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Why Is Earth’s Axis Tilted?
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Earth's axis
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Why Is Earth’s Axis Tilted?

Written by
Aman Kumar
Aman Kumar is an editor at Encyclopaedia Britannica.
Aman Kumar
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Britannica Editors
Encyclopaedia Britannica's editors oversee subject areas in which they have extensive knowledge, whether from years of experience gained by working on that content or via study for an advanced degree....
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History
Earth's axis
Earth's axisRelative to the plane of Earth's orbit of the Sun, the planet's axis is inclined at an angle of about 23.44°.
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Earth rotates eastward, a counterclockwise motion when viewed from a point above the North Pole. The hypothetical line about which Earth spins is its axis. Though invisible, the axis is said to be tilted, because of the way Earth aligns with the Sun in Earth’s orbital path: one of Earth’s poles, north or south, is always closer to the Sun than the other at any given point in the planet’s orbit. This tilt, measured relative to the plane of the orbit, is determined to be about 23.44°.

The dominant hypothesis for the origin of this tilt is related to the giant-impact hypothesis, which seeks to explain the formation of the Moon. Astronomers think that 4.5 billion years ago, early in its genesis, Earth was struck by a traveling celestial body about the size of Mars called Theia. This collision is thought to have contributed to the formation of the Moon. Furthermore, Theia is thought to have knocked into Earth at an angle, turning Earth so that its axial orientation shifted.

The degree to which Earth’s axis tilts relative to its orbital plane is not constant. It changes over tens of thousands of years between 22.1° and 24.5°. The relatively low range of variation in the axial inclination—in other words, its relative stability—is attributable to the gravitational force exerted on Earth by other celestial bodies, most notably by Earth’s moon. The gravitational pull the Moon exerts on Earth helps cause the oceans on the planet’s surface directly toward and directly opposite the Moon to accumulate water and to be depleted of it elsewhere (see tide). The resulting bulges increase Earth’s rotational inertia (the extent to which a spinning body resists changes in its angular momentum), which contributes to the relative stability of the planet’s axis.

Aman Kumar