the orbital element that determines the amplitude of the seasonal variation of incident solar radiation. The present value is 23°2' to 24°27'.

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 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.

The angle between the plane of the ecliptic and the plane of the equator. It now measures about 23°27', and is decreasing at the rate of one minute in 128 years.