How does the Moon affect the tides?
Short answer: The Moon's gravity pulls on Earth, and vice versa. This causes Earth's oceans to bulge out towards and away from the Moon.
Long Answer: High and low tides happen about twice a day for most coastline areas on Earth. As Earth rotates, the parts of Earth passing through one of the bulges experiences high tide. The low points are low tide.
The explains the high tide on the side of Earth facing the Moon, but what causes the high tide on the the opposite side of Earth? This is a bit more complicated. There are a couple of ways to think about it.
- Gravity is stronger when the two things pulling each other are closer together. The side of Earth facing the Moon is pulled more strongly towards the Moon because it's close. Next, the center of the planet is pulled a bit less. Finally, the opposite side of Earth is pulled towards the Moon the least of all. It's a bit like the far side of Earth is "left behind" as the rest of it is pulled towards the Moon.
- You can also think of this in terms of centrifugal force. Just like when you spin a bucket a water around and the water gets pushed away from you, Earth and the Moon are spinning around their combined center off mass, and the sides facing away from this are "pushed" outwards by centrifugal force.
Note that there are many other factors that affect the tides, especially local geography, the shape of the ground under the coastline water, etc. The Sun also has an effect, although the Sun's influence is much smaller than the Moon's because the Sun is much farther away.
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