By Dr. Marc Rayman
Asked by our friends at the Virginia Living Museum in Newport News, Virginia.
Well, first let's think about why it is the Moon has phases.
What people sometimes call "moonlight" is really sunlight reflecting off the Moon's surface. The Moon itself puts out no light at all. It takes about four weeks for the Moon to orbit once around Earth. During this time, the Moon's position relative to Earth and the Sun is constantly changing. During part of its journey, the Moon is between Earth and the Sun. Then, a few days later, it is off to one side. Still later, the Moon moves around so that Earth is between it and the Sun. And so on.
Try this little demonstration. You will need a lamp and a small ball (like a tennis ball or a softball). If the lamp has a shade, take it off. Pretend the lamp is the Sun, the ball is the Moon, and your head is Earth. Darken the room except for the lamp.
Now, hold the ball straight out in front of you. Stand facing the lamp. The ball will appear dark because the lighted side of the ball is facing away from you. This position represents the New Moon, dark and about to be born. Now, turn just a little tiny bit to your left, still holding the ball straight out. You will see only a thin lighted crescent on the right side of the ball. Now turn to the left a little more until the light is on your right. You will see half the lighted side of the ball. This position represents the First Quarter phase of the Moon. Now turn to the left again, so the lamp is behind you. (If the ball is directly in the shadow of your head, raise the ball up a little higher.) You will see the entire lighted side of the ball. Now you are looking at the Full Moon. Now turn again so the lamp is on your left. Now you are looking at the Last Quarter phase.
The Moon's orbit actually dips a little bit above and below an imaginary line drawn between Earth and the Sun. That is why Earth seldom blocks the Sunlight from reflecting off the full Moon. Once in a while, though, Earth does get directly in the way, and we have the exciting event called a lunar eclipse.
This month, go outside each day or evening and observe the Moon moving through its phases. At night, look carefully, especially when the Moon is just a crescent, and find the dark part of the Moon. We can see the dark part because it reflects Earth light-that is, Sunlight reflected from Earth.
So, how long does each phase of the Moon last? Well, the phases are just names we give to certain points along the Moon's smooth path around Earth. Technically, each phase, just like the one called a New Moon, when the Moon is exactly between Earth and the Sun, lasts only a brief instant. But to our eyes, a New Moon can last for a few days, representing the time that the Moon appears in the sky too near the Sun's position for us to see it at all. The time it takes the Moon to go through all its phases is about a month, and that was so important to our ancestors that they created the period of time we call a month. Maybe you've even noticed that the word month is like the word moon.
It isn't just the Moon that appears to have phases. The planets also have phases. For example, the orbit of Venus is inside the orbit of Earth. When Venus is in the part of its orbit between Earth and the Sun, we can't even see it, because its sunlit side will be away from us. This position is similar to the New Moon phase. As Venus progresses around the Sun, more and more of its illuminated side is visible to us on Earth. When Venus is on the far side of the Sun, we will see its Full Phase. At this time, we will see Venus only around sunset or sunrise, when the sky is just dark enough to see it even though Venus appears in the sky near the Sun.
Even Earth has phases, as viewed from the Moon. Photos taken of Earth by Apollo astronauts show part of Earth in darkness and part lit by the Sun.