27 Mar 2022

Tides twice a day

The best explanation I’ve heard for why there are tides twice a day appeared on The Curious Cases of Rutherford & Fry podcast.

Guest Helen Czerski explained there are two forces at work:

  • The gravitational pull of the Moon on the Earth (and its water)
  • Centrifugal force from the rotation of the Earth/Moon system, “pushing the water outwards”.


“On the side of the Earth closest to the moon, the gravitational pull of the moon is stronger than the centrifugal force. And so there’s a bulge of water towards the moon. But right the way round the other side, the gravitational pull is a little bit less, and the centrifugal force is even more. So the centrifugal force pulling outwards also makes a difference. And so you’ve got these two bulges on either side of the Earth, lined up with the Moon.

That led me to An Introduction to Oceanography, chapter 11, Tidal Forces, which refers to inertia (a property of mater, the cause) rather than centrifugal force.

The podcast episode is The Turn of the Tide, broadcast 17 March 2022.