41000year

41000year is a term used in astronomy and geoscience to denote the obliquity cycle of Earth's axis, the periodic variation of its tilt occurring on roughly a 41,000-year timescale. It is a shorthand often employed in discussions of Milankovitch cycles and their influence on climate over geological timescales.

The obliquity cycle describes changes in the angle between Earth's equatorial plane and its orbital plane, varying between about 22.1 and 24.5 degrees. The cycle is driven by gravitational interactions with other bodies in the solar system, particularly the Moon and the planets, and the exact period can drift slightly over long timescales. The term 41000year emphasizes the approximate duration of a full tilt cycle, rather than a fixed constant.

Climate relevance arises because changes in axial tilt modulate the distribution of solar radiation received at different latitudes. Higher tilt increases summer insolation in high latitudes and can promote melting, while lower tilt reduces summer insolation and can encourage cooling and glacier growth. The obliquity cycle operates alongside other orbital cycles, such as eccentricity and precession, to pace long-term climate shifts, including glacial and interglacial periods.

In practice, 41000year appears in paleoclimate research, climate modeling, and educational materials as a convenient label for the obliquity component of Milankovitch theory. It is not an official SI unit or constant, and modern analyses often treat it as an approximate cycle length subject to variations over millions of years.

See also: Milankovitch cycles, axial tilt, obliquity, climate cycles.