The Moon's gravity tugs on Earth's oceans, pulling the water nearest it into a bulge — a high tide. But here's the surprise: there's a second bulge on the opposite side of Earth too. That's because the Moon pulls the near water hardest, the solid Earth a bit less, and the far water least of all — so the far ocean gets 'left behind', bulging outward. Earth spins through both bulges each day, so your coast gets two high tides and two lows. Spin the Earth in the simulator and watch your beach dip into a bulge twice.
Most people think the Moon simply pulls the ocean into a single bulge. In fact tides come from the difference in gravity across Earth: the near ocean is pulled hardest and the far ocean least, stretching the seas into two bulges.
What's actually happening
Almost everyone knows the Moon causes the tides, and most people picture it simply: the Moon's gravity pulls the ocean toward it into a bulge, and that's high tide. That's half right, and it leaves a glaring puzzle. If the Moon just pulled the water to one side, every coast would get one high tide a day as it rotated past the bulge. But most coasts get two — a high roughly every twelve hours. Where does the second bulge come from?
The answer is that tides are caused not by gravity's strength but by its difference across the Earth. Gravity weakens with distance, so the Moon pulls the ocean on the near side of Earth a little harder than it pulls the solid Earth at the centre, and pulls the far-side ocean weaker still. Think of three things in a row being tugged by different amounts: the near water is pulled ahead, the far water lags behind. The result is that the oceans get stretched into two bulges — one heaped toward the Moon, and one heaped away from it on the opposite side, where the water has effectively been "left behind". The simulator shows both bulges tracking the Earth–Moon line as the Moon orbits.
Now spin the Earth, and the two-tides-a-day pattern falls out. Your stretch of coast rotates through one bulge (high tide), then a low, then the second bulge twelve hours later (high again), then another low — two highs and two lows every day, which is exactly what tide tables show for most places. The Sun joins in too, with its own weaker tidal stretch (it's far more massive than the Moon but vastly farther away, and it's the difference that matters): when Sun and Moon line up, at full and new moon, their bulges add to make extra-large "spring" tides; when they pull at right angles, the tides are gentler "neaps". And there's a slow epilogue — the same tidal tug is gradually braking Earth's spin and nudging the Moon away, lengthening our day by a couple of milliseconds a century. The tides are quietly stealing time.
Tides are caused by gravity's difference across the Earth, not its strength — the two bulges give most coasts two highs a day, and the same tug is quietly slowing our day.
- 1Hold a loop of stretchy string or an elastic band with both hands and pull your hands apart: the loop stretches into an oval — two bulges, not one.
- 2That's what differential gravity does to the oceans: it stretches them along the Earth–Moon line, bulging both toward and away from the Moon.
- 3Now turn the oval slowly while keeping it stretched: any point on it passes through a wide part (high), a narrow part (low), the other wide part (high), and the other narrow part (low) — two of each per turn, just like a day of tides.
Common questions
Because the oceans are stretched into two bulges — one heaped toward the Moon and one on the opposite side, where the far water is effectively left behind. Your coast rotates through both roughly every twelve hours.
Yes, a weaker one. When the Sun and Moon align at full and new moon, their bulges add to make large "spring" tides; when they pull at right angles, the tides are gentler "neaps".
Yes. Tidal friction is gradually braking Earth's spin and nudging the Moon away, lengthening the day by about 1.7 ms per century and pushing the Moon some 3.8 cm farther off each year.