Earth’s Rotation Day Date in the current year: January 8, 2025

Today, it is common knowledge that the Earth rotates around its own axis once every 24 hours. But you might be surprised to learn that this knowledge is relatively recent: the Earth’s diurnal rotation was proven in the mid-19th century, less than 200 years ago.

Of course, scientists have been theorizing about the Earth’s daily rotation since ancient times. Among the ancient scholars who believed in the Earth’s rotation were the Greeks Philolaus, Hicetas, Heraclides, Ecphantus, and the Indian astronomer Aryabhata. The idea that the Earth rotates around its own axis was also supported by some medieval Muslim astronomers. However, it was highly controversial in medieval and early modern Europe.

Early experiments to verify the Earth’s rotation were conducted in the late 18th and early 19th centuries by Giovanni Battista Guglielmini, Johann Friedrich Benzenberg, and Ferdinand Reich. However, it was Léon Foucault’s pendulum experiment that proved Earth’s rotation around its axis.

The experimental setup with a pendulum used by Foucault to demonstrate the Earth’s rotation was first used by the Italian scientist Vincenzo Viviani in the mid-1700s, but Viviani did not connect the irregularities in the pendulum’s swing to the Earth’s rotation. Foucault, on the other hand, observed that the plane of oscillation of a long and heavy pendulum seemed to change spontaneously over a long period of time, presumably due to the Earth’s rotation.

Foucault conducted his first experiment with a pendulum on January 8, 1851, in the cellar of his house. The first public demonstration of Foucault’s pendulum took place about a month later above the Paris meridian in the Paris Observatory.

But the most famous demonstration of diurnal motion took place a few weeks later in the Panthéon; Foucault used a 28-kg brass-plated lead pendulum suspended from the dome of the Panthéon on a 67-m long wire. Foucault had theorized that the pendulum’s plane of oscillation depended on latitude, and the experiment proved him right: at the latitude of Paris, the predicted and observed shift was about 11 degrees clockwise per hour.

The original Foucault pendulum used in the Panthéon experiment has been moved to the Musée des Arts et Métiers in Paris, but the Panthéon has a copy. Foucault pendulums can now be found in museums around the world: Argentina, Australia, Austria, Belarus, Belgium, Brazil, Bulgaria, Canada, Chile, China, Colombia, Czech Republic, Denmark, Estonia, Finland, France, Germany, Hungary, Iceland, India, Iran, Israel, Italy, Japan, Latvia, Liechtenstein, Lithuania, Luxembourg, Mexico, Moldova, Netherlands, New Zealand, Norway, Pakistan, Poland, Portugal, Puerto Rico, Romania, Russia, Serbia, South Africa, Spain, Sweden, Switzerland, Thailand, Tunisia, Turkey, Ukraine, United Kingdom, United States, and Venezuela.

Earth’s Rotation Day was created to celebrate Foucault’s discovery and to encourage people to learn more about the Earth’s rotation and how it affects our lives. For instance, the rotation of the Earth creates a diurnal cycle of light and darkness (day and night) with its temperature fluctuations, and is the reason why time zones were created. It also causes the tides and is responsible for our sleep-wake cycle.