Obliquity of the ecliptic

From Glossary of Meteorology

obliquity of the ecliptic

The angle between the plane of the ecliptic (or the plane of the earth's orbit) and the plane of the earth's equator; the "tilt" of the earth.

The obliquity of the ecliptic is computed from the following formula:
where t is the year for which the obliquity is desired. For 1999, the value was 23°26′21.89″. It is the oblique orientation of the earth's axis relative to its orbit that accounts for the seasons, for, in the period of a year, the angle of incidence of incoming solar radiation varies by nearly 47° at any one place. Particularly at high latitudes, this results in a great seasonal temperature contrast. M. Milankovitch has calculated that the obliquity of the ecliptic varies between 24.5° and 22° in the course of 40 000 years. This variation may be considered as a long-period climatic control and is included in the astronomical theory of ice ages.