# Orbital diagram or animation?

So I got out side and look up and in the West is the setting sun. Not set, just getting low.

I look to the East and there is the moon. Well, isn’t that cool.

Wait… The moon is a crescent. There is a shadow covering part of it. A shadow caused by the Earth? That was what I was taught.

Wait, I can see both at the same time. I am standing on the Earth. How can the Earth be making the shadow?

Is there a place or site that will have an animation or simple diagram that can explain this to me?

Or can some one just talk me through it?

If there was a lunar eclipse over your area, your observations are no coincidence. The moon MUST BE full to have a lunar eclipse, and a full moon rises as the sun sets. It seems you witnessed a lunar eclipse in progress as the (normally full) moon was rising. Does this help somewhat?

No eclipse was involved, if I’m reading the OP right. The moon doesn’t look like that because of the Earth’s shadow; it’s relatively uncommon for the sun, moon, and Earth to be lined up properly such that the Earth’s shadow does fall on the moon. When this happens, we see a lunar eclipse. The moon’s more normal state of being partly bright and partly shadowed is because the moon is illuminated by the sun. Imagine first just the moon and sun in space. The sun shines light on the moon, but half of the moon will be in shadow. That is, the moon has a “night side,” just like the Earth. Now for an observer in space near the moon, unless he is exactly toward the sun or away from the sun, when looking at the moon he will see some of the light side and some of the dark side and the line dividing them. As that observer moves around the moon, different fractions of it will be light or dark.

Now instead of an observer floating around the moon, we’ll change ths system a bit. Now we have Earth, and our observer is standing on it. From our observer’s point of view, now he is standing completely still and the moon moves around him. So our light source, very far away, keeps half the moon bright and half dark all the time, and as the moon revolves around the Earth we see more or less of the light half. Notice that this explains the (approximately) monthly cycle of the moon’s phases: the moon takes that amount of time to revolve once around the Earth. By contrast, in an eclipse, the Earth’s shadow passes across the moon quite quickly (relative to a month).

A simple diagram here.

Note that the diagram is a top-down view. What this leaves out is that the moon, Earth, and sun are usually not all in exactly the same plane. If they were, then every time the moon got to position 5 we’d have a lunar eclipse.

Anyway, by pretending the blue ball in the middle that represents the Earth is instead our observer, you can see (with a bit of mental shifting of perspective) how looking at the moon in those different positions would show you varying degrees of illuminated crescent. The text below the diagram explains roughly the same thing I did in my previous post.

Quote of OP: So I got out side and look up and in the West is the setting sun. Not set, just getting low. I look to the East and there is the moon. Well, isn’t that cool.
Wait… The moon is a crescent. There is a shadow covering part of it. A shadow caused by the Earth? That was what I was taught.

The OP says he saw a crescent moon in the East at sunset? This is not possible, whether a waxing or waning crescent. A waxing crescent is observed in the west with slight angular separation from the sun, and a waning crescent rises before the sun.

While the explanations given explain why the moon has phases, it does not explain how a crescent moon could be found in the east as the sun sinks in the west…unless the OP really means to say a waxing gibbous moon was observed.

When did this happen? The last lunar eclipse was on Sep 7. No others are due this year.

You’re right, Jinx, I completely missed the crescent-in-the-East problem. I was assuming the OP meant crescent moons in general from “Wait… The moon is a crescent. There is a shadow covering part of it. A shadow caused by the Earth? That was what I was taught.”

As others have said, the Earth wasn’t making the shadow. But even if it was (as in an eclipse) you’d still be able to see it. When I climb a tree, I can still see the tree’s shadow on the ground. The same is true for an Earthbound observer during a lunar eclipse.

Just to be sure GunNSpot is clear.

• The phenomenon you saw was NOT the earth’s shadow be cast from the sun onto the moon.

• If what you saw was a crescent moon “east” of the setting sun, then the moon was in fairly close proximity to the sun. The moon you saw was waxing toward Full, which means that when the moon is Full, it will be exactly opposite the sun (i.e. the sun will be setting in the west, just as the moon will be rising in the east).

• That explains WHY you saw a crescent. Imagine yourself in the center of a room (you’re the earth), and you’re holding a baseball (the moon). The west wall of the room is mounted a strong light, which simulates the setting sun. Now, hold the baseball out at arm’s length, and position it exactly between you and the light. You’re going to see only the ball’s silhouette. THIS is the New Moon. It’s completely in shadow. What makes the shadow? Not the earth, of course, it’s the “daylight” side of the baseball, that is in the way of the “nightside”, so it casts a shadow on itself. At any given time, the baseball (or moon) will be 50% illuminated.

Now, start slowly rotating yourself (keeping the baseball out at arms length) counter-clockwise. You’ll notice the baseball “waxing” a crescent shape. This arises because you’re starting to position the ball in a way where you can see the “daylight” side creeping in. That is, you’re moving the ball out from between yourself and the light. Keep turning CCW. The ball will continue rolling through these “phases” until the ball appears completely illuminated, this is the Full Moon. You’ll notice you’re the one that stands in between the ball and the light, as the ball is now pointed at the East wall. If your shadow is on the ball, that’s a lunar eclipse. But, in space, the moon, earth, and sun are relatively small, and far way from each other, so it’s not all that common for them to align so accurately. So typically the moon misses the earths shadow.

You can keep turning around until the baseball goes through all it’s phases (after Full it begins to wan). It’ll return back to a crescent right before it goes back to New Moon again, then the whole process repeats itself.

So, in this way, I’m able to completely understand where the moon is relative to the earth, just by looking at it’s phase. From now on, anytime you see a Full Moon, you’ll immediately notice that it’s rising just as soon as the sun is setting. Pretty cool.

Boy did I have a ton of misconceptions on this subject.

I’m sure I had it explained sometime in th edim past but I have been asleep since then.

Thanks guys. You all helped. I hope to keep this information in my head this time.

Hey, I was bored, and since you were originally looking for an animation… I made one real quick like…

Check it out.

Nicely done. Very nice.

thanks!

WOW !!!

Way cool …