3.12: Lunar Phases

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Can the Moon cast shadows?

Of course! A full moon is very bright. It is bright enough to cast shadows. If you are out away from city lights and the Moon is full, you might cast your own Moon shadow.

The Phases of the Moon

The Moon does not produce any light of its own. It only reflects light from the Sun. The Moon has phases because it orbits around Earth. One orbit takes about 28 days. As the moon moves around Earth, different parts of it appear to be lit up by the Sun. The Moon sometimes appears fully lit and sometimes completely dark. Sometimes it is partially lit. The different appearances of the Moon are referred to as phases of the Moon (Figure below).

A full moon occurs when the whole side facing Earth is lit. This happens when Earth is between the Moon and the Sun.

The phases of the moon

The moon's phases are a result of the moon's orbit around Earth.

About one week later, the Moon enters the quarter-moon phase. Only half of the Moon’s lit surface is visible from Earth, so it appears as a half circle.

Another week later, the Moon moves between Earth and the Sun. The side of the Moon facing Earth is completely dark. This is called a new moon. Sometimes you can just barely make out the outline of the new moon in the sky. This is because some sunlight reflects off the Earth and hits the Moon.

One week after that, the Moon is in another quarter-moon phase. Finally, in one more week, the Moon is back to full.

Before and after the quarter-moon phases are the gibbous and crescent phases. During the crescent moon phase, the Moon is less than half lit. It is seen as only a sliver or crescent shape. During the gibbous moon phase, the Moon is more than half lit. It is not full. The Moon undergoes a complete cycle of phases about every 29.5 days.

Summary

The appearance of the Moon from Earth has distinct phases.
A full moon is completely lit; a new moon is completely dark.
A gibbous moon is more than half lit; a crescent moon is less than half lit.

Review

Describe how the Sun, Moon, and Earth are aligned during a full moon.
Describe how the Sun, Moon, and Earth are aligned during a new moon.
Draw and label pictures of the Moon in its phases.

Explore More

Use the resource below to answer the questions that follow.

Why does the moon have phases?
What are the four main phases of the moon? How long does it take to change from one to another?
What is the name for a moon that is less than half full? More than half full?
What is the name for a moon that is getting larger? Getting smaller?
What causes a lunar eclipse? Why doesn't a lunar eclipse occur during each full moon?