The angle between a planet's rotational axis and the perpendicular to its orbital plane, determining the severity of seasonal changes. Earth's obliquity of about 23.5 degrees creates our four seasons.
From Latin 'obliquitas' meaning 'slantedness' or 'deviation from the perpendicular.' The astronomical usage developed from ancient Greek observations of the Sun's apparent path changing throughout the year, leading to understanding of axial tilt.
Earth's obliquity isn't constant - it oscillates between 22.1 and 24.5 degrees over 41,000-year cycles, helping drive ice ages! Mars has a much more chaotic obliquity that can swing wildly from 0 to 60 degrees, creating extreme climate variations that may have contributed to its loss of atmosphere and water.
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