1. High Tide Comes About 50 Minutes Later Each Day
Most coasts get two high tides and two low tides in a stretch of 24 hours and 50 minutes, with high tides about 12 hours and 25 minutes apart[1]. Those 24 hours and 50 minutes are called a lunar day (or tidal day): the time it takes a spot on Earth to turn from "directly under the Moon" to "directly under the Moon" again.
A lunar day is longer than a normal day because the Moon is moving too. It circles the Earth in the same direction the Earth spins, so after one full spin the Moon has crept ahead, and the Earth needs about 50 extra minutes to catch up[1]. The tides do not follow the Sun's clock. They mainly follow the Moon.
That is a strong hint, but a hint is not an explanation. How can a faraway Moon move the whole sea?
2. Why Galileo Left the Moon Out of His Tide Theory
Not everyone saw the Moon connection as obvious. The Italian scientist Galileo Galilei wrote a famous book in 1632, the Dialogue Concerning the Two Chief World Systems, and used the tides as his best evidence that the Earth really moves. His idea was that the Earth spinning and the Earth circling the Sun combine, so that the ground speeds up and slows down a little over each day, and the seas slosh back and forth like water in a bowl that is pushed and pulled[2].
The astronomer Johannes Kepler had suggested that the Moon is involved. Galileo turned that down. He wanted explanations built only from matter pushing on matter, and a pull reaching across empty space looked to him like a mysterious influence, the sort of thing he was trying to remove from science[2]. The account I read says his sloshing idea matched the observed tides poorly[2]. It is worth knowing why he said no, though, because it was a principled choice and not carelessness.
In 1687, Isaac Newton published his Principia and laid out a law of gravity in which every piece of matter pulls on every other, more weakly the farther apart they are. Tide theory has built on that work ever since[3]. So what does the Moon's pull do to the sea, and why does the sea not simply pile up on the side facing the Moon?
3. Why the Sea Bulges on the Side Facing the Moon and the Side Facing Away
Here is the surprising part. A tide does not come from the Moon's pull alone. It comes from the difference in that pull from one place on Earth to another[4]. The Moon's pull is a little stronger on the half of the Earth facing it, because that half is closer, and a little weaker on the far half.
A simple way to picture it is a row of three runners tied loosely together, with the front runner sprinting hardest and the back runner pulling least. The row stretches. In the same way, the whole Earth is stretched very slightly along the line toward the Moon. The solid ground barely gives, but the ocean is free to move, so water gathers into one bulge facing the Moon and a second bulge on the opposite side. NOAA's explanation says this stretching and squashing is what makes the two tidal bulges[4].
What actually moves the water is the sideways part of this pull. On a spot at the edge of the bulge, the difference in pull has a component along the sea surface, and it drags water toward the points directly under and opposite the Moon[5].
Now go back to the first section. The Earth turns once a day, carrying you through both bulges, so you meet a high tide twice per lunar day. A bulge is not a wave sweeping across the sea. It is the sea's shape, held roughly in place by the Moon, while the coast spins through it.
The Sun makes tides in the same way. It is far bigger but much farther away, and its tide-making force comes out at about half the Moon's, or a bit less, depending on which source you use[1][5].
4. Why Some Tides Are Bigger, and Why the Same Moon Gives Different Tides in Different Places
Twice a month, at new moon and full moon, the Sun, Earth, and Moon line up, and the Sun's tide adds to the Moon's. That produces extra-high high tides and very low low tides, called spring tides. (The name has nothing to do with the season.) At the quarter moons, the Sun and Moon pull from directions at right angles, partly cancel each other, and give smaller tides called neap tides[1]. Tides also get a little bigger when the Moon is at the closest point of its orbit[1].
Place matters as much as the Moon. If Earth were smooth and covered with water, the bulges would sweep around evenly. Real oceans have continents and basins of all depths in the way, so tides can be small in one place and dramatic in another[4][5]. The highest tides in the world are in Canada's Bay of Fundy[6], where the range between high and low water is roughly 15 meters (about 50 feet), against roughly 1 meter (3 feet) as a rough world average[7]. The Moon's pull in the Bay of Fundy is the same as anywhere else. The shape of the coast and the sea floor does the rest.
So the tide table on your wall is an answer to three questions at once: where the Moon is, where the Sun is, and what the local coast does with the water.
How to Look into the Tides Yourself
- Catch the 50-minute shift. Find a free online tide table for any coast (a national weather or ocean agency, or a local harbor site). Write down the time of the first high tide on five days in a row. Is each one about 50 minutes later than the one before? Now look at the day-to-day change on a different coast and compare.
- Tides and the Moon's phase. Next to your notes, write down the phase of the Moon on each day, from a calendar. Around new moon and full moon, is the gap between high and low tide bigger than at the half moon? Then you have found spring and neap tides in the table.
- Visiting the shore. Go with an adult, check the tide times first, and stay on safe, higher ground. A beach that is wide and open now can be much smaller in a few hours.
- Read the original. NOAA's tides tutorial has short pages on each idea above. Compare its explanation of the two bulges with the one in section 3.