Earth's rotation

For Earth's orbit around the Sun, see Earth's revolution.
"Earth's rotation period" redirects here. For the duration of daylight and night, see Daytime.

Earth's rotation or Earth's spin is the rotation of planet Earth around its own axis, as well as changes in the orientation of the rotation axis in space. Earth rotates eastward, in prograde motion. As viewed from the northern polar star Polaris, Earth turns counterclockwise.

The North Pole, also called the Geographic North Pole or Terrestrial North Pole, is the point in the Northern Hemisphere at which Earth's axis of rotation meets its surface. This point is distinct from Earth's north magnetic pole. The South Pole is the other point at which Earth's axis of rotation intersects its surface, in Antarctica.

Earth rotates once in about 24 hours with respect to the Sun, but once every 23 hours, 56 minutes, and 4 seconds with respect to other distant stars (see below). Earth's rotation is slowing slightly with time; thus, a day was shorter in the past. This is due to the tidal effects the Moon has on Earth's rotation. Atomic clocks show that the modern day is longer by about 1.7 milliseconds than a century ago, slowly increasing the rate at which UTC is adjusted by leap seconds. Analysis of historical astronomical records shows a slowing trend; the length of a day has increased by about 2.3 milliseconds per century since the 8th century BCE.

Scientists reported that in 2020, Earth had begun to rotate faster, after consistently turning slower than 86,400 seconds per day in the decades before. On June 29, 2022, Earth's rotation was completed in 1.59 milliseconds under 24 hours, setting a new record. Due to that trend, engineers worldwide are discussing a "negative leap second" and other possible timekeeping measures.

This increase in speed is thought to be due to various factors, including the complex motion of its molten core, oceans, and atmosphere, the effect of celestial bodies such as the Moon, and possibly climate change, which is causing the ice at Earth's poles to melt. The masses of ice account for the Earth's shape being that of an oblate spheroid, bulging around the equator. When these masses are reduced, the poles rebound from the loss of weight, and Earth becomes more spherical, which has the effect of bringing mass closer to its centre of gravity. Conservation of angular momentum dictates that a mass distributed more closely around its centre of gravity spins faster.

Earth's rotational speed is about 1,674.7 km/h (1,040.6 mph; 465.2 m/s; 1,526.2 ft/s) at the equator, but it decreases with latitude, becoming zero at the poles. This is because Earth's circumference is greatest along the equator and the shortest through the poles. The speed at any given latitude can be calculated by multiplying the equatorial speed by the cosine of the latitude. At the equator, the speed is about 1,674.7 km/h (1,040.6 mph), calculated by dividing Earth's equatorial circumference (c. 40,075 km (24,901 mi)) by the length of a sidereal day (c. 23.93 hours). At middle latitudes, the speed decreases; for example, at 45° latitude, the speed is roughly 1,184.2 km/h (735.8 mph) (1674.7  ×  cos (45°)). At the poles, the speed is effectively zero, as no distance there covers in a full rotation.

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