Why does the sea come in and go out twice a day?

Twice a day the water climbs the beach, then sneaks back out. The Moon is doing it from 380,000 km away — and the trick is sneakier than just "the Moon pulls the water up." Let's catch it in the act.

1Two things to know first

A pull that fades, and a planet that spins

You only need two simple ideas to crack this. Watch each one move:

The pull gets weaker far away

The closer you are to the Moon, the harder it tugs. Near things feel a strong pull; far things feel a gentler one. The arrows shrink as you move away.

Earth keeps spinning

Our planet turns all the way around once a day. So any one beach is carried past every side of Earth — facing the Moon, then away from it, every day.

2Two ways the Moon might pull the sea

The "even tug" idea vs the "stretch" idea

Here are the two stories people tell. Same Moon, same Earth — but they give very different oceans. Take a quick look at both:

The "even tug" idea

One bulge

If the Moon tugged every drop the same, water would just pile up on the side facing the Moon.

The "stretch" idea

Two bulges

If the near water is pulled harder than the far water, Earth gets stretched — a bulge on both sides.

3Your turn — bend the ocean

Drag the pull from even to uneven

Here's Earth wearing a thin coat of ocean, with the Moon off to the right. Drag the slider to make the Moon pull the near water harder than the far water, and watch the water change shape.

EVEN TUGVERY UNEVEN

4Now spin a beach around and count

How many high tides in one spin? 🌊

Let's mark one beach and carry it all the way around Earth for a full day. Each time the beach swings into a bulge, the tide goes high. But how many highs will it hit?

Guess before you spin

Does the Moon's uneven pull make the ocean bulge in one place or two? Pick first — then you'll spin a beach around and count the high tides yourself.