The Moon is basically a giant ball of rock 3500 kilometers across hanging in space. Its surface is actually pretty dark, with about the same reflectivity as a chalkboard or asphalt. However, it looks bright to us because it’s sitting in full sunlight; the Sun illuminates it, and it reflects that light down to us here on Earth. And because it’s a sphere, and orbiting the Earth, the way we see it lit by the Sun changes with time. That’s what causes its phases: geometry. The important thing to remember through all this is, because the Moon is a ball and in space, half of it is always illuminated by the Sun! This is true for the Earth, too, and every spherical object in space; half faces the Sun, half faces away. We call the part facing the Sun the daylight or bright side, and the half facing away the night or dark side.
The phase of the Moon refers to what shape the Moon appears to us; how much of it we see illuminated from the Earth. The key to all this is this line, dividing the lit day side from the unlit night side. We call that line the terminator. If you’re facing the moon, with the sun behind you, you’re seeing the half of the moon that is fully illuminated by sunlight and it looks full. If you’re off to the side you see half of the lit side and half of the dark side and we say the moon is half full. If the sun is on the other side of the moon, you’re look at the unlit half, and it looks dark. Now, mind you, I haven’t moved anything except our point of view here, so at all times the Moon is always half lit, and half dark. Remember that. The phase of the Moon we see depends on from what direction the sunlight’s hitting it, and the angle we see that from Earth. The Moon orbits the Earth roughly once per month. In fact, that’s where the word “month” comes from; “month” and “Moon” are cognates, words that have similar etymological histories, and in most languages, including English, the two words are very similar. The length of time we call the month is derived from the length of time it takes the Moon to go through all its phases -- 29.5 days.
The Moon is basically a giant ball of rock 3500 kilometers across hanging in space. Its surface is actually pretty dark, with about the same reflectivity as a chalkboard or asphalt. However, it looks bright to us because it’s sitting in full sunlight; the Sun illuminates it, and it reflects that light down to us here on Earth. And because it’s a sphere, and orbiting the Earth, the way we see it lit by the Sun changes with time. That’s what causes its phases: geometry. The important thing to remember through all this is, because the Moon is a ball and in space, half of it is always illuminated by the Sun! This is true for the Earth, too, and every spherical object in space; half faces the Sun, half faces away. We call the part facing the Sun the daylight or bright side, and the half facing away the night or dark side.The phase of the Moon refers to what shape the Moon appears to us; how much of it we see illuminated from the Earth. The key to all this is this line, dividing the lit day side from the unlit night side. We call that line the terminator. If you’re facing the moon, with the sun behind you, you’re seeing the half of the moon that is fully illuminated by sunlight and it looks full. If you’re off to the side you see half of the lit side and half of the dark side and we say the moon is half full. If the sun is on the other side of the moon, you’re look at the unlit half, and it looks dark. Now, mind you, I haven’t moved anything except our point of view here, so at all times the Moon is always half lit, and half dark. Remember that. The phase of the Moon we see depends on from what direction the sunlight’s hitting it, and the angle we see that from Earth. The Moon orbits the Earth roughly once per month. In fact, that’s where the word “month” comes from; “month” and “Moon” are cognates, words that have similar etymological histories, and in most languages, including English, the two words are very similar. The length of time we call the month is derived from the length of time it takes the Moon to go through all its phases -- 29.5 days.
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