Twice a day, everywhere there is a coast, the whole ocean rises and falls again. It is the Moon doing it — everybody knows that much. But the usual explanation, that the Moon pulls the water up on its side, cannot be right, and one look at a tide table shows why: there are two high tides a day, and one of them is on the side of the Earth facing away from the Moon. Nothing is pulling that one.
"The Moon's gravity pulls the sea towards it, so the water piles up underneath the Moon." If that were the whole story there would be one bulge, and one high tide each time your bit of coast turned to face the Moon — so one high tide roughly every 25 hours. There are two, about 12 hours and 25 minutes apart, and the second one happens when the Moon is on the far side of the planet, underneath your feet.
So the real explanation has to produce a bulge at both ends. That turns out to need one idea, and it is the same idea that runs the Moon's real path: what matters is not how hard gravity pulls, but how much harder it pulls on one part of a thing than on another.
Gravity gets weaker with distance. The Earth is wide — about 12,700 km — so the side of the Earth facing the Moon is noticeably closer to it than the far side, and gets pulled harder. The middle gets pulled by something in between.
Now, everything is falling towards the Moon together, and the Earth as a whole moves at the middle rate. So compared with the ground under them: the near water is pulled forward and pulls ahead. The far water is pulled least, so it is left behind. Both ends move away from the middle. Two bulges — and the far one is not being pushed by anything. It is simply falling more slowly than the planet it is sitting on.
The left picture is what the Moon does. The right picture is what you feel, because you are falling along with the middle. Nothing lifts the far bulge — it gets left behind, and being left behind looks exactly like being lifted.
The Sun is vastly heavier than the Moon. It pulls on the Earth about 179 times harder. So the Sun should own the tides completely. It does not — it produces less than half of the Moon's.
This is the one bit of arithmetic on the page worth keeping. A tide comes from the difference in pull across the Earth's width, and that difference falls off far faster with distance than the pull itself does. The Sun is 390 times further away than the Moon, and being further away hurts a tide much more than it hurts a pull. Enormous but distant loses to small but close.
The Sun's 46% is not nothing, though, and it explains something you can watch on any tide table. When the Sun and Moon are lined up — at new Moon and full Moon — their two tides add together and you get the biggest tides of the month. A week later, at half Moon, they are pulling at right angles and partly cancel, giving the feeblest ones.
The big ones are called spring tides, which has nothing to do with the season — it is spring as in "spring up". They come round every two weeks, and since new and full Moon are also when eclipses happen, an eclipse always lands in a week of big tides.
If the Earth simply spun under two fixed bulges, high tides would be exactly 12 hours apart. They are 12 hours and 25 minutes apart, and the extra 25 minutes has a familiar cause.
While you spin round once, the Moon has moved on a bit in its own orbit. You have to keep turning a little further to get back underneath it. One full turn takes 23 hours 56 minutes; one turn plus catching up with the Moon takes 24 hours 50 minutes, and that is a lunar day. Halve it, because there are two bulges, and you get the 12 hours 25 minutes between high tides.
That is exactly the argument on why a day is longer than one spin, with the Moon in the Sun's place. The same 4-minute-a-day slippage that makes a solar day longer than a spin makes the tide arrive later each day — which is why the good rock-pooling time drifts through the week.
Everything above would give every coast on Earth a tide of about half a metre, arriving neatly on schedule. Real tides are nothing like that tidy.
So the two bulges are the engine, not the outcome. It is a good example of something this site runs into a lot: the physics is clean and the world is not, and predicting a real tide at a real harbour needs the local geography as well as the Moon.
Dragging two bulges of ocean round the planet takes energy, and it has to come from somewhere. It comes out of the Earth's spin. Every day is very slightly longer than the one before — not by enough to notice in a lifetime, but by enough to add up: around 600 million years ago a day was about 22 hours long, and the year had over 400 of them.
The energy is not lost. It goes into the Moon's orbit, which means the Moon is slowly climbing away from us at about 3.8 cm a year — roughly the rate your fingernails grow. That number is not an estimate. Apollo astronauts left mirrors on the Moon, and observatories bounce lasers off them and time the round trip, which is why rung 2's distance keeps quietly getting bigger.
The Earth raises tides in the Moon far stronger than the Moon raises in us, and it has been at it for billions of years. It finished the job long ago: the Moon's spin has been dragged until it exactly matches its orbit, so it now keeps one face pointing at us forever. That is why nobody on Earth saw the far side of the Moon until a spacecraft went round the back in 1959. The near side is not the side facing us by luck. It is the side tides left facing us.
The hard part of tides is believing that one pull, all in the same direction, can stretch something at both ends. Three coins and a ruler settle it in a minute.
Now stand on the middle coin, because that is where you live. From there, the near coin has moved away from you towards the Moon, and the far coin has moved away from you in the opposite direction. Two bulges, from one pull, with nothing pushing outward at all. That is the tide, and you have just built it on a table.
If you live near the sea, there is a better version that takes a fortnight. Note the time of high tide on the same beach on several days running, and watch it slip about fifty minutes later each day — the Moon getting away from you. Note how big the tide is too, and you should find the range swelling and shrinking on a two-week rhythm that follows the shape of the Moon: biggest around new and full, weakest at half.