Obliquity

Obliquity, in astronomy, is the angle between a body's axis of rotation and the normal to its orbital plane. For planets, this tilt is commonly called axial tilt and it determines how seasons are expressed by changing how sunlight is distributed across latitudes during the year.

Earth’s obliquity is currently about 23.439 degrees and varies over a cycle of roughly 41,000 years, swinging between about 22.1 and 24.5 degrees. This variation changes the intensity of seasons, with higher tilt amplifying summers and winters and lower tilt reducing seasonal contrast. Obliquity is a central element of the Milankovitch cycles, which also include orbital eccentricity and axial precession, and together these factors influence long-term climate patterns.

The obliquity of a planet evolves because gravitational torques from the Sun, Moon, and other bodies act on the planet’s equatorial bulge, slowly altering the orientation of its rotation axis relative to the orbital plane. This is related to precession, the gradual shift of the orientation of the axis, and to nutation, shorter-term oscillations. In the solar system, obliquities vary widely among planets: Mercury has almost no tilt, Venus has a very large obliquity due to retrograde rotation, Earth about 23.4 degrees, Mars about 25 degrees, and Uranus about 98 degrees.

Obliquity is also a factor in planetary habitability studies, as extreme tilts can produce pronounced climates or long-term climatic instability that may affect the development and persistence of life.