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The Moon does not make its own light, but it reflects light from the Sun.
The dusty lunar surface reflects only about 12 percent of the Sun’s rays. But that’s enough to make the Moon shine brightly. However, we don’t see all of it from Earth all the time.
The only part of the Moon we see is what’s lit by the Sun. During a full Moon, the Sun is shining right on the Moon. But Earth is in the way at all other times, so only part of the Moon can be seen. The Moon waxes, or grows, while going from new Moon (dark) to full Moon. It wanes, or decreases, when going from a full Moon to a new Moon. ▼
In a waning Moon, the light decreases from right to left across its surface. In a waxing Moon, the light increases from right to left.
▲ The time the Moon takes to turn on its axis is the same as the time it takes to orbit Earth, or 27.32 days. Thus, the Moon is in a synchronous rotation with Earth. That means one side of the Moon, called the near side, always faces Earth. The part of the Moon we can’t see is called the far side. ▼
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◀ A new Moon is nearly or totally invisible from Earth. When the Moon is between Earth and the Sun, the light from the Sun falls on the far side of the Moon. The side that faces Earth is dark, so we can’t see the Moon.
▲ Earth goes around the Sun while the Moon goes around Earth in an oval, or elliptical, orbit. Our view of the sunlit part of the Moon changes as the Moon circles Earth. That’s why the Moon’s shape seems to change. These changes are called phases. One phase cycle takes about 29.5 days, from one new Moon to the next. Why is there a difference between the time it takes the Moon to finish a phase cycle and the time it takes to orbit Earth? It’s because of the relationship of the Moon to the Sun. The Moon has to move the extra days to catch up with the moving Sun.
Eclipses
Every so often, part or all of one heavenly body is hidden from view by the shadow of another. That’s called an eclipse. A solar eclipse happens when the Moon blocks the Sun’s light from some area on Earth. A lunar eclipse is when Earth blocks the Sun’s light from the Moon. When do eclipses occur? Only when the Moon crosses Earth’s orbital plane at a time when the Sun, Moon, and Earth are lined up. That means they only happen during a new Moon or a full Moon. ▼
Total Solar Eclipse
The Moon blocks out the entire Sun. The only part of the Sun that shows is a faint halo of light called a corona.
Annular Eclipse
Sometimes the Moon is at a far point in its orbit of Earth. Then, it can’t totally block the Sun. This leaves an annulus, or ring, that is brighter than a corona.
Total Lunar Eclipse
The Moon moves into Earth’s shadow, also called an umbra. Most of the time, the Moon does not totally disappear during a lunar eclipse. Instead, it usually turns red. A little bit of sunlight reflects off Earth. It refracts, or bends, in the atmosphere, and then bounces off the Moon.
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Partial Solar Eclipse
In a partial solar eclipse, the Moon covers only part of the Sun.
Partial Lunar Eclipse
The Moon passes through part of Earth’s shadow.
