The Moon does not produce 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 is enough to make the Moon shine brightly, although we don’t see all of it from Earth all the time.
We see only the part of the Moon that is lit by the Sun. During a full Moon, the Sun is shining directly 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 (grows) while going from new Moon (dark) to full Moon. It wanes (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 and the time it takes to orbit Earth are equal—27.32 days. Thus, the Moon is in a synchronous rotation with Earth, so 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 the phase of the Moon in which it 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 orbits the Sun while the Moon orbits Earth in an elliptical, or oval, orbit. Because our view of the sunlit part of the Moon changes as the Moon circles Earth, the shape of the Moon seems to change. These changes are called phases. A complete phase cycle takes an average of 29.5 days, from one new Moon to the next. The difference between the time it takes the Moon to complete a phase cycle and the time it takes to orbit Earth is due to 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. This is called an eclipse. A solar eclipse occurs when the Moon blocks the Sun’s light from some area on Earth. A lunar eclipse occurs when Earth blocks the Sun’s light from the Moon. Eclipses occur only when the Moon crosses Earth’s orbital plane at a time when the Sun, Moon, and Earth are aligned (at a new Moon or a full Moon). ▼

Total Solar Eclipse
The Moon blocks out the entire Sun, except for a faint halo of light called a corona.


Annular Eclipse
When the Moon is at a far point in its orbit of Earth, it cannot 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 (umbra). The Moon seldom disappears completely during a lunar eclipse but instead usually turns red. A small amount of sunlight reflects off Earth, 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
In a partial lunar eclipse, the Moon passes through part of Earth’s shadow.
