Why does Earth appear to move in the lunar sky if the moon is tidally locked?

[StephenPrivitera]

Since the moon always has the same side facing Earth, shouldn't the Earth always be in the same position in the sky when observing from the moon? There's a photo taken by the Apollo 8 crew in a book I have that claims the Earth is rising above the horizon of the moon. The book is "The Rough Guide to the Universe" by John Scalzi in case you have it and want to look it up (page 197). It struck me as wrong. Let me know what you think.

 

[mathman]

I haven't seen the picture, but could it be seeing the Earth at sunrise on the earth?

 

[Ivan Seeking]

Originally posted by StephenPrivitera
Since the moon always has the same side facing Earth, shouldn't the Earth always be in the same position in the sky when observing from the moon? There's a photo taken by the Apollo 8 crew in a book I have that claims the Earth is rising above the horizon of the moon. The book is "The Rough Guide to the Universe" by John Scalzi in case you have it and want to look it up (page 197). It struck me as wrong. Let me know what you think.

I would think this is taken from the orbiting craft; not from the surface. I am not aware of any expeditions into the dark side of the moon...except of course by Floyd.

 

[Phobos]

check this link by astronomer Sten Odenwald
http://itss.raytheon.com/cafe/qadir/q1905.html

 

[StephenPrivitera]

It's a picture of a gibbous Earth with the lit limb facing upwards (above the horizon). I guess I'm having trouble interpreting the picture. Since the gibbous faces upwards doesn't that mean the Sun is above the horizon? Then it must be day on the moon, but the background of the picture is black. How can you have a picture of the Earth from the moon during the day on the moon without having a bright background? Is the brightness of the Earth day due to our atmosphere? If you were in empty space nearby the Sun (say, 1AU away), would it be bright like the day? Would you simply turn around to make it be night time? Whenever I picture space outside the Earth's atmosphere I picture darkness.

What is the "lunar Libration effect?"

 

[Phobos]

Originally posted by StephenPrivitera
Is the brightness of the Earth day due to our atmosphere?

Yes. The parallel light rays from the sun get scattered once entering our atmosphere, thereby bouncing off lots of stuff and illuminating the whole area.

If you were in empty space nearby the Sun (say, 1AU away), would it be bright like the day? ... Whenever I picture space outside the Earth's atmosphere I picture darkness.

The sun would still appear bright like it does in our sky, but space would be black in other directions (except of course for the bright stars).

Check out some of the photos from the space shuttle or the Hubble telescope.

What is the "lunar Libration effect?"

As you know, the Moon keeps the same face toward the Earth. However, there is a slight wobble...you get to see a bit more (few degrees) on one side or the other from time to time...due to the Moon's slightly non-circular orbit.

 

[StephenPrivitera]

Originally posted by Phobos
The sun would still appear bright like it does in our sky, but space would be black in other directions (except of course for the bright stars).
[/B]

Could it be then that on the moon during the moon day the background appears black even though the sun in in the sky? From what I know, the moon has a very minimal atmosphere. That would explain the picture.

 

[I, Brian]

Without any appreciable atmosphere to refract light rays from the sun, the moon sky will remain as black - essentially, deep space. The stars generally aren't visible in the photos because of brightness and contrast issues (ie, they are drowned out by the light of a lunar day).