Earthly Rotation

Like a whirligig Earth rotates on its own axis.

Primitive observers from the inhabited middle latitudes of Earth would've have mused about something we rarely contemplate:

The Sun rises in the East
The Sun sets in the West

Like most celestial phenomena, Sunrises and Sunsets occur over a repeatedly observable and predictable cycle. During one day, the Sun appears to follow a half-circular arc through the sky.

As observers of the night sky our ancestors would've also noticed that stars moves along a similar circular path.

Our ancestors might have reasonably believed that the bespeckled night sky was an extraterrestrial backdrop that rotates.

"Before we invented civilization our ancestors lived mainly in the open out under the sky." ― Carl Sagan

Whirling Southern Star Trails over ALMA by ESO

Ancient ancestral astronomers might have hypothesized a stationary Earth centered about a rotating backdrop of stars.

"Mortal as I am, I know that I am born for a day. But when I follow at my pleasure the serried multitude of the stars in their circular course, my feet no longer touch the earth."
― Ptolemy, Almagest, a 2^nd century astronomical treatise

Today we know it's the Earth that's rotating on its axis that creates this illusion. How might we devise a model that matches our observations and proves the Earth is rotating?

Supporting Model

Foucault pendulum
at the North Pole
by Theresa Knott

A less intuitive but valid hypothesis is that the Earth is rotating on its own axis. Of course we know now that Earth rotates on its own axis, but we were helped by a model to clinch the deal of corroborating what others had observed for generations.

The classic model that helped prove Earth is rotating on its own axis is a simple experiment with a pendulum known as the Foucault pendulum.

Foucault Pendulum

The Foucault pendulum (pronounced foo-koh) was introduced in 1851 to visually demonstrate Earth's rotation. The experiment is named after French physicist Léon Foucault.

An experimental Foucault pendulum, commonly suspended 39–98 feet and freely swinging back and forth, will be visibly affected by Earth’s rotation.

Simulated pendulum rotation
by Dominique Toussaint

The affect is not visible after a few swings, rather becomes gradually visible by a matter of degrees over the course of a day.

To maintain the bob of a pendulum swinging in museum demonstrations, an electromagnetic or other drive is used, or the pendulum is manually restarted.

A pendulum day is the time needed for the plane of a Foucault pendulum to complete 360 degree rotation. The duration of a pendulum day is one day divided by the sine of the latitude.

"I have come to believe that the whole world is an enigma, a harmless enigma that is made terrible by our own mad attempt to interpret it as though it had an underlying truth." ― Umberto Eco

REFERENCES

• But it moves! How we know the Earth rotates, Ethan Siegel,17 September 2010.
• Foucault pendulum, Wikipedia.
• Pale Blue Dot: A Vision of the Human Future in Space, Carl Sagan, Random House 1994.