How does the Moon work?
The other day I realized that I couldn't explain why the Moon has phases (full moon, half moon, etc.). I looked it up on Wikipedia and thought I'd figured it out: the Sun always (except for eclipses) lights up half the Moon, and we see that half from different angles. For example, if we see a full moon, that means that the Sun is behind us (us = viewer and the Moon), and if we see a new moon, that means the Sun is behind the Moon. I also found a cool Flash animation that helps visualize the process.
In an attempt to apply my newfound knowledge, I concluded that the angle of the shadow on the Moon's surface must indicate where the Sun is, like this:
That's an exaggerated drawing of a "gibbous" moon. In simple terms, you're looking at a ball that's illuminated by a light, so if you recognize the boundary between the lit and unlit sides of the ball, you can tell where the light must be.
This sounds great in theory. But tonight just after sunset I saw this in the sky:
If my theory were correct, then this observation would be impossible. Either the Sun must be above and to the right of the Moon to explain the shadow boundary (which it plainly isn't, because I just saw the Sun set below the horizon), or the shadow boundary should obscure the upper-left region of the Moon (which it plainly doesn't). I know what I saw, so my theory must be wrong.
This is probably going to be one of those problems I figure out the moment I press the Publish button on this entry. Otherwise, I'd appreciate an explanation about exactly how I'm confusing myself.
Update, 10:30 a.m. 10/23/2007: After some discussion with smart people involving whiteboards, flashlights, and borrowed computer-mouse trackballs, I have learned nothing new and realized nothing new. My basic premise is correct that the shadow on the Moon indicates where the Sun must be in the sky, or at least that a given shadow precludes certain Sun positions. Moreover, my observation was indeed that the Sun was in one of the precluded positions based on where I judged the shadow to be. So the most likely explanation at this point is that my observation was wrong.
One point that one person didn't get was that my second diagram above really represents what I saw -- that the Sun's setting point on the horizon was only about 10-20 degrees to the right of where I saw the Moon. The picture does not condense two different parts of the sky into a single view.
The moon's shadow points correctly towards the sun; we're on a rotating ball. I had to do the whole pretend my left hand was me standing vertically on a ball, and my right hand was the moon, then vary the latitude my left hand stood at and whether the moon was to the left or right of my left hand (the sun was under me in my imagination of this).
Right--it's hard to remember that it's the earth that is turning, not the sun. We need different terms for sunrise and sunset--what is the Latin for "toward the sun" and "away from the sun"?
What happened to the Quotes to be Whirled section? They were pure gold! Thanks.
Mike your daughter is already so smart. Can't believe she posted this :)
The "horns of the moon" always point away from the sun.
Imagine the 'horns' (the narrow tips that are lit up ) are the ends of a bow, and draw a mental line between them, like a bow string. Notch an arrow at the center of this bowstring and the arrow will point towards the sun.
Quote:
Because the "horns" of the moon at the ends of the crescent are always facing away from the setting or rising sun, they always point upward in the sky. It is fun to watch for paintings and pictures which show an "impossible moon" with the horns pointed downwards.
End of Quote.
Source of quote:
http://liftoff.msfc.nasa.gov/Academy/UNIVERSE/MOON.HTML