1. Seeing the Moon
You only see the Moon because light from the Sun reflects off the Moon and into your eyes.

2. The Moon’s Formation
According to the giant impact hypothesis, shortly after Earth formed about 4.6 billion years ago, an object about the size of the planet Mars collided with Earth.

3. The Moon’s Formation (cont.)
The impact ejected vaporized rock that formed a ring around Earth.

4. The Moon’s Formation (cont.)
Eventually, the material in the ring cooled and clumped together and formed the Moon.

5. The Moon’s Formation (cont.)
The surface of the Moon was shaped early in its history.
Moon’s craters were formed when objects from space crashed into the Moon.

6. The Moon’s Formation (cont.)
The large, dark, flat areas on the Moon are called maria.
When the maria formed, lava flowed up through the Moon’s crust and solidified, covering many of the Moon’s craters and other features.
The light-colored highlands are too high for the lava that formed the maria to reach.

7. The Moon’s Motion
The Moon revolves around the Earth. One revolution takes days.
The Moon rotates as it revolves around Earth.
One complete rotation of the Moon takes 27.3 days, meaning the Moon makes one rotation in the same amount of time that it makes one revolution around Earth.

8. The Moon’s Motion (cont.)
Because the Moon takes the same amount of time to orbit Earth and make one rotation, the same side of the Moon is always facing Earth.
This side of the Moon is called the near side.
The side of the Moon that cannot be see from Earth is called the far side of the moon.

9. The Moon’s Motion (cont.)

10. Phases of the Moon
The lit part of the Moon or a planet that can be seen from Earth is called a phase.

11. Phases of the Moon (cont.)
The sequences of phases is the lunar cycle.
During the waxing phases, more of the Moon’s near side is lit each night.
During the waning phases, less of the Moon’s near side is lit each night.

12. The motion of the Moon around Earth causes the phase of the Moon to change.
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13. Phases of the Moon (cont.)
It takes 27.3 days for one revolution of the moon so a lunar month is 28 days long.
It takes 7 days for a moon to go from new moon to first quarter moon.
It takes 7 days for a moon to go from first quarter moon to full moon.
It takes 7 days for a moon to go from full moon to third quarter moon.
It takes 7 days for a moon to go from third quarter moon to new moon.

14. Phases of the Moon (cont.)

15. Lesson 2: Earth’s Moon
The gravitational pull of Earth on the Moon makes the Moon revolve around Earth. The Moon rotates once as it makes one complete orbit around Earth.
The lit part of the Moon that you can see from Earth—the Moon’s phase—changes during the lunar cycle as the Moon revolves around Earth.
Jason Reed/Photodisc/Getty Images