Dec 09 2008
Tides
So what does the moon really have to do with tides? Imagine the moon as sort of a water magnet. The water on the side of the earth facing the moon will bulge out due to the moons gravitational pull.
OK so it’s easy to picture that bulge following the moon as the earth rotates. If it were that simple, there would only be one high tide and one low tide a day. But there are two.
The water on the side of the earth opposite the moon also bulges out. This happens because the earth and the moon revolve around a common “center of gravity”. It’s the same way the hood on your jacket might puff out if you picked up a little kid and swung them around by their arms.
So the two “bulges” of water are the high tides, and in between (the water had to come from somewhere) are the low tides. Therefore most places on earth have 2 high and two low tide. Physical barriers, like um continental crust for example, sometimes make things interesting.
The sun also has a pull on the earth’s water, so when the sun, moon and earth are all lined up (like during a full and new moon), tides are exaggerated (extra high and extra low). This is called a spring tide.
When the sun and moon are at 90 degree angle to each other (1st & 3rd quarter) they work against each other, and we have a neap tide, in which there is a much smalled difference between high and low tide.
