Tropic

The tropics or tropical circles of a planet are the circles that mark the maximal latitude in which its Central Star can pass through Zenith at that latitude. So, every planet has two tropics (degenerated, in some cases), since we can consider symmetry between the two hemispheres. The region comprehended between the two tropics in a planet is called intertropical zone.

In a planet whose rotation axis have had a value o $\phi$ of inclination, with respect to the perpendicular line at orbital plane, the tropics are the latitude circles with lat. $+\phi$ and $-\phi$. The Earth, for example, has an approximated inclination axis of 23,5°; that means, its tropics have latitudes +23,5° and -23,5°. The first, in earth's northern hemisphere, is called Tropic of Cancer; the last, on erth's southern hemisphere, is Tropic of Capricorn.

When the planetary inclination is zero, both tropics coincide with the Equator; the Central Star stays at high noon only in zero latitude. On the other hand, when a planet lays in its orbital plane, that means, its axial inclination is 90°, each tropic coincide with its respective pole, anth the Central Star can be seen at Zenith in every place on planet's surface.