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
A shared pull, or a changing pull?
Compare two idealized rules. An equal pull moves Earth and water together. A difference in pull can change the ocean's shape.
No tidal bulge
If Earth and every drop felt the same pull, they would accelerate together. There would be no Moon-made tidal stretch.
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.
4Now spin a beach around and count
How many highs in half a turn? 🌊
A marked beach starts at low water between the two bulges. Carry it through half a turn. Count each high-water peak it passes.
Guess before you spin
In this model, the beach starts between the bulges and turns halfway around Earth. How many high-water peaks will it pass?