structural:expansion, structural:scholar_edition, information:extraction
Astronomical definition

The tropics are defined as the region between the Tropic of Cancer in the Northern Hemisphere at 23°26′09.9″ (or 23.43608°) N and the Tropic of Capricorn in the Southern Hemisphere at 23°26′09.9″ (or 23.43608°) S; these latitudes correspond to the axial tilt of the Earth.

The Tropic of Cancer is the Northernmost latitude from which the Sun can ever be seen directly overhead, and the Tropic of Capricorn is the Southernmost. This means that the tropical zone includes everywhere on Earth which is a subsolar point at least once during the solar year. Thus the maximum latitudes of the tropics have equal distances from the equator on either side. Likewise, they approximate the angle of the Earth's axial tilt. This angle is not perfectly fixed, mainly due to the influence of the moon, but the limits of the tropics are a geographic convention, and their variance from the true latitudes is very small.