What if Earth had no Moon?

At the beach, the sea comes up the sand. Later it goes back down.

The Moon helps make that happen. What if Earth had no Moon at all?

If you spend a day at the beach, you can watch the water creep up the sand and then slide back down again. These rises and falls are called tides, and most coasts get two high tides and two low tides each day.

The Moon is the main cause. So what would the tides be like if Earth had no Moon? In this lab you can remove the Moon from a tide model and find out.

Most coastlines see two high and two low tides roughly every 24 hours and 50 minutes, a rhythm tied to the Moon's motion across the sky. NOAA describes the Moon as the dominant force behind Earth's tides, with the Sun playing a smaller part.

This lab asks what would remain if the Moon's mass were zero: would the oceans stop rising and falling, or would some tide still be left, and how large would it be?

The Moon gently pulls on the sea. The water piles up a little on the side near the Moon.

It piles up on the far side too. Earth spins through both piles each day.

The Moon pulls a little harder on the side of Earth that faces it than on the far side. That difference stretches the oceans into two bulges, one toward the Moon and one away from it. As Earth spins, your beach passes through both bulges, so you get two high tides a day.

The Sun stretches the oceans too, but only about half as much. When the Sun and Moon line up, at new and full moon, their tides add up to big spring tides. When they pull at right angles, the tides are smaller neap tides.

Tidal stretching is the difference in a body's pull across Earth's diameter, roughly 2GMR/d³, so it scales with mass over distance cubed. The Sun is about 27 million times more massive than the Moon but about 390 times farther away, so its tide is only about 0.46 of the Moon's.

In the model, today's Moon gives a tide strength of 1. Lined up with the Sun (spring tide) the total is 1.46; at right angles (neap tide) it is 0.54. On an imaginary planet-wide ocean the spring-tide range would be about 0.78 meters.

In our pretend lab, Olly takes the Moon away.

The sea still goes up and down a little. The Sun is making small tides!

Set the Moon's mass to zero and the tide meter drops from 1.46 to 0.46. The tides do not vanish, because the Sun is still stretching the oceans. But they are much smaller: about a third of today's biggest tides.

Something else disappears too. Without a Moon to line up with the Sun, there are no extra-big spring tides and no small neap tides. The tides are the same size every day, and the moon-phase readout says there is no Moon.

With the lunar term set to zero, the model's combined tide T = √(L² + S² + 2LS cos 2θ) reduces to S ≈ 0.46 at every angle. Spring and neap become identical, so the fortnightly spring-neap cycle disappears. The ideal open-ocean range falls from about 0.78 m to about 0.25 m.

This is a thought experiment carried out with a model. The Moon cannot simply vanish, and the model only answers one question: how strong the tidal stretching would be.

The Moon is small, but it is close to Earth. Close things make big tides.

The Sun is huge, but very far away. So its tides are smaller.

Tides are not made by how hard something pulls, but by how much its pull changes from one side of Earth to the other. That change shrinks very fast with distance: twice as far gives only one eighth as much stretching.

The Moon is small but close, so its pull changes a lot across Earth. The Sun is enormous but so far away that its pull is almost the same on both sides. That is why the Sun's tides are only about half as big as the Moon's.

The Sun's gravitational pull on Earth is about 177 times the Moon's, yet its tide-raising effect is smaller, because tidal acceleration depends on the gradient of the field, which scales as M/d³ rather than M/d².

The model's limits: it uses equilibrium tide theory, an ocean covering the whole planet that takes its shape instantly, and circular orbits. Real tides depend on coastlines, ocean depth and basin resonances, so the meter figures are for an imaginary global ocean, not any real beach.

Some beaches have tiny tides. Some bays have giant tides!

The shape of the coast makes a big difference.

Real tides vary enormously from place to place. In the Bay of Fundy in Canada, the water can rise and fall by more than 16 meters, while in most of the Mediterranean Sea the tide rises and falls by less than half a meter. The shape of the coast and the depth of the sea can make the tide much bigger or smaller.

So a world with no Moon would still have tides, but every coast would get only the Sun's small, steady share of them, shaped by its own bays and beaches.

Observed tidal ranges run from well under a meter on many coasts (the Mediterranean's tides average about 40 cm) to about 16 meters in the Bay of Fundy, because coastlines, ocean depth and basin resonances reshape the tide. Real high tides also lag the Moon by hours and vary from place to place.

The model also leaves out the long-term story: tidal friction is slowly lengthening Earth's day (by about 0.002 seconds per century) and pushing the Moon outward. A Moon-free Earth would have lost that braking, an idea the model does not calculate.