The Earth rotates by spinning around an imaginary line called its axis, which runs from the North Pole to the South Pole. This rotation happens from west to east, causing the Sun to appear to rise in the east and set in the west. The Earth completes one full rotation approximately every 24 hours, which defines the length of a day
. The reason Earth spins is rooted in its formation about 4.54 billion years ago. Initially, a large cloud of dust and gas collapsed under gravity, flattening into a spinning disk. As this disk spun faster, particles collided and clumped together, forming the Sun and planets. A significant collision with a Mars-sized body likely sped up Earth's rotation early on, resulting in a much shorter day than today
. Earth's rotation is maintained by inertia, meaning it resists changes to its state of motion. Although gravitational forces from the Moon, Sun, and other bodies exert torque that gradually slows Earth's spin, this effect is very slow, lengthening the day by only about 1/500th of a second every 100 years
. The Earth's axis is tilted about 23.5 degrees relative to its orbit around the Sun, which, combined with its rotation, causes the cycle of day and night and the changing seasons
. In summary:
- Earth spins on its axis from west to east once every ~24 hours
- This spin originated from the angular momentum of the early solar system's dust cloud and was influenced by early collisions
- The rotation axis points roughly toward the star Polaris in the Northern Hemisphere
- Earth's rotation is gradually slowing due to tidal forces but remains steady due to inertia
This rotation is fundamental to the cycle of day and night experienced on Earth.
