Did you know that the entire Earth is being ripped out from under us? Or that the Earth's tides are like a massive, cosmic game of juggling? Find out more after the jump!
The way tides work isn't particularly hard to understand, but it does take mental juggling. In this case, the mental juggling is quite literal, because there are at least three ‘balls' in the air, and the tide depends on the position of all of them.
The first ball is, of course, the Earth, which spins on its axis once every twenty-four hours. That means that wherever we are on the Earth, we're directly underneath the Moon once a day. And when we are, that's when the tide is high. The gravity of the Moon pulls on the Earth. Since we mostly interact with solid objects, we don't much notice it. The oceans, though, are a huge collection of loose molecules. Those molecules are pulled towards the Moon, and don't have anything to anchor them. As the Moon gets closer, and exerts more of a force on the sea, the ocean level rises. That's why there's one high tide per day.
The Moon, however, is traveling, too. It revolves around the Earth roughly once every month. When the Moon is new, it is lined up with the third player in our game, the Sun. The Sun also exerts its gravity upon our oceans, and during that one time of the month when the Sun, Moon and Earth all line up, the tide is highest.
Perhaps the landlubbers among io9 readers have read this post with equanimity. After all, there is only one time per day when any particular slice of the Earth is closest to the Moon. And it's only one time per month when the Sun and the Moon are lined up on one side of the Earth.
The salty sea dogs out there, however, have probably stopped singing their shanties, cutting their jibs, and burying their various booties to fire up their computers and send in a complaining email. As anyone who has lived near the sea knows, there are two high tides per day, and there are two exceptionally high tides, or spring tides, per month. What's more, those two tides are evenly spaced. The tide is highest when the Moon and Sun are directly above it and also when they are directly across the Earth from it. If gravity is responsible for the tides, how can that be?
It's because the Sun and Moon's gravity affects the earth, as well. The Moon exerts a pull on the Earth. The Earth stays and, for the most part, moves in one piece. The water on the Earth, however, is loose, and inertia causes it to lag behind the Earth. From the point of view of someone on the shore, it rises in relation to the Earth. From the point of view of someone in the water, the Earth is ripped out from underneath the sea.
Perhaps what's most amazing about the Moon's effect on the tide is not that it's so powerful, but that it is so stable. The thing twirling around us in space is enough to rip the Earth and the ocean apart every day, twice a day, and yet it's been orbiting us for billions of years. It's enough to make one feel secure about one's place in the universe. As long as one isn't standing on a beach when the tide rushes in.