest known "classical moons" of the outer planets
and any of innumerable asteroids elsewhere in the solar
system of similar size, is the fact that they have been
captured by a planet.
Yes, that is exactly what characterizes a moon: A moon
orbits a larger body. Nothing prevents an asteroid from
being a moon. Dactyl is a moon of Ida.



There is no theoretical minimum mass for a planetary
satellite.
And no need for one.



As far as nature is concerned, it's all just clumps of atoms
of different amounts and compositions. But should we not
draw a line somewhere for the sake of convenience?
What convenience?

-- Jeff, in Minneapolis

Jens
2013-Jun-19, 10:37 PM
Has there never been a discussion about the need for a definition of the terms 'moon' and 'satellite' at the IAU? I'm really surprised that there is no official definition, and the one suggested here seems eminently sensible.
But - how does it get proposed to the IAU?!

Just a couple of things. First I think a proposal would have to come from the membership of the IAU as a professional organization. But also, I think it's important to keep in mind that the IAU is a body of astronomers, and does not have any jurisdiction per se over the use of English or any other language. I think their jurisdiction is really over the use of terms by astronomers in their working language, which happens to be English today.

Incidentally, many languages use different terms for our moon and those of other planets.

mapguy
2013-Jun-19, 10:40 PM
OK, let's not have a repeat of the unproductive 2006 thread (http://cosmoquest.org/forum/showthread.php?42786-The-quot-What-is-a-moon-quot-definition-thread&highlight=). The OP has made his point, and Jeff has made a case for why he disagrees. Let's give other people a chance to voice their opinions. Maybe you guys can agree to refrain from jumping back in for a few days? I will do the same.

Matthias
2013-Jun-19, 10:56 PM
I see.

I have long considered that desire to be counterproductive. The number of moons a planet has is pure trivia, having nothing at all to do with understanding of the planet.

I must disagree with this. While you are essentially correct, you miss the point. There is nothing wrong with random facts. Astronomy is one science which has no shortage of data, but whose information lies outside of conventional human experience. Except in purely intellectual or mathematical sense, humans cannot truly wrap their brains around such concepts as how far away Proxima Centauri is, how much mass the Sun has, or how old the planet Earth is. It's all just units with numbers and powers of ten.


Giving people the idea that the number of moons orbiting a planet is an important fact that they should memorize is something I very much want to avoid, to the point of taking proactive steps to prevent it.

Part of learning astronomy (or any science) is starting with things the layman can deal with. Everyone has seen the Moon. Everyone might know there are other planets that go around the Sun, or even all their names. Did you know there are other moons like our Moon that go around each planet? Here's their names too. Etc. That's why we start out the newbies with seemingly-useless information. The harder stuff comes later.



I made a simple chart comparable to the Wikipedia pie charts posted by mapguy. Its purpose was to show my rationale for grouping the moons into three classes by size -- large, medium and small. It shows the 28 largest moons, plus the two largest asteroids and two smallest planets for comparison.

There isn't anything wrong with your classification method for planetary satellites. For the sake of "convenience", you put them into groups to better grasp them as a class of objects. But I think your classification scheme--based as it is on diameters--is purely arbitrary. Is there some natural process on which your diameter ranges are based, or is it random chance? If we found a star system whose moons suggested different ranges of "large" and "medium", would your system still apply? What if we found a star system whose moon diameters were fairly evenly distributed? Your definition of "large moons" and "medium moons" cannot be applied universally.

Defining a moon as being a body of a certain minimum mass (hydrostatic equilibrium) and in orbit of a planet can be applied to accept or reject any planet-orbiting body in any star system, and it would make sense, being based on a natural process and nothing arbitrary like the ranges of diameters which have clumped together by chance. Any natural body in orbit of a planet, that's too small for hydrostatic equilibrium, would be a minor satellite. You can still call them all moons if you really want to; like you can call Pluto a planet, or even a double planet, if you really want to.

Asteroids don't have to have moons, but they do have satellites, and that's good enough for me.

Jeff Root
2013-Jun-20, 02:22 AM
Naming and classifying things fills a human
(non-scientific) need to break information down into
digestible and comprehensible pieces.
If you read the thread that Ara Pacis linked to
(http://cosmoquest.org/forum/showthread.php?42786),
you will find that I twice stated that "I have a penchant
for classifying things. Probably everyone who engages
in this argument has a penchant for classifying things."
I agree that classifying things is helpful in understanding
them. I totally disagree that what you are suggesting
is at all helpful in understanding anything. Entirely the
contrary. As I said in my previous post, I think it is
counterproductive, an impediment to understanding.



For example, the electromagnetic spectrum is broad and
continuous, and nature doesn't care about the limits of
human perception of light. but for the sake of human
understanding we have broken down the EM spectrum in
different ranges such as radio, visible light, ultraviolet
light, gamma rays, and so on.
That isn't historically correct. These various bands were
each discovered independently, then integrated into one
continuum as it was realized they were actually all the
same thing.

I wrote a compact summary of it just over a year ago:

http://cosmoquest.org/forum/showthread.php?133739

-------

Why does the Visual Spectrum exist?
Post #7
2012-May-30, 01:33 PM
Jeff Root

The different bands of the electromagnetic spectrum
are distingiushed or defined in various different ways,
most of which allow a great deal of overlap.

The visual band is defined by what humans can see.
It is visible because it can change the electron excitation
of some chemical substances, as in photography.

Infrared is anything longer than what humans can see,
but shorter than microwaves. Infrared waves are short
enough to be dealt with by lenses and photography.
They have enough energy to be detected as individual
photons by appropriate detectors.

Microwaves are too long for lensing or photography, but
short enough to be handled by waveguides, and long
enough to be detected by radio receivers. They don't
have enough energy to be detected as individual photons
or to cause electron excitation in chemical substances.
They are generally absorbed by nonmetals.

Radio waves are too long to be conveniently handled by
waveguides. They generally pass through nonmetals.

Ultraviolet is anything shorter than humans can see,
and longer than X-rays. Ultraviolet generally is able
to ionize many chemical substances. The shorter the
wavelength, the more substances it can ionize.

X-rays are capable of penetrating matter that absorbs
ultraviolet. The shorter the wavelength, the greater
the penetration.

Gamma rays have the same properties as X-rays, but
often originate in atomic nuclei rather than in electron
shells, and generally are shorter, more energetic, and
more penetrating than X-rays.

-------

The distinctions are based on how different wavelengths
of light interact with other matter.

Your suggestions, in contrast, are completely arbitrary.

-- Jeff, in Minneapolis

Jeff Root
2013-Jun-20, 02:23 AM
It is for human convenience that I suggest a definition of
what a moon is. I do not believe it to be convenient to
maintain a list of a dozen or more moons (or several dozen)
for each of the outer planets.
You don't have to maintain it. Somebody else will do the
work, and you can relax.



Children of every modern generation have been coached to
memorize all the planets in order. They can do this because
an eight-item list is convenient. It would not be convenient
to memorize the names of more than sixty moons of Jupiter,
sixty-plus moons of Saturn, 25+ moons of Uranus, and a
dozen-or-so moons of Neptune, each in their order (and not
all actually have names). It will be more convenient for
children to memorize the names of only 18 or 19 moons
spread among five planets, and their order.
We should be able to find some way to dispose of the extra
planets and moons. We could call it a "Death Star"!



Convenience, and giving names to things, helps us grasp the
universe from our limited human perspective. This need for
convenience better summarizes the original point I was trying
to reach by my expression of distate for the ever-expanding
catalog of identifiable natural satellites that orbit the outer
planets. Planets with sixty, one hundred, or one thousand
moons are not convenient.
We'll get rid of them. We'll destroy them all! Bwahahaha!!!



Now if I understand your last reply correctly, your definition
reads as follows:



a moon is an object in gravitational orbit around another,
larger object which may be as large as a planet but not as
large as a star.
I disagree with this. Taking your definition to its logical
conclusion will result in an unmanageably large, inconvenient
list of "moons" for each planet in the solar system, whether
they have the mass of Titan or the mass of a grain of rice.
I don't expect you would want to name them all, or even to
try numbering them. You might enjoy having such a broad
definition of "moon" as a convenient shorthand for saying
"this object is in orbit around a planet", but the term
"planetary satellite" or "minor satellite" could perform that
job just as well.
But so does the term "moon", in current useage, as well as
historical useage. I'm not advocating that anything change,
I'm advocating that people not wreck things that have been
working just fine for hundreds of years by trying to "improve"
them without understanding the consequences.

I want to ask you if you really think your argument makes
even the least little bit of sense. There is no value in people
memorizing names of moons. That is just trivia. A total
waste of time, effort, and brain synapses. If you think that
defining "moon" to mean "anything orbiting a planet" implies
inconveniently long lists of moons, then I will inform you
that defining "building" as "any structure that people can
enter" makes for very inconvenietly long lists of buildings.
I mean, how are little children going to memorize the names
of all of the buildings just in Minneapolis, not to mention
those in St. Paul, Bloomington, Edina, Minnetonka, Chicago,
Miami, New York City, London, Paris, Oslo, Dubai, and so on
and on and on. There's just too many! We need to restrict
the definition of "building" so that there are only about eight
or nine in any one city. That will make it more managable.

And people! Oh my! Memorizing the names of all the people
in the world is going to keep the children occupied day and
night. There are more than 7 billion people in the world!
We have got to change the definition so that a person is at
least 300 pounds, maybe 350 or 400. We need to set a
reasonable cutoff to keep the list from being absurdly long.
Seven billion is way, way, too many!

Don't you agree?



From a linguistic point of view, the word "moon" has more
gravitas than the word "satellite".
Really? How so?



I would think that for something to be deemed a "moon", it
would need to be a special class of planetary satellite. Given
that the Moon is round and exhibits phases, other objects
which share its characteristics (orbiting a planet, exists in
hydrostatic equilibrium, able to exhibit phases from the POV
of its orbited body) can also be called "moons" with the Moon
being the archetypal member of that class of object.
It seems to me that "orbiting a planet" is sufficient.

As I said in post #70 of the other thread,

Attempts to define something by multiple characteristics
generally cause problems when it turns out that not all the
defined things have all the characteristics.

You are trying to convince me to replace a good definition
with a poor definition so that little children don't have to
memorize thousands or millions, or billions, or trillions, or
who knows how many names of moons, as if it were an
actual imminent problem.

The real problem is that some people think memorizing
names of moons has any value in learning about the
Universe they inhabit.



I am not overly concerned with traditional or historical
definitions of astronomical terms as you seem to be.
Wanting to change a perfectly good, long-established
definition in order to replace it with an arbitrary one
that you invented to solve an imaginary (and freaky)
problem is motivating me. I am not afraid of change,
in general, but changing something good to something
bad for no real reason is a loss, not a gain.



Language should be consistent, but we should not let
history or tradition dictate the usage of language--or
else words could never change or modify their meaning,
and we would have to invent a new word each time we
understood a certain concept a little more clearly!
I understand what you are saying. You do not.

The meaning of the term "moon" has evolved and grown
to suit the needs of the times. You want to chop it apart
in order to avert a catastrophe in which children are forced
to memorize names of individual specks of dust.

Is that your only objection? It is the only objection I have
seen here, or in the other thread, and it makes no sense
at all.



As our understanding of things improves, we often find
language incongruent with, or even contradictory to science.
For example, the term "planet" has referred to a collection
of celestial bodies which at various times included the Sun,
the Moon, several asteroids, and two objects now currently
defined as dwarf planets, and was a name for things which
by the Greeks' understanding was nothing remotely similar
to our understanding of what a planet is today. The Greeks'
original definition of a planet, or any definition of a planet
that would include asteroids and dwarf planets may be a
historical or traditional definition, but it is one that no
serious scientist would want to rely on today.
The current definition of "planet" is not much different from
what it was two thousand years ago. Lucian of Samosata
understood that the planets were other worlds. He thought
people might live on them, and wrote fantasy fiction about
traveling to them, just as people do today. I would not
generally classify the Sun as a planet, but it really is just
a very big, very massive, very hot planet. I don't have any
problem with classifying the Moon as a planet, or Pluto, or
asteroids. You just classified Pluto as a planet yourself,
in the quote above. A dwarf planet is a planet.

The Greeks saw seven objects in the sky that wandered
against the background of fixed stars. They called them
"planets", for that is the meaning of the word "planet" in
Greek: "wanderer". We know now that the Sun is more
like the distant stars than like the other planets, so we
call it a star rather than a planet, even though it wanders.
As smaller or more distant wandering objects were found,
they were added to the list of known planets. But except
for reclassification of the Sun, the definition of "planet"
remains largely as it was in Lucian's time.

-- Jeff, in Minneapolis

Ara Pacis
2013-Jun-20, 06:12 AM
In the interest of my own laziness, and since it's your thread. What was the outcome?

If you mean have I concluded my mission of finding everyone of the 400 voters in that late night debacle and strung them upside down by their shoelaces and them whipped then with celery sticks until they recanted the absurdity they foisted upon this world, then no, the outcome has not been achieved. I mention the "planet" issue because in my thinking, if we had used my preferred definition of a planet, then there would be no moons as this OP suggests: those satellites would be planets.

Jens
2013-Jun-20, 06:28 AM
If you mean have I concluded my mission of finding everyone of the 400 voters in that late night debacle and strung them upside down by their shoelaces and them whipped then with celery sticks until they recanted the absurdity they foisted upon this world, then no, the outcome has not been achieved.

You know, they're just a group of astronomers. We don't have to follow their directives. We are very free to call Pluto a planet if we so please. They have zero official jurisdiction to force changes to the English language (nobody does, actually).

Ara Pacis
2013-Jun-20, 04:41 PM
You know, they're just a group of astronomers. We don't have to follow their directives. We are very free to call Pluto a planet if we so please. They have zero official jurisdiction to force changes to the English language (nobody does, actually).

Then I wish they'd stop trying to correct people. I find it annoying and arrogant that certain types of scientists take a word that has a common meaning, redefine it so that it has a neologistic and specific meaning, and then start lambasting people for using the historic and common meaning. It's one thing if it's an organic change, or if the new meaning catches on over time, but to make sudden pronouncements that upset applecarts seems to be counterproductive.

Hornblower
2013-Jun-20, 05:03 PM
I see no compelling need to restrict the definition according to size and shape, but I would be agreeable to classifying them according to likely type of origin. Specifically, small bodies in weakly bound retrograde orbits appear likely to be captured asteroids that previously were unbound to the respective planets, while bodies in tight prograde orbits appear likely to have formed with their planets. Triton clearly is an oddball in this scheme.

Jens
2013-Jun-21, 12:22 AM
Then I wish they'd stop trying to correct people. I find it annoying and arrogant that certain types of scientists take a word that has a common meaning, redefine it so that it has a neologistic and specific meaning, and then start lambasting people for using the historic and common meaning.

I'd agree if that is really true. But is it? If they want to redefine it for their own working use, then I don't see any problem at all. Now, if they start lambasting people for using the more common reason, then I'd have a problem. But to be honest, I've never been lambasted by an astronomer about that. I've only heard it hyped up by the media. Isn't it the media that is creating a problem here? Or have you actually been lambasted by an astronomer about it?

There are professional astronomers here, so I could ask: can anyone who is a professional astronomer chip in about whether they would lambast somebody for calling Pluto a planet?

EDG
2013-Jun-21, 05:31 AM
There are professional astronomers here, so I could ask: can anyone who is a professional astronomer chip in about whether they would lambast somebody for calling Pluto a planet?

I would. (I have been a professional planetary scientist at one stage...). Well, I wouldn't "lambast" them per se, but I'd certainly correct them.

Though it's more complicated than that. I would say that at best Pluto is a "Minor Planet". Basically that's the same as a "Dwarf Planet" but unlike the latter it's actually a sub-definition of "planet" (the 'big 8' are Major Planets) - Stern & Levison called them "Uberplanets" and "Unterplanets" in their 2000 paper. (I explain more in this article on my website (http://evildrganymede.net/2010/11/19/what-is-a-planet/), if anyone wants the full explanation and links to the papers I referred to).

I would still say that there are 8 (major) planets in the solar system though. Pluto isn't one of them.

The funny thing about the Pluto controversy is that people seem to forget that even before the IAU stepped in, scientists didn't really think Pluto fit into the solar system as a planet. It was tiny, out on a weird elliptical orbit, definitely wasn't "Planet X" that was supposedly perturbing Neptune and Uranus' orbits... it seemed more like an asteroid if not for the fact that we hadn't found any other objects out there like it in 60 years or so. But then we started to find other KBOs, and it became really obvious very quickly that it was essentially just a big icy asteroid, and one of many. So I'm rather bemused that people got so up in arms about it being 'demoted' from being a 'real planet'.

Jens
2013-Jun-21, 06:09 AM
The funny thing about the Pluto controversy is that people seem to forget that even before the IAU stepped in, scientists didn't really think Pluto fit into the solar system as a planet. It was tiny, out on a weird elliptical orbit, definitely wasn't "Planet X" that was supposedly perturbing Neptune and Uranus' orbits... it seemed more like an asteroid if not for the fact that we hadn't found any other objects out there like it in 60 years or so. But then we started to find other KBOs, and it became really obvious very quickly that it was essentially just a big icy asteroid, and one of many. So I'm rather bemused that people got so up in arms about it being 'demoted' from being a 'real planet'.

And just to add to that, in what context would the issue even come up? The only two times I really remember talking about Pluto in a situation where it was important for it to be a planet was (1) in grade school, remembering the names of the planets, and (2) talking about the IAU decision! Even if you go hunting for it with a big telescope, you will say "I'm looking at Pluto" not "I'm looking at the planet called Pluto," so there really wouldn't be any reason to correct somebody in the first place.

EDG
2013-Jun-21, 06:12 AM
And just to add to that, in what context would the issue even come up? The only two times I really remember talking about Pluto in a situation where it was important for it to be a planet was (1) in grade school, remembering the names of the planets, and (2) talking about the IAU decision! Even if you go hunting for it with a big telescope, you will say "I'm looking at Pluto" not "I'm looking at the planet called Pluto," so there really wouldn't be any reason to correct somebody in the first place.

Don't forget that it'd be described as a planet in all the text books. But yeah, recently people only talk about it because of the IAU decision.

I tend to talk about it more because every year I show people around a scale model of the solar system for Astronomy Day, and it's actually given me something to talk about when I get to the Pluto exhibit because now I can talk about what is or isn't a planet, and why it makes sense that Pluto isn't considered to be a 'major planet'. :). And people do walk away understanding why it isn't.

Ara Pacis
2013-Jun-21, 06:41 AM
I would. (I have been a professional planetary scientist at one stage...). Well, I wouldn't "lambast" them per se, but I'd certainly correct them.

Though it's more complicated than that. I would say that at best Pluto is a "Minor Planet". Basically that's the same as a "Dwarf Planet" but unlike the latter it's actually a sub-definition of "planet" (the 'big 8' are Major Planets) - Stern & Levison called them "Uberplanets" and "Unterplanets" in their 2000 paper. (I explain more in this article on my website (http://evildrganymede.net/2010/11/19/what-is-a-planet/), if anyone wants the full explanation and links to the papers I referred to).

I would still say that there are 8 (major) planets in the solar system though. Pluto isn't one of them.

The funny thing about the Pluto controversy is that people seem to forget that even before the IAU stepped in, scientists didn't really think Pluto fit into the solar system as a planet. It was tiny, out on a weird elliptical orbit, definitely wasn't "Planet X" that was supposedly perturbing Neptune and Uranus' orbits... it seemed more like an asteroid if not for the fact that we hadn't found any other objects out there like it in 60 years or so. But then we started to find other KBOs, and it became really obvious very quickly that it was essentially just a big icy asteroid, and one of many. So I'm rather bemused that people got so up in arms about it being 'demoted' from being a 'real planet'.

But that's not really the IAU position either. What you describe sounds like a classification system, which is not what they made. They made up a few definitions based on multiple specific characteristics instead of a taxonomy like a hierarchical schema.

Ara Pacis
2013-Jun-21, 06:43 AM
I'd agree if that is really true. But is it? If they want to redefine it for their own working use, then I don't see any problem at all. Now, if they start lambasting people for using the more common reason, then I'd have a problem. But to be honest, I've never been lambasted by an astronomer about that. I've only heard it hyped up by the media. Isn't it the media that is creating a problem here? Or have you actually been lambasted by an astronomer about it?

There are professional astronomers here, so I could ask: can anyone who is a professional astronomer chip in about whether they would lambast somebody for calling Pluto a planet?

Someone found a new favorite word! :)

And yes, I've seen it happen. Star-gazers can be a smug lot.

EDG
2013-Jun-21, 07:47 AM
But that's not really the IAU position either. What you describe sounds like a classification system, which is not what they made. They made up a few definitions based on multiple specific characteristics instead of a taxonomy like a hierarchical schema.

Yes, but the IAU position is frankly garbage - it's nonsensical and largely useless as a definition (for reasons explained in the link I posted). I may agree that Pluto isn't a proper planet, but I absolutely don't support the IAU's approach.

Jens
2013-Jun-21, 08:12 AM
Someone found a new favorite word! :)


Sorry, I thought you were being a bit melodramatic so I used it a bit much perhaps! :)

Ara Pacis
2013-Jun-22, 05:59 AM
Yes, but the IAU position is frankly garbage - it's nonsensical and largely useless as a definition (for reasons explained in the link I posted). I may agree that Pluto isn't a proper planet, but I absolutely don't support the IAU's approach.

On the IAU, we agree.

On Pluto, there are ways we might agree. If we use hydrostatic equilibrium as a basis for segregating planets from smaller objects, then Pluto could be a planet. If we make a distinction between composition or population or plane of the ecliptic or eccentricity of orbit, then we might sub-classify it as being in a planet of a certain phylum or family or class. A "regular" planet might be a taxonomic hierarchy, or it could be a shorthand category useful for combining groups of objects by context for convenience.

EDG
2013-Jun-22, 05:49 PM
On Pluto, there are ways we might agree. If we use hydrostatic equilibrium as a basis for segregating planets from smaller objects, then Pluto could be a planet. If we make a distinction between composition or population or plane of the ecliptic or eccentricity of orbit, then we might sub-classify it as being in a planet of a certain phylum or family or class. A "regular" planet might be a taxonomic hierarchy, or it could be a shorthand category useful for combining groups of objects by context for convenience.

I think there have been arguments against Stern/Levison and Soter's definitions because they're "too arbitrary" or mix up physical nature with orbital dynamics, but frankly I think there has to be some level of arbitrariness in there somewhere - even if it's by saying "a planetary discriminant greater than X makes it a major planet, and less than X makes in a minor planet".

To get back on topic though, given the pig's ear the IAU made of the planet definition, I'd hate to see the mess they'd make of defining moons...;)

Ara Pacis
2013-Jun-24, 06:01 AM
I think there have been arguments against Stern/Levison and Soter's definitions because they're "too arbitrary" or mix up physical nature with orbital dynamics, but frankly I think there has to be some level of arbitrariness in there somewhere - even if it's by saying "a planetary discriminant greater than X makes it a major planet, and less than X makes in a minor planet".Which would be fine if "minor planet" indicated a type of planet, but the IAU states that only planets are planets and even "dwarf planets" are not planets despite having the word "planet" in the name, just like "minor planets" are not "planets". It's like saying "houseplants" aren't "plants" because "houses" aren't "plants". What are they gonna say next, that "tree frogs" aren't "trees"?


To get back on topic though, given the pig's ear the IAU made of the planet definition, I'd hate to see the mess they'd make of defining moons...;)On that we agree.

EDG
2013-Jun-25, 12:23 AM
Which would be fine if "minor planet" indicated a type of planet, but the IAU states that only planets are planets and even "dwarf planets" are not planets despite having the word "planet" in the name, just like "minor planets" are not "planets". It's like saying "houseplants" aren't "plants" because "houses" aren't "plants". What are they gonna say next, that "tree frogs" aren't "trees"?

Yes, that's one reason why the IAU's attempt at a definition is so stupid IMO :). (note - when I say "minor planet" I mean it in the Stern/Levison sense, I just prefer that name to "unterplanet")
(another reason is that they just explicitly define what the eight 'actual' planets are by name, so they don't even have to conform to the rest of the definition!).

Ara Pacis
2013-Jun-25, 07:00 AM
Yes, that's one reason why the IAU's attempt at a definition is so stupid IMO :). (note - when I say "minor planet" I mean it in the Stern/Levison sense, I just prefer that name to "unterplanet")
(another reason is that they just explicitly define what the eight 'actual' planets are by name, so they don't even have to conform to the rest of the definition!).

Not to mention that they limit it to one star, so that scheme isn't even usable for other solar systems.

And I was just thinking about the Giant Impact Theory and now I think I just realized that we'll have to change all the textbooks again. We'll have to remove some time from the age of Planet Earth, because the Earth can't have been a planet until it cleared it's neighborhood and that didn't happen until the GI. Similarly, the theoretical object Theia can't have been a planet either. Imagine explaining to kids how the moon was created when an asteroid hit the Earth.

mapguy
2013-Jun-27, 04:54 PM
There appears to be plenty of disagreement with the IAU's way of defining the word "planet", and by extension, some of you also disagree with the OP's idea to more narrowly define the word "moon". Among these - and representing what I believe is the opposite extreme - is Jeff Root, who seems to be saying (please correct me if I'm wrong) that any pebble orbiting a slightly larger pebble is a "moon"... and presumably by logical extension, any pebble orbiting a star is a "planet". For others of you who disagree with the IAU's definition, I'd like to hear whether or not you agree with that... and if not, then what would be your proposed definition of the word "planet"? (Or you could link to views you've already expressed in another thread.) And then, to bring it back on-topic: how would you propose defining the word "moon"? Is every pebble in a ring system a "moon"?

NEOWatcher
2013-Jun-27, 05:00 PM
... any pebble orbiting a slightly larger pebble is a "moon"... and presumably by logical extension, any pebble orbiting a star is a "planet". [...] Is every pebble in a ring system a "moon"?
No, those would be called turtles.

Jens
2013-Jun-27, 11:16 PM
For anyone worried about having to change geology textbooks, I'd point out again that the IAU doesn't have a police force to arrest people in other disciplines who don't respect their definitions. They don't have any power to arrest museum curators either, so if curators choose to slavishly follow those guidelines it's a problem with the curators, not the IAU.

In Japanese, incidentally, Pluto (as well as the rock that the Little Prince came from) are "stars," (星) and as far as I know the IAU has yet to make any arrests.

EDG
2013-Jun-28, 05:10 AM
You could probably define satellites in the similar way to how Stern/Levison or Soter define planets. Major satellites need to be in hydrostatic equilibrium and need to have cleared their orbits by some quantitative definition (e.g. by a "satellite discriminant" that is the equivalent to their Planetary Discriminants. Minor satellites would be below those definitons. Ring systems would be the equivalent of asteroid belts, full of objects that aren't in hydrostatic equilibrium that certainly haven't cleared their orbits of similarly sized objects.

neilzero
2013-Jun-28, 03:08 PM
I have mixed emotions about a tight definition of Moon, as that would likely exclude Dimos and Phobos (the two tiny moons of Mars) and other moons where there may soon be a human colony. Neil

EDG
2013-Jun-28, 11:27 PM
I'd rather have a decent definition of a moon than keep things as they are just because of some emotional value. That's the only reason people still think Pluto is a 'full' planet after all.

(Phobos and Deimos and the other 'irregular satellites' of gas giants would be 'minor satellites' in that kind of definition anyway).

Jeff Root
2013-Jun-29, 03:27 AM
I'd rather have a decent definition of a moon than keep things
as they are ...
Are you saying that we don't have a decent definition of a moon
right now? If so, in what way is it indecent? I don't see anything
wrong with it as it stands, and can't think of any change that
would be better and not worse.

-- Jeff, in Minneapolis

Jeff Root
2013-Jun-29, 04:00 AM
There appears to be plenty of disagreement with the IAU's way
of defining the word "planet", and by extension, some of you also
disagree with the OP's idea to more narrowly define the word
"moon". Among these - and representing what I believe is the
opposite extreme - is Jeff Root, who seems to be saying (please
correct me if I'm wrong) that any pebble orbiting a slightly larger
pebble is a "moon"...
I would define a moon as any object orbiting a larger object,
where the larger object is less massive than a star. If somebody
wants to talk about pebbles, I won't try to stop them. "Orbiting"
in this context means an orbit due to the gravitational attraction
between the objects, and the smaller object is within the larger
object's Hill sphere.

-- Jeff, in Minneapolis

slang
2013-Jun-29, 04:52 PM
Are you saying that we don't have a decent definition of a moon
right now? If so, in what way is it indecent?

Look up "mooning".

neilzero
2013-Jun-29, 06:09 PM
How about: It is a deci moon if the small dimention is less than 1000 kilometers, a centi moon if less than 100 kilometers, a millimoon if less than 10 kilometers, a micromoon if less than 10 meters, a nano moon if less than 10 millimeters, a pico moon if less than 10,000 pico meters, a fento moon if less than 10,000 fentometers etc down to planc size moons? Some sarcasm intended. Micromoons do not provide adequate radiation protection for center of mass habitats, and usually burn up in the atmosphere. Neil

Jens
2013-Jul-01, 08:47 AM
As an idea for definitions, what about something like, "a moon is a significant natural body orbiting around a planet," and "a planet is a significant natural body orbiting around a star." The "natural" is important to avoid the ISS being a moon (I think it's easy to agree it isn't, and the "significant" means that Phobos would be included, whereas the meter-large rocks going around Saturn in its rings are not (they are not significant individually, but are only significant as a "ring"). "Significant" may seem somewhat subjective, but it's supposed to be subjective. Then we don't have to tie ourselves up in knots trying to make a definition that includes everything we want to include and excludes what we want to exclude. It also means that Pluto could be considered a planet (if we want), whereas yet-to-be-discovered Oort body objects don't have to be, because they are not significant.

Jeff Root
2013-Jul-02, 02:23 AM
As an idea for definitions, what about something like,
"a moon is a significant natural body orbiting around a planet,"
and "a planet is a significant natural body orbiting around a star."
The "natural" is important to avoid the ISS being a moon
(I think it's easy to agree it isn't, ...
I agree that you think "natural" is important to avoid
the ISS being a moon, but why do you think it? What
about the non-naturalness of the ISS makes you feel
that it shouldn't be a moon? Why is it important to
you that the ISS not be a moon?



... and the "significant" means ...
Big enough to orbit, rather than be blown away.

-- Jeff, in Minneapolis

Jens
2013-Jul-02, 05:32 AM
Why is it important to
you that the ISS not be a moon?


It's not "important" to me, it's just that I don't think the ISS is a moon, so I'd prefer it to be out of the definition. Just as if I said, "I define a horse as a xxx without stripes," and you say, "Why is it important to you that a horse not have stripes?" Well, it's not important, it's just that in general, the ones with stripes I call zebras. If we want to call them horses, that's OK but my definition is that they're not included in horses. Similarly, the ISS to me is an artificial satellite rather than a moon. It's just how I decide to define the term myself, like any other word.

NEOWatcher
2013-Jul-02, 12:52 PM
IMO: We already have the word "satellite" which would need to be prefaced with natural, or man-made(or whatever word you want to use).
So, to me, it seems like something reserverd for a natural satellite seems appropriate.

Wolf-S
2013-Jul-02, 02:35 PM
I think too that the ISS should not be considered a moon, for the same reason that a city cannot be considered a mountain and a truck cannot be considered a boulder.

Jeff Root
2013-Jul-02, 03:02 PM
Wolf-s,

In what way(s) is the ISS not a moon?

Let's say I agree that a city should not be considered
a mountain and a truck should not be considered a
boulder. Why should the ISS not be considered a moon?

-- Jeff, in Minneapolis

Jens
2013-Jul-02, 10:40 PM
With regard to my proposal, I think a better question would be what is the difference between a pyramid and a mountain? The dictionary I looked used the term natural in defining a mountain. But in fact, you could give me a difficult question: what if humans created a huge pile of rock, or what if people built a huge round pile of rock in orbit around some planet? Then I would tend to use mountain or moon despite the definition.

Jens
2013-Jul-02, 10:45 PM
I suppose it may seem odd (though it shouldn't), but although I am a wordsmith by profession, so definitions are very important to me, I am very much agnostic with regard to the real truth value of definitions. As is true with the dust/gas question as well, we try to apply categories to a world that cannot be neatly categorized.

Matthias
2013-Jul-03, 05:16 AM
I am thankful that the human race got around to naming most of the elements way before our time. We would probably have a whole lot of elements with names starting with the letters "eka" or "un" otherwise, as those were "just as good" as what we use now, and there's no "real" reason for them to have the names they do.

EDG
2013-Jul-03, 06:04 AM
Are you saying that we don't have a decent definition of a moon
right now? If so, in what way is it indecent? I don't see anything
wrong with it as it stands, and can't think of any change that
would be better and not worse.

As far as I'm aware, we don't have ANY definition of a moon beyond "something that orbits a planet" - maybe that works for you, but for me it's kinda useless. That means any old bit of junk or pebble orbiting a planet could be considered a "moon", and that's patently ridiculous (in that case, Earth has thousands and thousands of "moons"). It's rather like the situation we had before the IAU came along and attempted to 'fix' a planet definition - people before then apparently were just supposed to 'know' what a planet was.

EDG
2013-Jul-03, 06:06 AM
In what way(s) is the ISS not a moon?

Let's say I agree that a city should not be considered
a mountain and a truck should not be considered a
boulder. Why should the ISS not be considered a moon?

Because it's artificial.

Jens
2013-Jul-03, 07:49 AM
Because it's artificial.

Just for the sake of argument, then, if some weird artist started collecting minerals and clumped them all together and made this HUGE spherical object (twice as big as Phobos) and put it in orbit around Mars, you wouldn't call it a moon?

Wolf-S
2013-Jul-03, 09:19 AM
Wolf-s,

In what way(s) is the ISS not a moon?

Let's say I agree that a city should not be considered
a mountain and a truck should not be considered a
boulder. Why should the ISS not be considered a moon?

-- Jeff, in MinneapolisI would not put the ISS (anthropogenic) into the same category as a naturogenic object, or something anthropogenic resembling a natural object to the point it cannot be distinguished from natural. Like if we discover an object in orbit around the Earth and it turns out to be a spent rocket stage (http://en.wikipedia.org/wiki/J002E3), it's really discovering an old, abandoned oil rig instead of a genuine volcanic island.

But:


Just for the sake of argument, then, if some weird artist started collecting minerals and clumped them all together and made this HUGE spherical object (twice as big as Phobos) and put it in orbit around Mars, you wouldn't call it a moon?
I would call this object a moon, more precisely an artificial moon. It could be compared to artificial vs natural water reservoir.

I actually think that the popular usage of moons, moonlets, asteroids, satellites, rings, etc is generally quite consistent with the properties of these objects. When one talks about a moon of Saturn, it can be assumed that they are not talking about one of the multitude of small objects in the rings, but they are referring to Titan, for example. However, something more specific, like a temporary moonlet in the ring, which would refer to something like statistically larger-than-average temporary aggregate of icy material, would get the point across.

Jeff Root
2013-Jul-03, 03:06 PM
Are you saying that we don't have a decent definition of a moon
right now? If so, in what way is it indecent? I don't see anything
wrong with it as it stands, and can't think of any change that
would be better and not worse.
As far as I'm aware, we don't have ANY definition of a moon
beyond "something that orbits a planet" - maybe that works for
you, but for me it's kinda useless. That means any old bit of junk
or pebble orbiting a planet could be considered a "moon", and
that's patently ridiculous
To me, it doesn't seem ridiculous at all. Not even slightly.
Not even the tiniest little bit.

Why does the fact that "any old bit of junk or pebble orbiting
a planet could be considered a 'moon'" seem ridiculous to you?



(in that case, Earth has thousands and thousands of "moons").
It's rather like the situation we had before the IAU came along
and attempted to 'fix' a planet definition - people before then
apparently were just supposed to 'know' what a planet was.
Like every other word in every human language, people were
and are expected to learn the meaning of the word "planet" by
hearing or seeing it used in context or by looking it up in a
dictionary, encyclopedia, or other reference, such as an
astronomy textbook.

My dictionary has this entry:

planet (plan'it), n. [M.E. & OFr. planete; LL. planeta;
Gr. planetes, wanderer < planen, to lead astray, wander],
1. originally, any of the heavenly bodies with apparent
motion (as distinguished from the fixed stars), including
the sun, moon, Mercury, Venus, Mars, Jupiter, and Saturn.
2. now, any heavenly body that shines by reflected
sunlight and revolves about the sun: the major planets,
in their order from the sun, are Mercury, Venus, Earth,
Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto; the
minor planets are the asteroids, or planetoids, which
move in orbits between Mars and Jupiter. 3. In
astrology, any heavenly body supposed to influence
a person's life.

That is from Webster's New World Dictionary of the
American Language, College Edition, 1964.

One of the volumes in the spectacular Time/Life Science
Library is titled "Planets" (Carl Sagan, Jonathan Norton
Leonard, and the editors of LIFE, 1966). Naturally, the
entire book tells about what planets are. I can't quote
the entire book. My parents subscribed to this series of
books while I was in junior high school.

My textbook from the introductory astronomy course I
took at the University of Minnesota (An Introduction to
Astronomy, 7th Edition, Baker and Frederick, 1968) way
back in 1971 has a glossary with this entry:

Planet. A satellite of a star, in nearly circular orbit,
which was never capable of shining by a self-sustained
energy reaction.

Of course it also has a good deal of information about
the planets, their relations to the rest of the Universe,
and how they are observed.

A somewhat more recent astronomy textbook (Exploring
the Cosmos, Second Edition, Berman and Evans, 1977)
has this glossary entry:

planet: one of the principle nonluminous bodies in
orbit around the sun or another star. There are nine in
the solar system.

I have two other general astronomy textbooks which
describe what planets are, and a small book by William
J. Kaufmann III titled "Planets and Moons" which goes
into more detail.

So it isn't necessary to "just know".

-- Jeff, in Minneapolis

Jeff Root
2013-Jul-03, 03:18 PM
In what way(s) is the ISS not a moon?

Let's say I agree that a city should not be considered
a mountain and a truck should not be considered a
boulder. Why should the ISS not be considered a moon?
Because it's artificial.
Why does the fact that it is artificial imply that the ISS
should not be considered a moon?

-- Jeff, in Minneapolis

Jeff Root
2013-Jul-03, 03:44 PM
The very first description of an artificial satellite referred
to it as a moon:

http://en.wikipedia.org/wiki/The_Brick_Moon

The story is available online:

http://www.gutenberg.org/ebooks/1633

I have no sort of objection now to telling the whole story.
The subscribers, of course, have a right to know what became
of their money. The astronomers may as well know all about it,
before they announce any more asteroids with an enormous
movement in declination. And experimenters on the longitude
may as well know, so that they may act advisedly in attempting
another brick moon or in refusing to do so.

It all began more than thirty years ago, when we were in
college; as most good things begin...

-- Jeff, in Minneapolis

.

Wolf-S
2013-Jul-03, 04:02 PM
My general perspective is that the intelligent beings* who study the natural phenomena in the universe, as observers, in order to correctly identify said phenomena, must account for any artificial interference they may have created.

This was the case with the spent Saturn V rocket stage detected around the orbit of Earth in 2002. There are also many artificial objects left on Mars and the Moon that are however right now too small to be mistaken for geological features.

I think it is always important to make distinction between natural and artificial, because in a sentient-being-capable universe, we may think one of the moons that we observe is perfectly natural, but it may be, stretching the possibility here, a cosmic waste dump!

Whether we'll use "moon" for natural, "satellite" for artificial; put "artificial" or "natural" in front of the "moon", or use some other combo, does not really matter if it is clear that abandoned spaceship is one thing and naturally accreted rock is another.

As for the ultimate definition, I would use more or less the same system as is currently used for making distinction between planets and dwarf planets, which would disqualify ring particles from being moons, however assign "moonlet" status to stable moons in the gaps. More or less what is used popularly today.

--
* - humans and any other possible extraterrestrial civilizations

NEOWatcher
2013-Jul-03, 04:54 PM
Why does the fact that it is artificial imply that the ISS
should not be considered a moon?

EDG drew the line at artificial as do I. It's my opinion of a cutoff point which makes sense to me.

I wouldn't know how to explain why. At least that's before Wolf-S posted.
Yes; I believe the fact that santient beings constructed it is important.

It also seems like a subset of satellite which includes both artificial and natural. So; why not reserve Moon for natural, and some other word for artificial? (Noom?)

As far as a constructed moon out of natural material, it does get kind of fuzzy. I doubt we can come up with a lint trap that doesn't let some of that fuzz through. In that case, it's probably going to have to deal with the ability to detect it's artificial.

I asked before. For what purpose are we naming it? That's going to have a bearing on my opinion. Right now I'm running on objectivity and not subjectivity.

Jeff Root
2013-Jul-03, 09:19 PM
EDG drew the line at artificial as do I. It's my opinion
of a cutoff point which makes sense to me.
Why does anyone want to make a distinction between
objects in orbit around planets based on whether they
are natural or artificial? That distinction seems to be
totally arbitrary and irrelevant, and makes no sense
to me.

NEOWatcher,

This is off on a tangent, but why did you use the terms
"drew the line" and "cutoff point"? First-- and maybe
completely trivially-- are you talking about a line or a
point? Is a line the same thing as a point in this case?

More meaningfully, "drawing a line" suggests making an
arbitrary line (or point) of demarcation between two areas
or ranges of values. For example, a line was drawn at
49 degrees north latitude to separate the area of the USA
from the area of Canada. The location at 49 degrees was
informed by geopolitical considerations, but was basically
arbitrary. It was needed, though, to separate the region
into two different countries. Similarly, a cutoff point of
495 nanometres can be set to distinguish between the
range of spectrally-pure light considered to be blue and
the range considered to be green. That point is informed
by lots of people's opinions, but it, too is arbitrary. Many
people would say it should be closer to 490 nanometres,
because 495-nm light looks too green. Many other people
would say it should be closer to 500 nm, because 495-nm
light looks too blue. In both examples the division is
between two areas or ranges.

Your suggested "cutoff point", however, is between just
two binary values: Natural or artificial. There is no range
or continuum involved, so the expressions "drew the line"
and "cutoff point" seem inappropriate. They don't apply.

I'd say it is sloppy use of terminology, and precision of
terminology is the whole point and purpose of this thread.


I'll repeat what I said before:

Attempts to define something by multiple characteristics
generally cause problems when it turns out that not all the
defined things have all the characteristics.

By adding on restrictions to the definition of "moon", you
make it more difficult to determine whether a newly-found
object is a moon or not. The simple definition I advocate,
which is already in use, requires only that the object be in
orbit around another, larger object which is smaller than a
star. The definition you advocate requires that the object
also be examined to determine whether it is natural or
artificial in order to decide whether to call it a moon or
something else. In most cases, that would probably be
easy to do, but why should it ever be necessary? It has
no advantage over the existing definition which does not
distinguish between natural versus artificial.

-- Jeff, in Minneapolis

Jens
2013-Jul-03, 10:46 PM
Jeff, according to your definition, if a large object were orbiting around a black hole, would it be a planet or a moon? A black hole isn't often as big as a star.

Jens
2013-Jul-03, 10:52 PM
.

My textbook from the introductory astronomy course I
took at the University of Minnesota (An Introduction to
Astronomy, 7th Edition, Baker and Frederick, 1968) way
back in 1971 has a glossary with this entry:

Planet. A satellite of a star, in nearly circular orbit,
which was never capable of shining by a self-sustained
energy reaction.
-- Jeff, in Minneapolis

That's interesting, because doesn't that make something like Ulysses a planet?

Ara Pacis
2013-Jul-03, 11:11 PM
With regard to my proposal, I think a better question would be what is the difference between a pyramid and a mountain? The dictionary I looked used the term natural in defining a mountain. But in fact, you could give me a difficult question: what if humans created a huge pile of rock, or what if people built a huge round pile of rock in orbit around some planet? Then I would tend to use mountain or moon despite the definition.

I think this is a good opportunity to introduce and maintain a distinction between natural, artificial and synthetic. Luna is a natural moon. The ISS is an artificial moon. A human made pile of rock in space is a synthetic moon, because it resembles a natural object using natural, but human aided processes of accumulation (i.e. gravity) similar to those processes used to create certain classes of natural objects that are referred to as moons.

Jeff Root
2013-Jul-03, 11:31 PM
All known black holes are much, much, much, much, much
bigger than any star. Stellar-mass black holes, predicted
long before supermassive black holes were found but not
yet actually detected, would still be far more massive than
Jupiter, even if their event horizons are smaller than many
asteroids. The size of a black hole's event horizon is not
an accurate measure of its overall size!

If a planet was orbiting a star when the star went supernova
and collapsed into a black hole, and the planet survives in
orbit, then it remains a planet.


If somebody wants to call Ulysses a planet, that's okay with
me, but I wouldn't have any particular reason to do so. But...

What were Suisei, Nozomi, and Akatsuki called before they
were given proper names?

-- Jeff, in Minneapolis

.

Jens
2013-Jul-03, 11:33 PM
Why does anyone want to make a distinction between
objects in orbit around planets based on whether they
are natural or artificial? That distinction seems to be
totally arbitrary and irrelevant, and makes no sense
to me.



I don't know why, but it seems common. In Japanese, there is a word for moon, but it is used exclusively for our moon. The moons of other planets are called satellites. So technically in Japanese, moons other than ours and artificial satellites can be called by the same term. But they aren't. For whatever reason, an artificial satellite is always prefaced with artificial. It makes the term long, but people still seem to make the distinction.

EDG
2013-Jul-04, 01:55 AM
Why does the fact that it is artificial imply that the ISS should not be considered a moon?

-- Jeff, in Minneapolis

Honestly, I think we should just pick one of "moon" or "satellite" and drop the other (I prefer satellite), because it can get confusing two have two words that mean roughly the same thing. So you'd end up with "natural satellites" and "artificial satellites" - the ISS is an artificial satellite, the Moon is a natural satellite.

With that, the question of "should an artficial satellite be considered a 'moon' or not" becomes irrelevant.

Jens
2013-Jul-04, 03:33 AM
Honestly, I think we should just pick one of "moon" or "satellite" and drop the other (I prefer satellite), because it can get confusing two have two words that mean roughly the same thing.

Unfortunately, dictionaries don't have a "delete" function!

Jens
2013-Jul-04, 03:49 AM
The simple definition I advocate,
which is already in use, requires only that the object be in
orbit around another, larger object which is smaller than a
star.

Actually, though you say it's already in use, it seems in practice that the distinction between natural and artificial is the current usage. For example, Wikipedia has an entry for "natural satellite", which includes "moon" as an alternative, and another for "satellite," referring to artificial ones, which does not include moon as an alternative. Heavens above has a section on "satellites" which shows the time of passage of only artificial satellites (it doesn't include the moon, for example). If you look up the Wikipedia page for Ganymede, it states that it is a moon of Jupiter, whereas the entry for the ISS states that it is an artificial satellite, but does not use the term "moon" to describe it. My attempt at a definition was descriptive rather than prescriptive, which is why I have trouble responding to the question about why I feel a "need" to make a distinction. I am merely trying to come up with a definition that fits what I see as the real world usage of the term. And that's also why I mentioned that in Japanese, the term satellite is used for both natural and artificial ones (though almost always preceded by "artificial" in the case of an artificial one). So I don't have a problem with whatever one is used.

EDG
2013-Jul-04, 05:18 AM
Unfortunately, dictionaries don't have a "delete" function!

No, but definitions can be pretty specific. And if an accepted definition specifically states "the term 'moon' is not to be used in reference to satellites" then as far as I'm concerned that's that.

Jeff Root
2013-Jul-04, 08:35 AM
Do you know of any general dictionary definition of *any*
word which specifically states it is *not* to be used to refer
to some other specified thing? My dictionary does distinguish
between different terms in the synonym entries which follow
the entries for many words, and also in useage notes for many
words, but as far as I recall, not in the definitions themselves.

Also, I'm not going to retype the whole entry for "moon" from
my dictionary, but the definition that appears to be relevant is:

moon 6. any planetary satellite.

-- Jeff, in Minneapolis

Ara Pacis
2013-Jul-04, 11:20 PM
Do you know of any general dictionary definition of *any*
word which specifically states it is *not* to be used to refer
to some other specified thing? My dictionary does distinguish
between different terms in the synonym entries which follow
the entries for many words, and also in useage notes for many
words, but as far as I recall, not in the definitions themselves.

Also, I'm not going to retype the whole entry for "moon" from
my dictionary, but the definition that appears to be relevant is:

moon 6. any planetary satellite.

-- Jeff, in Minneapolis

Many general usage dictionaries will mention if a particular definition is archaic or out of style.

Jens
2013-Jul-04, 11:35 PM
Many general usage dictionaries will mention if a particular definition is archaic or out of style.

That's not what Jeff was asking. He was asking whether there are definitions like "horses are animals that do not have stripes."

Hornblower
2013-Jul-05, 12:46 AM
Much of this thread strikes me as an attempt at a solution in search of a problem. To me it is perfectly natural and sensible to draw a distinct line between artificial satellites and naturally occurring bodies orbiting planets or asteroids, and to use the generic term "moon" for examples of the latter which resemble our own Moon. I have seen the diminutive "moonlet" used for tiny, irregularly shaped ones. I see no cause for referring to a space station, spacecraft or other artificial satellite, which have nothing in common with the natural bodies except orbital motion, as moons.

Sometimes linguistic evolution does very well without a lot of invented busywork.

EDG
2013-Jul-05, 04:39 AM
That's not what Jeff was asking. He was asking whether there are definitions like "horses are animals that do not have stripes."

What I was suggesting was more akin to taking that definition of "a moon is a satellite" out of the dictionary, not to saying "a moon is something that is not X".

Jeff Root
2013-Jul-05, 05:31 AM
Jens,

I have a book titled "Planets and Moons", by William J.
Kaufmann, III. Could you translate that title into Japanese
and post the result? Windows 7 seems to be able to display
all manner of languages without my having any say in the
matter. Then translate it back into English as it would most
commonly be translated, and post that.

You can post the bill for your services in Japanese, too.

-- Jeff, in Minneapolis

NEOWatcher
2013-Jul-05, 04:20 PM
Why does anyone want to make a distinction between
objects in orbit around planets based on whether they
are natural or artificial? That distinction seems to be
totally arbitrary and irrelevant, and makes no sense
to me.
Did you read "opinion" in that? It makes sense to me, it doesn't make sense to you. So what?


This is off on a tangent, but why did you use the terms
"drew the line" and "cutoff point"? First-- and maybe
completely trivially-- are you talking about a line or a
point? Is a line the same thing as a point in this case?
Oh come now... Now you're just being argumentitive. Didn't you ever hear of idioms?




By adding on restrictions to the definition of "moon", you
make it more difficult to determine whether a newly-found
object is a moon or not.
I notice you avoided using the word satellite which I have repeatedly mentioned in this thread.
In my thinking, a newly-found object is a satellite. As we learn about it, then the satellite category is further defined as moon or man-made.

Jeff Root
2013-Jul-05, 05:46 PM
Why does anyone want to make a distinction between
objects in orbit around planets based on whether they
are natural or artificial? That distinction seems to be
totally arbitrary and irrelevant, and makes no sense
to me.
Did you read "opinion" in that? It makes sense to me,
it doesn't make sense to you. So what?
I don't dispute that it makes sense to you, but since
it makes no sense to me, I want to understand why it
makes sense to you.

And to be clear: We are talking about the meaning of
the word "moon". My question in the quote above is
"Why does anyone want to build a distinction into the
word "moon" between objects in orbit around planets
based on whether they are natural or artificial? I can
completely understand wanting to distinguish between
natural moons and artificial moons, but in that case
one can just add the adjective "natural" or "artificial"
onto the general-purpose word "moon", rather than
changing the meaning of the word "moon" to make it
refer only to natural objects.




This is off on a tangent, but why did you use the terms
"drew the line" and "cutoff point"? First-- and maybe
completely trivially-- are you talking about a line or a
point? Is a line the same thing as a point in this case?
Oh come now... Now you're just being argumentitive.
Didn't you ever hear of idioms?
I said it might be trivial. I couldn't tell why you used
two different idioms. If it was unintentional -- if you
did it without realizing what you were doing -- I'd say
it is slightly confusing because it is a mixed metaphor.
If it was an intentional redundancy, to try to make what
you were saying more clear by saying it in two different
ways, then the idea evidently was important to you.
The idea of drawing a line between different areas or
a cutoff point indicating a maximum or minimum value
in a continuum. But that doesn't apply, because what
you are doing is distinguishing between two binary
values, not setting a dividing line or cutoff point, so it
doesn't make sense that you put such importance on
it to say it twice.

So I ask again, why did you use the expressions "drew
the line" and "cutoff point"? Was there a reason, or
was it just sloppy use of language?




By adding on restrictions to the definition of "moon", you
make it more difficult to determine whether a newly-found
object is a moon or not.
I notice you avoided using the word satellite which I have
repeatedly mentioned in this thread.
The subject of the thread is the meaning of the word
"moon". Sometimes it does seem more appropriate to
use the word "satellite", but since the subject in this
case is the word "moon", I had no reason to use the
word "satellite" in its place.



In my thinking, a newly-found object is a satellite. As we
learn about it, then the satellite category is further defined
as moon or man-made.
That appears to be logical and as consistent with existing
usage as my suggestion. I'm not sure I can object to it.

-- Jeff, in Minneapolis

NEOWatcher
2013-Jul-05, 06:37 PM
I said it might be trivial. I couldn't tell why you used
two different idioms.
Then please just ask and don't give me a lecture on what a line is.


Was there a reason, or
was it just sloppy use of language?
Call it sloppy if you will. I'm not a wordsmith, so those mistakes are hard to avoid.


That appears to be logical and as consistent with existing
usage as my suggestion. I'm not sure I can object to it.
I'm happy I was able to finally find the right words to convey my message. Thanks for understanding.

Jens
2013-Jul-06, 12:32 AM
I think for the book Jeff mentioned, 惑星と衛星 would be a good translation, and Planets and Satellites is the literal back-translation.

Jens
2013-Jul-06, 12:40 AM
Jeff, what I have a hard time understanding is, the common usage to me seems to use the term moon to refer to natural satellites, and to use "satellite" or "artificial satellite" to refer to an artificial one. Do you disagree? Or do you agree, but want to change it?

neilzero
2013-Jul-06, 12:53 AM
We have duplicate (or almost duplicate) names for almost everything, especially if you consider that humans speak and write in thousands of languages and dialects. Solutions are futile in my opinion.
I find I can think of an alternate meaning for almost every sentence, no matter who said or wrote it. If you bore us with details, we are more likely to understand the intended meaning.
If we invest 10% of our waking hours on learning spelling, grammer, word meanings, definitions and math, we possibly will be seriously deficient, in compassion, intertainment, comrady, physics, chemistry, astronomy, business and sociology/one or more other important things especially if forgetting means we have to relearn occasionally. Moderate is often best. Neil

Jeff Root
2013-Jul-06, 03:24 AM
I think for the book Jeff mentioned, 惑星と衛星 would be a
good translation, and Planets and Satellites is the literal
back-translation.
Thanks. I can't read the Japanse, but at least I can look
at it, and perhaps copy it and show it to others. Oooh.
I just noticed that it is two kanji followed by one kana
follwed by two kanji. I'll bet the first two characters
translate as "planets", the next character translates as
"and", and the last two translate as "satellites"!



Jeff, what I have a hard time understanding is, the
common usage to me seems to use the term moon to
refer to natural satellites, and to use "satellite" or
"artificial satellite" to refer to an artificial one. Do you
disagree? Or do you agree, but want to change it?
I agree, but I don't want to change it. I think that use
of the term "moon" to refer to artificial satellites, while
not common now, was common in the late 1950's when
artificial moons were a new thing, and I see no good
reason to try to stop that useage and call it obsolete
or somehow "wrong".

-- Jeff, in Minneapolis

DonM435
2013-Jul-06, 02:34 PM
A fellow in our college dorm once got busted for a "mooning" offense. I wonder if a redefinition would help his Permanent Record retroactively? ;)

Centaur
2013-Jul-06, 04:23 PM
I've always thought of moon as natural, and satellite as man made.

Unfortunately many people born after the first artificial satellites were launched in 1957 do not realize that previously all satellites were natural. When I was young the word satellite was only applied to the Moon and the natural satellites of planets and stars. At that time there were no artificial satellites. In fact the planets are satellites of the Sun. This misunderstanding among young people led to the popular use of the word moon in application to the natural satellites of any planet. Journalists finding moon simpler to spell than satellite compounded the problem. Of course the others are only moons by analogy to the Moon. Before the first artificial satellites were launched, the word moon was much less frequently applied to natural satellites other than the Moon. Nowadays we occasionally hear people asking why the Moon has no name of its own. Of course it has a name: Moon. Before the invention of the telescope in the early seventeenth century, it was the only object that people called Moon. I never refer to the natural satellites of other planets as moons. For me there is only one Moon.

Ara Pacis
2013-Jul-07, 03:51 AM
Unfortunately many people born after the first artificial satellites were launched in 1957 do not realize that previously all satellites were natural. When I was young the word satellite was only applied to the Moon and the natural satellites of planets and stars. At that time there were no artificial satellites. In fact the planets are satellites of the Sun. This misunderstanding among young people led to the popular use of the word moon in application to the natural satellites of any planet. Journalists finding moon simpler to spell than satellite compounded the problem. Of course the others are only moons by analogy to the Moon. Before the first artificial satellites were launched, the word moon was much less frequently applied to natural satellites other than the Moon. Nowadays we occasionally hear people asking why the Moon has no name of its own. Of course it has a name: Moon. Before the invention of the telescope in the early seventeenth century, it was the only object that people called Moon. I never refer to the natural satellites of other planets as moons. For me there is only one Moon.

Good points. I refer to the satellites of other planets as either satellite planets or satellite asteroids, depending on size, composition or suspected origin.

Jens
2013-Jul-07, 06:59 AM
Unfortunately many people born after the first artificial satellites were launched in 1957 do not realize that previously all satellites were natural.

I have a couple on nitpicks. First, I was born after 1957 but I never thought that there had always been artificial satellites. Second, satellite isn't a particularly hard word to spell. In any case, editors have dictionaries so that they can even use words like rhythm. And then also, the online etymology dictionary I often refer to says that moon was used to refer to the moons of other planets as late back as 1665, so it isn't particularly new.

Matthias
2013-Jul-09, 05:26 AM
I would have little difficulty referring to an artificial satellite with enough mass to collapse into a sphere from its own gravity. But when I think of the word "moon" I think of something round, but most so-called "moons" in our solar system are not. Our own Moon is in the minority! Drawing the line at hydrostatic equilibrium keeps the number of known moons to a more or less fixed and small number (which is, if you think about it, is partly why the IAU defined what a planet was, because we might have been left with more than a dozen 'planets' on "The List" depending how many near Pluto-mass or larger KBOs and detached-disk objects may be out there). So we have less than two dozen actual "moons", and finding a new one will be really big news no matter where or how it is found--and astronomers can track down all the dozens and dozens of "minor satellites" that orbit the outer planets to their heart's content.

Ara Pacis
2013-Jul-10, 04:55 AM
I would have little difficulty referring to an artificial satellite with enough mass to collapse into a sphere from its own gravity. But when I think of the word "moon" I think of something round, but most so-called "moons" in our solar system are not. Our own Moon is in the minority! Drawing the line at hydrostatic equilibrium keeps the number of known moons to a more or less fixed and small number (which is, if you think about it, is partly why the IAU defined what a planet was, because we might have been left with more than a dozen 'planets' on "The List" depending how many near Pluto-mass or larger KBOs and detached-disk objects may be out there). So we have less than two dozen actual "moons", and finding a new one will be really big news no matter where or how it is found--and astronomers can track down all the dozens and dozens of "minor satellites" that orbit the outer planets to their heart's content.

Who cares how many ?-class objects there are of something in the solar system? Maybe you can start a movement to limit the number of asteroids by redefining what that word means.

Matthias
2013-Jul-11, 04:32 AM
Or we could assign asteroid numbers to every substellar object in the solar system treat them no differently than any other miscellaneous debris in the solar system. /sarcasm Really, there's no point to a name or a number or classification that does not help advance science. Yet human beings, love doing such frivolous things all the time.