Earth v. Moon

**bcmarshall** · Nov21-09, 04:03 AM

Sorry, I'm a newbie here and didn't know how to start a new thread, so I posted a question in another forum and then figured out how. I'm trying to redirect answers here.

I'm arguing with a friend about how a half-Earth would appear from the Moon. Since we see the Moon in phases, it makes sense that watching the Earth from the Moon would also offer the same phenomenon, Full Earth, Half Earth, New Earth. My question is if one is looking at a Half Earth from the Moon, would the shadow line appear horizontal or vertical.

My belief is that since the Moon is in a roughly equatorial orbit around the Earth, then the equators of the two objects are facing each other, with their axes within a few degrees of being parallel. Since we see the Half Moon as vertical from here, I conclude since there is no mechanism I can envision which would alter the perception, it should appear as a vertical line on the Earth as well.

My friend believes the shadow line should appear horizontally across the Earth.

I'd welcome any comments or explanations.

**Janus** · Nov21-09, 12:12 PM

It depends on where on the Moon you are viewing the Earth. Assume that you are standing at a point where the Earth is near the horizon. If you were standing near the Moon's equator, the line would look horizontal. If you were standing near the poles, it would look vertical.

**DrGreg** · Nov21-09, 12:54 PM

If you use Google Images to search for "Earth from Moon" you'll find some photos. Many of these (which show both Earth and Moon) were taken near the equatorial plane and show a horizontal line, but some show a tilted line.

And, from Earth, the half-Moon looks vertical only when its at its highest in the sky (for viewers not near the equator). It looks tilted at moonrise and moonset (by how much depending on your own latitude and whether it's summer or winter). The same applies to the half-Earth from the Moon.

**Vanadium 50** · Nov21-09, 01:04 PM

Is there earthrise and earthset on the moon? I would naively suppose not.

**HallsofIvy** · Nov21-09, 01:14 PM

Again, that depends on exactly where you are on the moon. The moon does have a slight "apparent" rotation- while it always has one fact toward the earth, the "edges" of what we see vary slightly over a month. If you are in one of those edge regions you will see an "earthrise" and "earthset".

**Janus** · Nov21-09, 01:16 PM

Originally Posted by Vanadium 50

Is there earthrise and earthset on the moon? I would naively suppose not.

If you are standing near enough to the limb of of the Moon (say a little East of Grimaldi crater), libration will cause the Earth to rise and set. It won't cross the sky, but will rise up above the horizon and then set again on the same horizon.

**Janus** · Nov21-09, 01:17 PM

Halls beat me to it.

**DrGreg** · Nov21-09, 01:22 PM

Originally Posted by Vanadium 50

Is there earthrise and earthset on the moon? I would naively suppose not.

Good point. The position of the Earth in the "Moon sky" ought to be more-or-less constant (although I guess there may be some wobble). So you can't really talk about earthrise and earthset, but there are places on the Moon where the Earth is permanently near the horizon. And then the same principle applies as when the Moon is near the Earth's horizon.

EDIT: ... Halls & Janus both beat me to it with a rather better answer than mine!

**bcmarshall** · Nov21-09, 03:29 PM

Originally Posted by Janus

It depends on where on the Moon you are viewing the Earth. Assume that you are standing at a point where the Earth is near the horizon. If you were standing near the Moon's equator, the line would look horizontal. If you were standing near the poles, it would look vertical.

I don't understand what mechanism would cause a 90 degree shift in perception. Perhaps you can explain.

We see the line vertically on the Moon from the Earth. What could possibly cause a horizontal appearance of the shadow line on Earth? There is of course only one source of light, the Sun. The two objects are moving roughly equator to equator in relation to each other.

I just can't figure why there should be any difference.

**ideasrule** · Nov21-09, 04:09 PM

Originally Posted by bcmarshall

I don't understand what mechanism would cause a 90 degree shift in perception. Perhaps you can explain.

We see the line vertically on the Moon from the Earth. What could possibly cause a horizontal appearance of the shadow line on Earth? There is of course only one source of light, the Sun. The two objects are moving roughly equator to equator in relation to each other.

I just can't figure why there should be any difference.

We DON'T see the shadow line as vertical. Go outside and take a look. The orientation of the shadow line changes as the Moon moves across the sky.

As for what might cause a 90-degree difference, somebody standing at the poles would be standing at 90 degrees relative to someone at the equator, hence the 90-degree difference.

Interesting trivia: In many African mythologies, the Moon is said to be a celestial boat because the the shadow line appears mostly horizontal near the equator.

**bcmarshall** · Nov21-09, 05:28 PM

Originally Posted by ideasrule

We DON'T see the shadow line as vertical. Go outside and take a look. The orientation of the shadow line changes as the Moon moves across the sky.

As for what might cause a 90-degree difference, somebody standing at the poles would be standing at 90 degrees relative to someone at the equator, hence the 90-degree difference.

Interesting trivia: In many African mythologies, the Moon is said to be a celestial boat because the the shadow line appears mostly horizontal near the equator.

I guess I'm just thick, but in either case we're looking at an object a quarter million miles away. Moving toward the pole on either object would certainly change the height in the sky that the other would appear, but we're talking about a perception of a shadow line on that object. It just makes no sense to me that the shadow line cast on an object at essentially infinite distance would appear differently because you're standing at a different spot on one object while looking at the other. Even in the case of the Earth looking at the Moon, the distance from pole to equator is only about 6k miles (10 k km). That's a mighty small percentage of change.

I'm not arguing that I'm right. I'm saying I don't understand the mechanism involved.

**Janus** · Nov21-09, 06:27 PM

Originally Posted by bcmarshall

I guess I'm just thick, but in either case we're looking at an object a quarter million miles away. Moving toward the pole on either object would certainly change the height in the sky that the other would appear, but we're talking about a perception of a shadow line on that object. It just makes no sense to me that the shadow line cast on an object at essentially infinite distance would appear differently because you're standing at a different spot on one object while looking at the other. Even in the case of the Earth looking at the Moon, the distance from pole to equator is only about 6k miles (10 k km). That's a mighty small percentage of change.

I'm not arguing that I'm right. I'm saying I don't understand the mechanism involved.

It's not the difference in distance its the difference in what direction your head points relative to the shadow line on the Earth.

Check out the attachment showing two people standing on the Moon, one on the Equator and the other at the pole. Notice that they are at right angles to each other. Looking at the same shadow line on the Earth, they will see it differently.

**russ_watters** · Nov22-09, 01:32 AM

Well it also matters where the sun is. If the moon is due south and the sun is due west, the shadow line will be near vertical, but if the moon is due east and the sun is below you/it, the shadow line will be near horizontal.

As was said above, go outside and look! the moon rotates as it moves across the sky. The next few days are actually a good time to view this phenomena.

~~Sorry!~~ · Nov22-09, 12:01 PM

Here is the moon from India:

and from I believe Australia:

I am certain from the way you've described seeing the moon from your position that you have probably never seen the moon like this before.

**DrGreg** · Nov22-09, 01:29 PM

The attached diagram shows how the angle between the moon and the observer's vertical varies at different times as the Earth rotates.

Photo credits:
Earth: Visible Earth, NASA.
Moon: Kopfjäger (Away) on flickr, CC BY 2.0

**jambaugh** · Nov22-09, 01:50 PM

Originally Posted by bcmarshall

It just makes no sense to me that the shadow line cast on an object at essentially infinite distance would appear differently because you're standing at a different spot on one object while looking at the other.

Think about it this way. Suppose you are standing on the Moon at the point nearest the Earth. Thus the Earth is directly overhead of you. You lay on your back to see it clearly. Now on your back you can rotate so the "half-Earth" appears in any direction you like. Right?

Now pick and orientation you want to view the Earth. As you lay on your back on the Moon I walk in the direction your head is pointing until I'm a quarter way round the moon. I'll be standing up but oriented exactly as you are laying down and I'll see the Earth's phase at the same random orientation you chose when you laid down.

It is a matter of relativity of orientation. Forget the moon is there and imagine you are floating in space. Pick the orientation you want to observe the Earth and then imagine the Moon appears below your feet.