1. Formal Definition
| Planet | Axis Tilt (Obliquity) | Notable Effect | | :--- | :--- | :--- | | Mercury | ~0.03° | No seasons | | Earth | | Temperate seasons | | Mars | 25.19° | Similar seasons, but more extreme due to eccentric orbit | | Jupiter | 3.13° | Negligible seasonal variation | | Saturn | 26.73° | Pronounced seasons (7+ years each) | | Uranus | 97.77° | Extreme seasons; poles face Sun directly | axis tilt definition
, formally known as obliquity , is the angle between an object's rotational axis and a line perpendicular to its orbital plane. More commonly, it is described as the angle between the rotational axis and the orbital plane itself (the complement of the former). For a planet, this tilt determines the seasonal variation on that body. For a planet, this tilt determines the seasonal
It is a common pedagogical error to define axis tilt as "the angle of the Earth's axis from the vertical." While functionally equivalent, the precise astronomical definition references the orbital plane (the ecliptic). Thus, Earth’s axis is tilted 23.44° away from the perpendicular to its orbital path. For Earth, the current mean obliquity is approximately
For Earth, the current mean obliquity is approximately (or 23°26′). This value is not constant; it varies between 22.1° and 24.5° over a cycle of approximately 41,000 years due to gravitational perturbations from other planets (a Milankovitch cycle).