[photo credit: NASA]

January 8 is Earth’s Rotation Day. The day celebrates the moment when French physicist Léon Foucault (pronounced fuko) showed that the Earth rotates on its axis, in 1851.

Foucault Pendulum


Without any formal scientific training, Foucault came up with this understanding using what is now known as the Foucault pendulum.

The pendulum is like the weight that hangs from the bottom of a clock and moves back and forth. ‘Pendulum’ means ‘to hang.’

This is a print of the Foucault pendulum from 1895.



In this video, physics and astronomy professor Jim LaBelle discusses the science behind the Foucault pendulum, while seated next to Dartmouth University’s pendulum.

Learn about gravity

Did you know that the speed of the Earth’s rotation can change from day to day and from year to year?

Also, since the Earth’s rotation is fastest at the equator, more force is pushing at you from the Earth’s center (called centrifugal force). When you stand on the equator, you weigh slightly less than when you stand on the north or south poles!

Gravity is everywhere, even in space. Spend some time this week learning about how gravity affects our lives. Here are a few videos and free courses to get you started:

Help scientists

NASA has a project open to educators and students called the Earth Rotation Detector. To take part, you use an app on your smartphone. The app lets you measure the increase of gravity at your location. The data you enter is then shared with NASA.