Clime

Ptolemy gives a list of parallels, starting with the equator, and proceeding north at intervals, chosen so that the longest day (summer solstice) increases in steps of a quarter of an hour from 12 hours at the equator to 18 hours at 58° N, and then, in larger steps, to 24 hours at the Arctic Circle.

But for the purposes of his geographical tables, Ptolemy reduces this list to eleven parallels, dividing the area between the equator and 54°1' N into ten segments, at half-hour intervals reaching from 12 hours to 17 hours. Even later in his work, he reduces this to seven parallels, reaching from 16°27' N (13 hours) to 48°32' N (16 hours).

Ptolemy's system of seven climes was primarily adopted in Arabo-Persian astronomy, by authors such as al-Biruni and al-Idrisi, and eventually by Amīn Rāzī, the author of the 16th-century haft iqlīm (lit. 'seven climes'), while in Europe, Aristotle's system of five climes was more successful. This view dominated in medieval Europe, and existence and inhabitability of the Southern temperate zone, the antipodes, was a matter of dispute.^{[citation needed]}

To identify the parallels delineating his climes, Ptolemy gives a geographical location through which they pass. The following is a list of the 33 parallels between the equator and the polar circle (39 parallels between the equator and the pole)^[5] of the full system of climes; the reduced system of seven climes is indicated by additional numbers in brackets. Both Ptolemy's latitude estimates and modern values are provided. Ptolemy never considered the impact of atmospheric refraction on his calculation of the length of the day. If you account for that and for the drift in orbital parameters since Ptolemy's time, his values are remarkably accurate, off by less than 30 arcseconds in most cases.

• Climate
• Four continents
• Geographical zone
• Roof of the World
• Seven Seas

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