Moon
Though a satellite of Earth, the Moon is bigger than Pluto. Some scientists think of it as a planet (four other moons in our solar system are even bigger). There are various theories about how the Moon was created, but recent evidence indicates it formed when a huge collision tore a chunk of the Earth away. Because it takes 27.3 days both to rotate on its axis and to orbit Earth, the Moon always shows us the same face.
We see the Moon because of reflected sunlight. How much of it we see depends on its position in relation to Earth and the Sun.
https://www.youtube.com/watch?v=xcVwLrAavyA
http://vimeo.com/channels/staffpicks/108018156
The 27.3-day number is what scientists call a sidereal month, and it is how long it takes the Moon to orbit the Earth in relation to a fixed star. Another measurement, called a synodic month, is measured between in relation to the Sun and equals 29.5 days. Full moons and new moon are measured by the synodic month.
Earth's gravity keeps the Moon in orbit, while the Moon's gravity creates tides on our oceans.
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