Re: Why do we see the same phase of the moon as people do from other countries

The big thing to remember is that the phases of the moon don't have much to with where we are on Earth as much as where the Moon is in its orbit around the Earth. That might be a little confusing, so let me try to explain a little.

The Moon orbits the Earth the same way that satellites, the Space Shuttle, and the Space Station do - the further away something is the slower it orbits and the moon is far enough away that it takes (about) a month to orbit the Earth. Sometimes its orbit takes it between the Sun and the Earth and all that we can see is the side that is in shadow - this is the new moon that is almost completely black. When the Moon is behind the Earth the Sun lights up the whole side that we can see and we have a full moon. A half moon comes when the Moon is off to the side, and we also have other phases as the Moon circles our planet.

So - think about when the Moon is between the Sun and the Earth. We can only see half of the Moon - the side facing the Earth. With the Sun behind the Moon the only part of the Moon that we can see is the dark side - the side that is in shadow. It doesn't matter where on Earth you stand, there is only one side of the Moon that can be seen. The same thing holds for the full moon - again, there is only one side that can be seen from Earth and that is the sunlit side. In fact, no matter where the Moon is in its orbit around the Earth, there is only one side of the Moon that anyone on Earth can see - no matter where they happen to be located. This is why everyone on Earth always sees the same phases of the Moon.

Part of the reason for this is that the Earth and Moon are both fairly small and the Moon is far away compared to the size of the Earth. Try this experiment - when it's dark outside turn off all but one lamp in a room in your house and hold a ball in your hand while stretching your arm as far as you can. Now close your left eye and look very carefully at the ball to see exactly where on the ball is the line between shadow and light. Now close your right eye and look at the ball through your left eye. You should see close to the exact same picture as with your other eye and you should see exactly the same amount of the ball is illuminated (and in shadow) with each eye. This is because, compared to the length of your arm, your eyes are fairly close together and they both show you (almost) the same view of the ball - just as, compared to the distance between the Earth and the Moon, the distance from one side of the Earth to the other is very small.

[note added by MadSci Admin: A very similar question was answered a few years ago at this site. See this previous answer. In addition, there is a really good animation at sites such as this one at U. of Wisconsin.]