How Tides Work

Build a sandcastle near the water’s edge in the morning, and by afternoon the sea has swallowed it. Come back the next day and the water is low again — but a little later than yesterday. The ocean is breathing in and out, twice a day, like clockwork. What could possibly move that much water?

The answer is hanging in the sky: the Moon.

The Moon pulls on the ocean

You already know from Gravity & Orbits that anything with mass pulls on everything else, and that the pull gets stronger the closer you are. The Moon is small compared to Earth, but it’s close enough that its gravity gives our planet a real, measurable tug.

Rock doesn’t show that tug much — it’s stiff. But the ocean is liquid, free to flow, so it answers the Moon’s pull by piling up. That pile of extra water is a tidal bulge, and when your beach rotates into it, the water rises: high tide.

So far, so sensible. But here’s the puzzle: if the Moon simply dragged the ocean toward itself, there’d be one bulge and one high tide a day. Almost every coast gets two. Why?

The key idea: the near side and the far side feel different pulls

Remember the closer-means-stronger rule. Now apply it across the whole Earth:

• The ocean on the near side (facing the Moon) is closest, so it’s pulled hardest — it gets tugged toward the Moon more than the Earth does, and bulges toward the Moon.
• The solid Earth in the middle is pulled a medium amount.
• The ocean on the far side is furthest away, so it’s pulled least — the Earth is effectively tugged out from under it, and the water gets left behind, bulging away from the Moon.

It isn’t the Moon’s pull itself that makes tides. It’s the difference in that pull from one side of Earth to the other. Stretch a planet between a hard tug on one side and a weak tug on the other, and its ocean ends up with two bulges: one facing the Moon, one directly opposite.

Two bulges, and an Earth that spins all the way around once a day — so your beach sweeps through both of them. Two high tides and two low tides every day, roughly six hours apart from each other.

See it for yourself

Here’s the whole system from above the North Pole. The ocean is hugely exaggerated so you can see the stretch; the red flag is your beach, riding the spinning Earth, and the graph tracks its water level through the day.

Watch the graph as a day goes by: two peaks, two dips. Notice the peaks arrive when your beach points at the Moon — and also when it points away from it. That second peak is the far-side bulge, the “left behind” water.

Why high tide comes ~50 minutes later each day

If Earth simply spun beneath two frozen bulges, high tide would hit your beach at the same times every day. But the Moon isn’t frozen — it’s orbiting, in the same direction Earth spins, going all the way around in about 27 days.

So each day the bulges have drifted a little further along, chasing the Moon. Your beach has to spin a bit past one full turn to catch up with them — about 50 extra minutes’ worth. That’s why today’s high tide is roughly 50 minutes later than yesterday’s, and why tide charts track the Moon, not the clock.

The Sun joins in: spring and neap tides

The Sun is enormously more massive than the Moon — but it’s also about 400 times further away, and tides care about the difference in pull across Earth, which fades fast with distance. So the Sun raises its own pair of bulges, a bit less than half the size of the Moon’s.

Two sets of bulges means they can team up or trip each other up:

• Sun and Moon lined up (full moon or new moon): their bulges stack. Extra-high highs, extra-low lows — spring tides.
• Sun and Moon at right angles (half moon): the Sun’s bulges partly fill in the Moon’s dips. Gentle highs, mild lows — neap tides.

Flip the Sun on in the simulator above and try both buttons — you can watch the graph’s peaks grow and shrink.

Check yourself

Hold on to that one idea — near and far feel different pulls, so things get stretched — because it scales up in a spectacular way. Around a black hole, the same physics that gently lifts the sea twice a day becomes strong enough to pull anything that falls in into a long, thin strand.