Equinoctial hours

The measurement of the full day with equinoctial hours of equal length was first used about 2,400 years ago in Babylonia to make astronomical observations comparable regardless of the season.

Geminos of Rhodes reported the observation of Pytheas of Massalia that the duration of the night depended on the geographical latitude of the place in question.

[1] Otto Neugebauer cites this account as the oldest testimony to the concept of hour (¹ra)[clarification needed] as a defined measure of time.

[2] The Babylonian calendar knew no division of the day into 24 time units, so Ancient Egyptian influence for this system can be considered probable.

The period of its origin can be dated to the 4th century BC, since Pytheas of Massalia refers to the terminus[clarification needed] G¨j perÐodoj introduced by Eudoxus of Cnidus.

In the appendix to his commentary on Aratos of Soloi and Eudoxos of Knidos, he uses the well-known 24-hour circles and names stars whose rises are separated from each other by about one equinoctial hour in certain seasons.

[citation needed] With the invention of the Stroke clock, for the first time one could read equinoctial hours mechanically without having to perform astronomical calculations.