PDA

View Full Version : The Amazing Shrinking Earth



SkyEyeGuy
2003-Jun-16, 06:50 PM
All,

No doubt you've all seen the 'Why does the full Moon look so large as it rises, and seem to shrink soon after?' question before. I have a variation of it for you; I need to know this so I won't look silly to the editors at Analog (I normally write fantasy stories, but I'm about to strike out for an SF setting, thus my post here).

Now, my understanding of the extra-large full Moon as it hangs just over the horizon phenomena is that the Moon isn't really any bigger than it will be when it reaches the middle of the sky. The whole thing is an illusion -- caused not by refraction or a similar mechanism, but by a peculiarity of human perception.

Okay. That said, I plan to have a group of characters standing (yes, in pressure suits, etc.) out on the Moon (Mare Crisium, specifically). They watch the Earth rise.

Can I expect my Moon-based viewers to perceive that the Earth appears to shrink as it rises, as does the Moon, when observed from Earth?

Yeah, I plan to throw in the old image-refracted-through-thick-atmosphere trope, only so my protagonist (a Lunar citizen tired of bad astronomy) can comment pithily upon it.

Is this scene plausible? I think so, but the Lunar horizon may appear to be closer than Earth's, or. . . um, something else.

It should be apparent now why I make so few forays into SF.

Thanks!

QuagmaPhage
2003-Jun-16, 07:07 PM
Is this scene plausible?

No. Because the Moon always has the same surface towards the Earth it would not appear to rise over the horizon. From a point on the Lunar surface the Earth would appear to stand still in the sky. To get the desired effect the charachters on the Moon would need to travel over large distances and thus perceive the Earth as "rising" in the sky.

But otherwise it would be a good scene.

SkyEyeGuy
2003-Jun-16, 07:15 PM
See? Something TOLD me I had no business setting a story on the Moon!

Thank you, thank you, a million times thank you!

Donnie B.
2003-Jun-16, 08:03 PM
There is one exception. If your character is standing right at the limb of the Moon (as seen from Earth), a phenomenon known as libation could allow him to see the Earth rise and set once a month. The Earth would never get very high in the sky, though, maybe barely over the horizon if the location was just right. And it would be a slow process -- two weeks to rise, then two more weeks to set again in the same spot.

Not exactly the stuff of gripping drama...

SkyEyeGuy
2003-Jun-16, 08:33 PM
Thanks, Donnie B.

I'll just rewrite the scene -- it served only to annoy my protagonist anyway, and there are plenty of ways to annoy the man. I just thought it would be fun to toss in a bit of persistent Bad Astronomy, since I imagine misconceptions and fallacies will follow us into space as they have followed everywhere else.

What irks me is that I nearly pulled such an idiotic stunt.

Would Starry Night or Redshift or any other commercial astronomy software be of any help, do you think? I know that some of the packages claim to be able to produce views that would be seen from many places and many times -- even places and times off Earth.

I'll have to look into some of those.

Thanks!

tracer
2003-Jun-16, 08:36 PM
Just remember: Since it takes about a month for the moon to go arond the Earth once, and it's locked in synchronous rotation with the Earth, a solar "day" on the moon will be about a month long (29-and-a-half days for the sun to come around to the same point in the "sky"), and a sidereal "day" on the moon will also be about a month long (28 days for a given star to come around to the same point in the "sky").

Emspak
2003-Jun-16, 08:48 PM
Hey SkyEYe-

I thought of something if you want a quick "bad astronomy" way to annoy some guy on the Moon.

Try this -- an eclipse as seen from the Moon will last considerably longer than from Earth, since it takes longer to pass through the Earth's shadow. As such, it is also visible from anywhere becuase unlike te Lunar shadow, whose "cone" only just touches the Earth's surface, the Earth's shadow cone covers the whole Moon, and then some.

This would mean that the Solar Corona is a bit less visible, and also that the Earth would appear to pass over the Sun in an hour or so as opposed to the few minutes of totality on Earth. The movement would be more apparent than an Earthrise.

If viewed from the limb of the Moon, it would make a pretty dramatic sight.

It would be interesting to see if the light on the Lunar surface would look reddish to people on it the way it does from Earth (look at a picture of a Lunar eclipse and you will see what I mean). Red light bends around the Earth's atmosphere, so the edge of the Sun would redden as it passes behind the limb of the Earth.

Something you could do there, maybe.

tracer
2003-Jun-17, 12:22 AM
Try this -- an eclipse as seen from the Moon will last considerably longer than from Earth, since it takes longer to pass through the Earth's shadow. As such, it is also visible from anywhere becuase unlike te Lunar shadow, whose "cone" only just touches the Earth's surface, the Earth's shadow cone covers the whole Moon, and then some.
Except during a partial lunar eclipse.

Which brings up another point: A "lunar" eclipse, as seen from the moon, would be seen as an eclipse of the sun. A "solar" eclipse seen from the moon would appear as a tiny little eclipse of part of the Earth. Would moon dwellers call a lunar eclipse a solar eclipse, and a solar eclipse a terrestrial eclipse?

The Supreme Canuck
2003-Jun-17, 12:23 AM
It stands to reason that they would.

ToSeek
2003-Jun-17, 12:43 AM
Would Starry Night or Redshift or any other commercial astronomy software be of any help, do you think? I know that some of the packages claim to be able to produce views that would be seen from many places and many times -- even places and times off Earth.


Starry Night Backyard - the package I've got - will reproduce views from just about anywhere in the solar system.

Karthesios
2003-Jun-19, 08:22 AM
.

It would be interesting to see if the light on the Lunar surface would look reddish to people on it the way it does from Earth (look at a picture of a Lunar eclipse and you will see what I mean). Red light bends around the Earth's atmosphere, so the edge of the Sun would redden as it passes behind the limb of the Earth.

Something you could do there, maybe.

I saw a painting somewhere of what a lunar eclipse would appear like if you were standing on the moon, I wish I could find the image, but I could've sworn it was on Space.com or something, but anyway, the artist had the moon in a dull red light.

crazy4space
2003-Jun-19, 01:35 PM
[
quote="tracer"]Just remember: Since it takes about a month for the moon to go arond the Earth once, and it's locked in synchronous rotation with the Earth, a solar "day" on the moon will be about a month long (29-and-a-half days for the sun to come around to the same point in the "sky"), and a sidereal "day" on the moon will also be about a month long (28 days for a given star to come around to the same point in the "sky").[/quote]
Distance from Earth ,Mean, 238,840 miles or 0.0025695 astronomical units
Maximum - 252,700 miles
Minimum- 221,460 miles
Sidereal period - 27.321661 days
Synodic period - 29d 12h 44m 02s
Axial inclination of equator, referred to the ecliptic - 1 degree 32'
Orbital eccentricity - 0.0549
Orbital inclination - 5 degrees09'
Mean orbital velocity - 2,287 m.p.h. = 0.63 miles per second = 3,350ft per second.
Apparent diameter - max. 33'31", mean 31'5", min 29'22"
Magnitude of full moon - -12.7
Mean albedo - 0.07
Diameter - 2,160 miles (3,476 kilometers)
Mass 1/81.3 Earth=0.0123 Earth= 3.7 X 10 -8 Sun.
Volume - 0.0203 Earth
Escape velocity - 1.5 miles per second (2.38 km/sec).
Density 3.34 water = 0.60 Earth
Surface gravity 0.01653 Earth

Hope this helps. :D

Mainframes
2003-Jun-19, 02:00 PM
Surface gravity 0.01653 Earth



Forgive my pickyness, but shouldn't that be 0.1653 Earth?

eburacum45
2003-Jun-20, 10:14 AM
Sure the moon would be bathed in reddish light during an eclipse- when you are in the umbra, the only light that reaches you would be from the earth's atmosphere, which would appear as a reddish ring in the sky.
This light is sunlight scattered by Rayleigh scattering, and is really the light of all the sunsets on the earth at once.
Rayleigh scattering causes all the blue light to be scattered much more widely, so little of the blue light reaches the moon.

As an aside, there is a place near the tip of the earth's shadow cone which allows you to see the rim of the sun all the way round the earth- a terrestrial annular eclipse. Because of refraction the annulus would appear bright red.

crazy4space
2003-Jun-20, 03:56 PM
Surface gravity 0.01653 Earth




Forgive my pickyness, but shouldn't that be 0.1653 Earth?

Went back to check my data and that is how its printed - remember to start at tens on the right side of the decimal.

QuagmaPhage
2003-Jun-20, 07:19 PM
[
Magnitude of full moon - -12.7

Surface gravity 0.01653 Earth


0.01653 Earth = 1,653 % of Earth's surface gravity so this is wrong. It should be 1/6 of Earth's surface gravity which is about 16 %. Your data must be wrong.

While we are nitpicking it should also say:

Apparent magnitude of full moon - -12.7

to avoid confusion with absolute magnitude. :wink:

tracer
2003-Jun-20, 07:22 PM
0.01653 Earth = 1,653 % of Earth's surface gravity so this is wrong.
Darn right it's wrong! 1635% of Earth's surface gravity would be over sixteen times --

Oh, wait, you're from one of those countries that uses a comma for a decimal point, ain'tcha? :oops:

QuagmaPhage
2003-Jun-20, 07:35 PM
0.01653 Earth = 1,653 % of Earth's surface gravity so this is wrong.
Darn right it's wrong! 1635% of Earth's surface gravity would be over sixteen times --

Oh, wait, you're from one of those countries that uses a comma for a decimal point, ain'tcha? :oops:

Yes, sorry. I should have remembered that some countries are backwards in this respect. :oops:

crazy4space
2003-Jun-23, 12:52 PM
[
quote="tracer"]
0.01653 Earth = 1,653 % of Earth's surface gravity so this is wrong.
Darn right it's wrong! 1635% of Earth's surface gravity would be over sixteen times --
I have had this hanging in my office for over 10 years. I got this from a library book and from what I can remember he was an English selenologist who studied the moon for a living and had worked for the Royal Astronomical Society( hope this is right), wish I could remember his name. So now I'm confused which is it 0.01635 or 0.1635. I know it's 1/6th and when I do the math with the zero it works out to 6.12 and without it works out to 6.25?

Mainframes
2003-Jun-23, 01:01 PM
[
quote="tracer"]
0.01653 Earth = 1,653 % of Earth's surface gravity so this is wrong.
Darn right it's wrong! 1635% of Earth's surface gravity would be over sixteen times --
I have had this hanging in my office for over 10 years. I got this from a library book and from what I can remember he was an English selenologist who studied the moon for a living and had worked for the Royal Astronomical Society( hope this is right), wish I could remember his name. So now I'm confused which is it 0.01635 or 0.1635. I know it's 1/6th and when I do the math with the zero it works out to 6.12 and without it works out to 6.25?

It is roughly one sixth gravity - 1/6 is about equal to 0.1653 which is also equal to 16.53%. Simple calculation, just divide 1 by 6 on your calculator and you'll get the correct order of magnitude, if not the exact answer....