Real Moon doesn't move like in Celestia!

[ccm01]

Hello

I'm a new fan of Celestia,and it's amazing to see the universe and our solar system from such another point of view.

But I've remarked (I think) a very bad mistake.

When I watch the real Moon in the night I can only see one side of the real Moon, equal on which day or date I do this. That means if you're standing on the real Moon and you want to watch earth, the earth won't move from it's position in the "sky of the Moon".

Other explaination:
The real Moon needs approximetly 28 days to turn around itself and also approximetly 28 days to turn around the earth.

But in Celestia: If you're standing on the celestia moon (SyncOrbit) and want to watch the earth the earth is MOVING! through the "sky of the celestia moon".

!!!But that's impossible!!!

Can somebody explain this problem more detailed?
Is it really a mistake in Celestia?
Do I think wrong?
Is it possible to solve the problem in Celestia manually, if it is an mistake?


Hoping for replies
Bye

[HankR]

I haven't actually checked the latest version of Celestia, so it's possible there really is a bug. But in fact, the earth does not remain stationary in the lunar sky. It actually moves around quite a bit. This is because the moon does not present exactly the same face to the earth. It turns slightly, first this way then that. This phenomenon is called libration. As a result, somewhat more than half the lunar surface (about 60 per cent, I believe) is visible from earth over time. The change in the orientation of the moon's face isn't very noticeable unless you examine the limb carefully at the extreme positions. But from the moon, the change in the earth's location is very apparent. I was surprised by this myself when I first saw it using Starry Night.

So in spite of what you've been told, the earth does move!

- Hank

[erostosthenes]

well if you were surprised that the earth moves from the point of view of the moon, then you'll be shocked to know the moon doesn't actually orbit the earth so to speak. it is actually 2.3X more gravitationally attracted to the sun than to the earth and thus travels in its own orbit around the sun, though highly perturbed by the earth.

[ccm01]

Hi

it's ccm01 again
i've registered now

and I want to thank HankR for the explaination.
After reading, i checked up and searched a good position on moon to the earth. With fast running time I saw that the earth was moving like a 8 in the sky of the moon (this time I used the ZOOM-out). That confirms HankR's explaination.

Thank you again


PS: @erostosthenes ( what a difficult name!)
you're also right I think, but this isn't it common to all moons in the
solarsystem?

[erostosthenes]

PS: @erostosthenes ( what a difficult name!)
you're also right I think, but this isn't it common to all moons in the
solarsystem?

actually no, this is unique to the earth moon system. you can work it out pretty simply for yourself.

a=MG/r^2 and take the ratios of the acceleration toward the sun and toward the earth for the moon, so you have:

(Msun*d^2)/(Mearth*(1au-d)^2) where d is the distance between the earth and the moon = approximately 3E5km and Msun is approximately 333333.3*Mearth.

so then you get 333333.3*((3E5km)^2 )/((1au-3E5km)^2)

you'll find this ratio is something like 2.3 which means the moon is more than twice as attracted to the sun than to the earth. no other moon is like this.

[HankR]

It's true that the moon's acceleration due to the sun's gravitational attraction is greater than its acceleration due to the earth's gravitational attraction. However, since the earth is subject to nearly the same acceleration due to solar gravity as the moon, the effect of solar gravity on the motion of the moon relative to the earth is comparatively small. Specifically, it is smaller than the effect of the earth's gravity on the motion of the moon relative to the sun. In other words, the perturbation of the moon's geocentric orbit by the sun is smaller than the perturbation of the moon's heliocentric orbit by the earth. Thus, the moon's actual motion is closer to its unperturbed orbit when considered as orbiting the earth rather than the sun.

In the general case, orbital dynamicists define the "sphere of influence" (or "sphere of activity") of a planet as the region within which the planet's perturbation of the heliocentric orbit of a nearby body exceeds the sun's perturbation of the planetocentic orbit of that body. In this region it is more convenient to consider the body's motion as a planetocentric orbit disturbed by the sun than as a heliocentric orbit disturbed by the planet. The radius of the earth's sphere of influence is approximately 925000 km; the moon's orbit, at 384000 km, is well within this limit.

- Hank

[HankR]

In other words, the perturbation of the moon's geocentric orbit by the sun is smaller than the perturbation of the moon's heliocentric orbit by the earth. Thus, the moon's actual motion is closer to its unperturbed orbit when considered as orbiting the earth rather than the sun.

- Hank

I think this is somewhat subjective here. Plotting the motion of the moon against background stars over the course of a year can look either like a perturbed heliocentric orbit or an odd geocentric orbit. That's why I chose to just look at gravitational acceleration, but it was certainly easier for the Apollo astronauts and mission planners to approximate the orbit of the moon as simply geocentric. And I'm not so sure about this "sphere of influence". Were the moon's orbit highly inclined to nearly 90 degrees, it could orbit at a much further distance than as it is now.

[Eros]

the above post was by me, erostosthenes.

[HankR]

It's true, of course, that the moon orbits the sun along with the earth. But it is indeed a fact, as explained above, that the moon's orbit with respect to the earth is less perturbed by the sun's gravity than the moon's orbit with respect to the sun is perturbed by the earth's gravity. I didn't make this up; my explanation is based on the discussion in the book "Orbital Motion" by A.E. Roy (3rd ed. 1988), which includes a mathematical derivation of the relative magnitude of the respective perturbations. Nothing subjective about it.

- Hank

[Rocketman]

Woah. I just attempted to read the posts before this one. My brain hurts. Does every body on here talk like that?