Obliquity of the ecliptic
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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.