Science Puzzle
Why Are There Two High Tides a Day?
The Moon's gravity pulls on the oceans, so it makes sense that the sea bulges up on the side of Earth facing the Moon. As Earth spins, each coast passes under that bulge and gets a high tide.
But most coasts get two high tides a day, about 12 hours 25 minutes apart, so there must be a second bulge on the side of Earth facing away from the Moon. Why does the ocean bulge there, where the Moon's pull is weakest?
The Answer
Tides come from the difference in the Moon's pull across Earth, not from the pull itself.
Gravity weakens with distance. The ocean on the near side is closest to the Moon, so it's pulled hardest. Earth's centre, about 6,400 km further away, is pulled a little less. The ocean on the far side, another 6,400 km on, is pulled least of all. Compared with Earth's centre, the near ocean is pulled ahead towards the Moon and the far ocean lags behind it. Earth gets stretched along the line to the Moon, with a bulge at each end.
Earth turns under both bulges every day, so most places get two high tides and two low tides. They're 12 hours 25 minutes apart, rather than exactly 12, because the Moon moves along its orbit as Earth spins, and Earth has to turn a bit further to catch up with it.
The Sun stretches Earth in the same way, with a little under half the Moon's effect. When the Sun and Moon line up, at new and full moon, their effects add up and give the extra-high spring tides; when they're at right angles, we get the smaller neap tides. Earth's spin is the same all the way round, so it can't make a bulge on one side only.
The principle: Tidal forces. Because gravity weakens with distance, a large body is pulled unevenly by a nearby mass, and the difference stretches it into two bulges along the line between them.