The poem “Why the Beach Disappears Twice a Day” makes a claim that surprises almost everyone the first time they hear it. The Moon raises a bulge of ocean on the side of the Earth facing it, which makes sense, but it also raises one on the side facing away. That’s why most coastlines get two high tides a day rather than one. The poem gives the short reason. This is the long one.
The poem gets one phrase exactly right that is easy to read past. The far sea, it says, is “pulled least of all” and “left to swell behind”. Left behind is the whole idea, and the rest of this piece is about why.
Start with the fact that gravity weakens with distance. The Moon pulls on every part of the Earth, but not equally. The ocean on the side nearest the Moon is about 6,400 kilometres closer to it than the centre of the Earth is, so it’s pulled a little harder. The ocean on the far side is about 6,400 kilometres further away, so it’s pulled a little less.
What matters for the tides is not the pull itself, but the differences. Imagine the Earth as three runners, the near ocean, the centre and the far ocean, all being called towards the Moon. The near ocean is pulled hardest, so it pulls ahead of the centre: that’s the first bulge. The centre is pulled harder than the far ocean, so it pulls ahead of the far ocean, which gets left behind. Seen from the Earth’s centre, the far ocean is moving away from it: that’s the second bulge. The Earth is being gently stretched along the line towards the Moon. Physicists call this a tidal force, and it’s the same effect, only far stronger, that tears comets apart when they pass too close to a planet.
You’ll sometimes see the far bulge explained as being “flung out” as the Earth and Moon swing around each other. They do swing around a shared centre of mass, which sits about 4,700 kilometres from the Earth’s centre, still inside the planet. But the cleanest explanation is the stretching one: the far bulge exists because the far side is pulled less than everything nearer the Moon.
The Earth spins through both bulges, which is why a beach usually gets two high tides. But the Moon moves along its orbit in the meantime, so the Earth has to turn a little further each time to catch up with it. Successive high tides are therefore about 12 hours and 25 minutes apart, not exactly 12, and a full cycle of two high tides takes about 24 hours and 50 minutes. That’s why the same tide arrives about 50 minutes later each day: the “some fifty minutes late” in the poem’s fourth stanza.
The poem’s fifth stanza, about spring and neap tides, comes from the same stretching. The Sun stretches the Earth too, a little under half as strongly as the Moon. At new and full moon, the Sun, Moon and Earth line up, the two stretches add together, and we get the big spring tides. At the quarter moons, they pull at right angles and partly cancel, giving the gentler neaps. “Spring” here has nothing to do with the season; it comes from the idea of the water springing up.
Now the strange part. The Earth spins faster than the Moon orbits, so it drags the bulges slightly ahead of the line to the Moon. The Moon’s gravity tugs back on that leading bulge, which acts like a very gentle brake on the Earth’s spin. Our days are getting longer, by about two thousandths of a second per century. Coral fossils, which lay down daily growth rings, suggest that around 400 million years ago a year had roughly 400 days, because each day was shorter.
The bulge pulls on the Moon too, nudging it forward in its orbit, and that moves it slowly outward. We can measure this precisely. Astronauts on the Apollo missions left mirrors on the Moon’s surface, and observatories on Earth still bounce lasers off them and time the light’s return. The answer: the Moon is moving away from us by about 3.8 centimetres a year, about as fast as your fingernails grow.
So the ocean rising up a beach twice a day is not just a curiosity of the coast. It’s one visible edge of a slow exchange between the Earth and the Moon, which has been going on for billions of years and is still going on now.