Earth's rotation

Earth's rotation is the planet's spin around its own axis. This movement is known as prograde motion because Earth rotates toward the east.

This rotation creates our cycle of day and night. It also causes the apparent movement of stars across the sky. The axis of rotation connects the North Pole to the South Pole. The North Pole is a specific point in the Northern Hemisphere. It is different from the magnetic north pole.

Rotation speed varies depending on your latitude. At the equator, the Earth's circumference is at its largest. This means the rotational speed is highest there, at about 1,670 kilometers per hour. As you move toward the poles, the speed decreases. At 45 degrees latitude, the speed drops to roughly 1,180 kilometers per hour. At the poles, the speed is effectively zero. This happens because there is no distance to travel during a rotation at those points.

We measure the length of a rotation in two different ways. A solar day is the time from one noon to the next. This takes about 24 hours. A sidereal day is the time it takes to rotate relative to distant stars. This takes 23 hours, 56 minutes, and 4 seconds. The solar day changes slightly throughout the year. This is due to the eccentricity and inclination of Earth's orbit. For example, the true solar day is longer near perihelion. This is when Earth is closest to the Sun.

For centuries, thinkers have debated how the Earth moves. Some ancient Greeks, like Philolaus, believed the Earth rotated. However, Aristotle argued that the stars were fixed in a sphere. He believed the heavens rotated around a stationary Earth. This view lasted a long time. In 499 CE, the Indian astronomer Aryabhata suggested the Earth rotates. He used a clever analogy. He said a person in a boat sees the river banks moving backward.

Later, Nicolaus Copernicus established the heliocentric system in 1543. He showed that planets circle the Sun. Scientists like Galileo and Newton later provided more evidence for this rotation.

Scientists use precise tools to track changes in rotation speed. Atomic clocks show that the length of a day is not constant. For a long time, the Moon's tidal effects slowed Earth down. This increased the length of a day by 2.3 milliseconds per century. However, in 2020, Earth began to rotate faster. On June 29, 2022, Earth completed a rotation in 1.59 milliseconds under 24 hours. This was a new record.

Engineers are now discussing a "negative leap second" to manage this.

Several factors influence how fast the planet spins. The movement of the molten core, oceans, and atmosphere all matter. Climate change may also play a role. Melting ice at the poles changes the Earth's shape. Earth is an oblate spheroid, which means it bulges at the equator.

When ice melts, the poles rebound from the loss of weight. This makes the Earth more spherical. This brings mass closer to the center of gravity. The law of conservation of angular momentum says that mass spinning closer to the center moves faster.

Rotation also creates the Coriolis effect. This is an apparent path that moving objects follow in a rotating system. In the Northern Hemisphere, projectiles veer to the right. In the Southern Hemisphere, they veer to the left. This effect is very important for meteorology. It causes cyclones to rotate in different directions in each hemisphere. Scientists once tested this by dropping weights from tall towers. The most famous test was the Foucault pendulum in 1851. This device uses a heavy sphere to show the Earth rotating underneath it.