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Fooglmog
2010-Oct-07, 11:02 AM
So, I made an off the cuff remark in a thread on the CT forum a few days ago about how Cruithne is not actually Earth's second moon despite some people suggesting it to be. Jim took exception to this, and we started discussing whether or not Cruithne actually is a moon of Earth.

The discussion had nothing to do with the OP, so a mod asked us to take the discussion to a more appropriate venue. I'm going to copy what I believe to be the important excerpts from our discussion to give some context here (View the full thread Here (http://www.bautforum.com/showthread.php/108218-Hurrah-for-QI!)), though this discussion doesn't have to be a continuation of that one. I'm more interested in learning what the arguments in favour of calling Cruithne a "moon" are.

Fooglmog:
I'm sorry, I happen to hold onto the old fashion precept that to call an object a "moon" it must actually be orbiting the object which you claim it is a moon of.

Jim:
Actually, there is an ongoing debate as to whether Cruithne should be classified as a "temporary" moon or a "co-orbital" moon of Earth.

Fooglmog:
I personally have not seen either "temporary moon" or "co-orbital moon" suggested as names for objects like Cruithne. I would normally give you the benefit of the doubt (as I have strong suspicions that you have a much stronger background in astronomy than I) however, both of those terms are already in use as references to entirely different bodies. A co-orbital moon is a moon which follows the same orbital path around a planet as another moon (such as Janus and Epimetheus around Saturn), while a temporary moon is exactly what is sounds like -- a body which is captured by a planet, orbits it for a time, then escapes (such as Kushida-Muramatsu did around Jupiter).

***

That's where Captain Swoop suggested we move to a new thread the discuss Cruithne.

So, I ask, what are the reasons why it's being called a "moon" by some? It seems pretty clear cut to me. Cruithne does not orbit Earth, therefore it is not a moon of Earth.

AndreasJ
2010-Oct-07, 11:18 AM
I believe the basic reason to refer to Cruithne as a "moon" is to sell newspapers. "Second moon" makes for more eye-catching headlines than "itzy asteroid in a funky orbit".

Trakar
2010-Oct-07, 03:36 PM
I believe the basic reason to refer to Cruithne as a "moon" is to sell newspapers. "Second moon" makes for more eye-catching headlines than "itzy asteroid in a funky orbit".

While I largely agree with you, a moon is simply any body which orbits another. So the issue is really as simple as whether, or not, this orbit should be considered to be an Earth orbit. It definitely interacts with the Earth and is unstable over the long term, but the orbit is actually a Solar orbit rather than an Earth orbit, which would seem to discount the use of the term moon. However, it is in a 1:1 resonance orbit with the Earth and the term used for such objects is quasi-satellite.

http://www.astro.uwo.ca/~wiegert/quasi/quasi.html

Centaur
2010-Oct-07, 04:50 PM
The 5 km asteroid Cruithne orbits the Sun in its own elliptical orbit. Its orbital characteristics at first glance appear fairly normal for an Apollo type asteroid. That is one whose orbit carries it both nearer and farther from the Sun than Earth. Its aphelion is a little beyond the orbit of Mars, while its perihelion approaches the orbit of Mercury. As with all asteroids, its orbital characteristics oscillate a bit in a cyclical fashion over periods of centuries due to the gravitational influence of the planets.

What’s unusual about Cruithne's orbit is that in the long run its semi-major axis averages almost exactly the same length as that of Earth’s, meaning its orbital period averages almost exactly one year. This is undoubtedly due to the gravitational influence of the Earth. However, over a cycle of 770 years, Cruithne’s semi-major axis oscillates between 0.997 and 1.003 AU. That means that for about half of the cycle it has a period of slightly less than one year and for the other half a little more than a year.

The closest Cruithne ever gets to the Earth is about 12.5 million km. In the current era the annual minimum is about 50 million km and growing. Between 2050 and 2224 it will always be more than 1 AU (150 million km) from the Earth. In fact, between 2046 and 2234 Cruithne will appear in superior conjunction (other side of Sun) twice a year.

As noted, as viewed in a normal diagram of the solar system, Cruithne orbits the Sun in its own elliptical orbit. What’s interesting is if we plot Cruithne’s movement on a bipolar diagram in which both the Sun and Earth are kept in one position. This way we see Cruithne’s movement relative to a stationary Earth. In this case its orbit appears to be in the shape of a kidney bean. In the current era it is perpetually a “morning star” and from our point of view appears to be orbiting in a kidney bean shaped oscillation centered on a point well ahead of the Earth in its orbit, never circling the Earth. Currently, Cruithne’s orbital period is a little less than one year, so its net apparent movement leads to a gain further beyond the Earth over time.

Remember that in actuality (or inertially) Cruithne is moving in an elliptical orbit around the Sun, and as it repeatedly reaches a particular point in its real orbit, the Earth is a little further behind in its orbit than it was the previous year. This will continue until 2285 when Cruithne’s orbital period will become more than one year, and in the bipolar diagram the center of its kidney bean shaped orbit will seem to start regressing back toward the Earth and eventually a little behind it as it was for much of the twentieth century. During those periods separated by 770 years, Cruithne has the appearance of orbiting Earth in a kidney bean shaped orbit.

The horseshoe shape occurs at an even higher level of imagination. The horseshoe is the oscillation over 770 years of the center of the apparent kidney bean shaped orbit as viewed in the bipolar diagram.

The Moon is the only Moon in my book. I cringe when that term is applied to other natural satellites of planets. I agree that its secondary use is a media invention designed to sell (and save space and prevent spelling errors), especially when referring to a second Moon of Earth. Some have applied the term quasi-satellite to Cruithne, and I have no quarrel with that.

Agemegos
2010-Oct-08, 12:28 AM
Is Cruithne anything like an Earth Trojan?

Centaur
2010-Oct-08, 04:08 AM
Is Cruithne anything like an Earth Trojan?

Welcome to the discussion group, Agemegos.

That’s an interesting question. Of course this entire thread is in regard to questions of semantics. I would say that the Trojans are an intermediate class between a true planetary satellite and something like Cruithne. An individual Trojan orbits the Sun with a mean heliocentric longitudinal separation of 60° from Jupiter, although this can vary considerably during a nearly twelve-year orbital period. The mean orbital resonance is 1:1. It can be considered to be also orbiting Jupiter at the same time. The case of Cruithne is considerably looser and more complex, although still in a 1:1 mean orbital resonance with Earth.

Of course it can be argued that the Moon primarily orbits the Sun while being perturbed by the Earth. That’s because the Moon is the only planetary satellite whose orbit relative to the Sun is always concave. Nevertheless, I’ll vote to keep the Moon in the planetary satellite class.

Bearded One
2010-Oct-10, 01:23 AM
If Cruithne is a moon of Earth wouldn't that make Pluto a moon of Neptune?

Centaur
2010-Oct-10, 01:34 AM
If Cruithne is a moon of Earth wouldn't that make Pluto a moon of Neptune?

Only in a related quasi-satellite sense, although with a 3:2 orbital resonance rather than 1:1. What you mention is one of the arguments that led to a reclassification of Pluto as something other than a major planet. Neptune gravitationally controls that region of the solar system and Pluto must submit completely to Neptune's influence.

Fooglmog
2010-Oct-10, 01:36 AM
Not necessarily.

It all depends on what characteristics of Cruithne qualify it as a moon. Cruithne has plenty of characteristics that Pluto lacks, and it seems possible that the arguments in favour of its moonhood would rest (at least partially) upon these characteristics.

Of course, it's impossible to know definitively unless someone who actually believes that Cruithne is a moon of Earth joins this thread and makes the arguments to that effect.

caveman1917
2010-Oct-11, 05:34 PM
If we take the earth-moon system and isolate it, it would continue to do the same thing. If we do that with the earth-cruithne system it wouldn't (it might end up in some orbit - but it's not the same thing), so calling that a moon of earth seems stretching things a little.

Centaur
2010-Oct-11, 06:05 PM
If we take the earth-moon system and isolate it, it would continue to do the same thing. If we do that with the earth-cruithne system it wouldn't (it might end up in some orbit - but it's not the same thing), so calling that a moon of earth seems stretching things a little.

That’s very true regarding the Earth and Moon, as long as we’re willing to ignore significant perturbations from the Sun. And if we were to isolate the Sun and Moon, the Moon would orbit the Sun along a path not too different from its actual course. But back to your main point, indeed Cruithe may follow almost any path depending on the time that it hypothetically would become isolated with the Earth, and another good argument for not calling it a satellite of Earth (Yikes, never moon!)

tony873004
2010-Oct-13, 10:33 PM
As has been mentioned, Cruithne doesn't directly orbit the Earth. Its orbit is a horseshoe orbit as Curt mentions. Here is my page about Cruithne, complete with an animation of its 'kidney bean' shaped orbit tracing horseshoes around the Sun with the Earth centered on the horseshoe's gap.
http://www.orbitsimulator.com/gravity/articles/cruithne.html

I wouldn't use the term quasi-satellite to describe Cruithne. That terms describes asteroids like 2004 GU9, which when viewed in a rotating frame actually circle the Earth, rather than circling the Sun in a horseshoe pattern like Cruithne. Look at the 2nd post in this thread for an animation of 2004 GU9's orbit:
http://www.orbitsimulator.com/cgi-bin/yabb/YaBB.pl?num=1175113160/0#0

2004 GU9 still does not directly orbit the Earth. It only appears to in a rotating frame. So it would not be correct to call it a moon of Earth's either.

Earth does occasionally pick up temporary satellites, capturing them from interplanetary space. They complete a few direct orbits of Earth before departing back into interplanetary space.

Trakar
2010-Oct-14, 04:24 AM
As has been mentioned, Cruithne doesn't directly orbit the Earth. Its orbit is a horseshoe orbit as Curt mentions. Here is my page about Cruithne, complete with an animation of its 'kidney bean' shaped orbit tracing horseshoes around the Sun with the Earth centered on the horseshoe's gap.
http://www.orbitsimulator.com/gravity/articles/cruithne.html

I wouldn't use the term quasi-satellite to describe Cruithne. That terms describes asteroids like 2004 GU9, which when viewed in a rotating frame actually circle the Earth, rather than circling the Sun in a horseshoe pattern like Cruithne. Look at the 2nd post in this thread for an animation of 2004 GU9's orbit:
http://www.orbitsimulator.com/cgi-bin/yabb/YaBB.pl?num=1175113160/0#0
...

There are difference of opinon on the terminology, but for those interested, and in conjunction with my earlier link :

The non-rotating and the rotating frame (http://www.astro.uwo.ca/~wiegert/quasi/int1.mpg) (2.7 Mb) mpg

The relative motions of a quasi-satellite and its planet are shown. After two orbits, the lines of sight connecting the QS to the planet are drawn in green. Both the quasi-satellite and the planet orbit the Sun in the same amount of time, though the quasi-satellite, because its orbit is elliptical, moves faster when closer to the Sun and slower when farther away. Note that, though they both go around the Sun, both are always on the same side of the Sun and in relatively close proximity to each other. Then, the planet will begin "picking up" the lines of sight, constructing the apparent path of the QS as seen from the orbiting planet. Their motions are such that the QS appears to perform a kidney-shaped loop around the planet. Though the QS thus appears to orbit the planet, this is a result of the frame of reference we are using.
http://www.astro.uwo.ca/~wiegert/quasi/quasi.html

Ara Pacis
2010-Oct-19, 06:25 PM
The Moon is the only Moon in my book. I cringe when that term is applied to other natural satellites of planets. I agree that its secondary use is a media invention designed to sell (and save space and prevent spelling errors), especially when referring to a second Moon of Earth. Some have applied the term quasi-satellite to Cruithne, and I have no quarrel with that.

I would prefer to make the term "moon" generic, and make The Proper name of our moon Luna (or Selene). I don't care if moon is tied to the concept of the time-period we call a month. I'm okay with the word economy of the current convention over "natural satellite".

For Cruithne, I'd call it neither, as the term "companion" seems apt enough. But we might want to call it something else like "an object that the so-called "planet" has not yet cleared from its orbit but which is not massive enough to disqualify aforementioned planet from planetary status".

Fooglmog
2010-Oct-20, 07:27 AM
For Cruithne, I'd call it neither, as the term "companion" seems apt enough. But we might want to call it something else like "an object that the so-called "planet" has not yet cleared from its orbit but which is not massive enough to disqualify aforementioned planet from planetary status".
Would that be "AOTTSCPHNYCFIOBWINMETDAPFPS" for short?