2006-12-10 08:35:55 · 6 answers · asked by dennisjns 1 in Science & Mathematics ➔ Astronomy & Space
You're looking at an illuminated sphere. The only time there is a straight line on one side of the illuminated moon is half-way between new moon and full moon and half way between full moon and half moon. At other times, the non-circular edge is an ellipse.
2006-12-10 08:51:45 · answer #1 · answered by JJ 7 · 1⤊ 0⤋
Okay, this is easiest to understand with a visual aid.
Take three balls. Don't worry about sizes, it's really not important. Crumple up peices of paper if you have to.
Put on far to the left side, this represents the Sun.
On the other side, put another representing Earth.
Now put the third ball just above our ball representing Earth, this is, of course, the Moon.
Now imagine shining a light at the ball representing the Moon from the ball representing the Sun. The light only shines on half of it. Pretending you're standing on the surface of our little Earth and looking up at the moon. You look up and you see about half lighted and the other half dark. As you move the Moon around Earth, the part that is lighted by the Sun looks different. If you put the ball on the otherside of earth, it's still half, but it's the other half. It takes almost 28 days for it to get around earth, thusly the 28 day Moon cycle.
Now, it gets a little more complicated. Move the Moon to the far side of the Earth, so it's blocked by the sun. This would be a lunar eclipse, but that doesn't happen every twenty eight days, so what gives, right?
The way the Moon rotates around Earth isn't the flat surface the table or desk you've got it on. Sometimes it's lower than the 'surface' (the plane of Earth's rotation around the Sun) and sometimes it's higher. The angle is about 13 degrees, so at it's peak, it's 13 degrees up and at it's lowest, it's 13 degrees down. In the middle, it's even.
When everything lines up right, so it's perfectly even and behind earth at the same time, the Moon moves through the Earth's shadow and the Moon's light is blocked and you see a lunar eclipse. This doesn't happen incredibly often, so it's usualy above or below and then you see a full Moon.
Because of the distances involved, 13 degrees can put it fully out of the earth's shadow. But sometimes it's not at the full 13 when it's back there, when it's close enough to the middle, but not perfectly even, you see only a part of the Earth's shadow and you see a partial lunar eclipse. These are fairly common.
2006-12-10 17:36:41 · answer #2 · answered by socialdeevolution 4 · 0⤊ 0⤋
Depends on when in the cycle you see the moon
The first sight of the moon after new moon is where the crescent is most noticable. As the phases grow towards full, it looses the crescent look and takes on the straight edge.
2006-12-10 22:31:07 · answer #3 · answered by orion_1812@yahoo.com 6 · 0⤊ 0⤋
They don't. All phases of the moon have curved sides.
2006-12-10 16:38:40 · answer #4 · answered by Paul B 2 · 0⤊ 0⤋
They aren't flat; you need glasses:
http://en.wikipedia.org/wiki/Moon_phases. Only when there's a quarter-moon is the boundary straight. At all other times, the boundary is curved.
2006-12-10 18:05:51 · answer #5 · answered by arbiter007 6 · 0⤊ 0⤋
The moon is a sphere, not flat.
2006-12-10 16:38:13 · answer #6 · answered by Roman Soldier 5 · 0⤊ 0⤋