τροπικός (κύκλος)

The tropical circles are the parallel circles to the celestial equator that touch the maximum and minimum elongation of the ecliptic (with regard to the equator), i.e. the astronomical coordinates of northernmost and southernmost declination of the Sun.

As Ptolemy transferred the celestial coordinate system of orthogonal "longitudes" and "latitudes" from the sphere to the sphere of the Earth, it was also possible to define "tropical circles" on Earth. These circles of geographical latitude are the parallel circles north and south of the Earth's equator above which the Sun stands perpendicular (i.e. in zenith) during the June and December solstice, respectively.