How does the Moon
pull the tides?

The Moon’s gravity pulls hardest on the ocean nearest it and weakest on the far side, stretching Earth’s water into two bulges. Earth rotates through both — two high tides a day.

The puzzle with tides isn’t that the sea rises toward the Moon — it’s that there are two high tides a day, including one on the side of Earth facing away from the Moon. The explanation is that gravity weakens with distance.

The Moon pulls hardest on the ocean directly beneath it, less hard on Earth as a whole, and least on the ocean on the far side. The near ocean is pulled away from Earth; Earth is pulled away from the far ocean. The result is two bulges of water on opposite sides of the planet. As Earth rotates once a day, your coastline sweeps through both bulges — two high tides and two low tides roughly every 24 hours and 50 minutes (the extra 50 minutes because the Moon has moved along its orbit meanwhile).

The Sun raises tides too, at a bit less than half the Moon’s strength. When Sun and Moon line up at new or full moon their tides add up — the large spring tides. When they pull at right angles at the quarter moons, they partly cancel — the gentle neap tides.

And tides work both ways: the friction of all that moving water is slowing Earth’s spin (days lengthen by about 2 milliseconds per century) and pushing the Moon outward by 3.8 cm a year.

This is one of seven interactive lessons in Moon Academy inside the Moon Explorer app — there, you don’t read the answer, you drag the Moon around its orbit and watch it happen.

← All seven questions

Dates and times are in Universal Time (UT). The free Moon Explorer app for Android converts everything to your local time and your exact location — fully offline, with no ads or accounts.