From DT Online

Lines of Latitude are imaginery lines drawn around the Earth to help locate points on a North-South axis. Lines of Longitude are used to determine the East-West position.

Latitudes are generated by imagining angles drawn from the Earth's centre on to the Earth's surface such that the Latitude of the Equator is 00 and the Latitudes of each of the North and South Poles is 900.

The Latitudes of both the Tropic of Cancer (aka the ‘Northern Tropic’) and the Tropic_of_Capricorn (aka the ‘Southern Tropic’) are each approximately 23.500 (23°26′13.6″).