Viewing Earth from the Moon - Would it Stay in One Spot?

[KevinMWHM]

If I was stationary on the Moon looking at Earth, would it never appear to move from its location in the sky? I made a quick model using a tennis ball and a globe and I'm thinking that it would go through "phases" (cresent, full, etc.) as well as spin, but stay in one spot.

(which would look pretty cool I would think!)

 

[vanesch]

If I was stationary on the Moon looking at Earth, would it never appear to move from its location in the sky?

That's correct. As the moon has always the same face towards the earth, at least its azimuth wouldn't change. The altitude could wobble a bit if the moon orbit is not perfectly in the equatorial plane (don't know by heart).

 

[Janus]

That's correct. As the moon has always the same face towards the earth, at least its azimuth wouldn't change. The altitude could wobble a bit if the moon orbit is not perfectly in the equatorial plane (don't know by heart).

The moon doesn't always have exactly the same face towards the Earth. For one, its axis of rotation is tilted 1.5° to its orbit. This would cause the Earth to move 3° back and forth in one direction. Secondly, while its speed of rotation is constant, due to the eccentricity of its orbit, its orbital velocity is not. This, from our perspective, causes the moon to appear to "rock" slightly. From the Moon's perspective, it would cause the Earth to move back and forth across an arc of about 12°.

This means that if you are standing at a point near one of the limbs of the Moon, the Earth could actually set and rise. It would fall below the horizon, and then pop back up in almost exactly the same spot.

Just one more little point, since the period between perigee and perigee is about 2 days shorter than the synodic month (length of the lunar day), Earthrise would not stay in step with the lunar day or the phases of the Earth. They would follow a pattern that repeats in just under 403 days.

 

[negitron]

The moon doesn't always have exactly the same face towards the Earth. For one, its axis of rotation is tilted 1.5° to its orbit.

Right:

 

[Theelectricchild]

The moon doesn't always have exactly the same face towards the Earth. For one, its axis of rotation is tilted 1.5° to its orbit.

Right:

[PLAIN]http://antwrp.gsfc.nasa.gov/apod/image/9911/lunation_ajc.gif[/QUOTE][/URL]

Interesting, so what exactly causes these vibrations, and does it only occur with tidally locked satellites? That is, would the Earth do this from the perspective of the Sun?

 

[Janus]

Interesting, so what exactly causes these vibrations, and does it only occur with tidally locked satellites? That is, would the Earth do this from the perspective of the Sun?

The nodding up and down is caused by the fact the moon's axis is tilted to its orbit, just like the Earth's is tilted to its.

The side by side rocking is caused by the fact that the moon's orbital speed changes at different parts of its orbit.

Here's an example.

http://home.earthlink.net/~parvey/sitebuildercontent/sitebuilderpictures/librationanim.gif

In this example the Moon and planet rotate at the same rate. (this is to emphasize the fact that the moon's rotation rate stay the same throughout. As the moon travels around the planet, it gets closer ans further from the planet. As it does so, it speeds up and slows down. So at different times the orbit get ahead or behind the rotation. The "lit up"(vs. the red and white) part of the moon shows what would be visible from the planet. Notice how it changes over the course of the orbit.

It's something you would only notice with a tidally locked moon and one that it not in a circular orbit.

 

[Theelectricchild]

This graphic you created does a great job explaining this apparent wobbling. Thank you.