In space, the closest object to Earth is the Moon. It orbits Earth, and appears to change shape as it moves—the different shapes are called PHASES. The Moon has no light of its own but shines by reflecting sunlight. It is the only other world that humans have set foot on.
Table 4. MOON DATA
|Diameter (width) at equator||2,160 miles (3,476 km)|
|Average distance from Earth||238,900 miles (384,400 km)|
|Time to orbit Earth||27.3 days|
|Time to spin around own axis||27.3 days|
|Time to go through phases||29.5 days (1 month)|
|Mass||0.01 x Earth’s mass|
|Gravity||0.17 x Earth’s gravity|
|Average temperature||–4°F (–20°C)|
The Moon is rocky. It has no atmosphere to protect it, so anything heading toward the Moon will crash into its surface, which is covered with craters from meteorites. The Moon has a hard outer crust of granitelike rock. The typical rock of the mare (plains) regions is similar to volcanic basalt on Earth. The Moon’s core, or center, may be partly molten (liquid).
The Moon’s gravity pulls on Earth’s oceans and distorts them, causing tides. The water on the side of Earth closest to the Moon experiences the biggest pull, and bulges outward. The water on the opposite side also bulges, and the two bulges follow the Moon’s motion and Earth’s rotation.
The pull of the Sun and Moon affects the tides. The lowest (neap) tides occur when the Sun and the Moon pull at right angles to each other and their pulls partly cancel each other out.
Our changing views of light on the Moon are called phases. As on Earth, one half of the Moon is lit up by the Sun while the other half is dark. As the Moon orbits us, we see it from different angles, with its light side pointing toward us or away from us.
The phases begin when the Moon comes between the Sun and Earth. The bright side of the Moon is facing away from us, and we see the dark near side. We call this a “new moon.” As the Moon moves along its orbit, we see more and more of the near side lit up, until we see it all lit up at “full moon.” Then we see less and less of the Moon lit up, until it shrinks to a crescent and then disappears at the next new moon.
When the Moon seems to grow in size from night to night, we say it is waxing. When it seems to shrink, we say it is waning. A crescent moon is mostly dark, and a gibbous moon is mostly light. As the Moon orbits Earth, it spins on its own axis so the same half (the near side) always faces Earth—we never see the far side of the Moon.