Earth's axial tilt

Earth's rotation axis is tilted about 23.4 degrees from the perpendicular to its orbital plane. This tilt causes the seasons as Earth orbits the Sun.

Season: A recurring period of the year distinguished by characteristic patterns of temperature, daylight, and weather. Axial tilt causes seasons by changing sunlight angle and daylight duration over Earth's orbit.
Earth's rotation: Earth's spin about an axis through its geographic poles. The axis of this rotation is the direction whose orientation defines Earth's axial tilt.
Giant-impact hypothesis: The hypothesis that a collision between early Earth and a Mars-sized body formed the Moon. The impact likely helped set the Earth–Moon system's initial spin orientation.
Milankovitch cycles: Long-term variations in Earth's orbit and orientation that alter the distribution of sunlight reaching Earth. Axial tilt varies cyclically and contributes to these changes in insolation.
Solar declination: The angular position of the Sun north or south of Earth's celestial equator. Its annual shift tracks how axial tilt redirects direct sunlight between the hemispheres.
Ecliptic: The apparent annual path of the Sun across the sky, corresponding to Earth's orbital plane. Earth's tilt is measured relative to the perpendicular to this plane.
Theia: The hypothesized protoplanet that collided with early Earth in the leading model of the Moon's formation. Theia's collision is a proposed source of the angular momentum that shaped Earth's early spin.
Obliquity cycle: The roughly 41,000-year variation in Earth's axial tilt, between about 22.1 and 24.5 degrees. This cycle changes the contrast between seasons, especially at high latitudes.
Day length: The duration of daylight at a location during one rotation of Earth. Tilt makes daylight duration vary with latitude and season.
Celestial equator: The projection of Earth's equator onto the celestial sphere. Its orientation expresses the tilt between Earth's equatorial plane and orbital plane.

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