The Egyptians were first to have a 24-hour day, measured from dawn to dawn. Before that, days had been divided into twelve segments, regardless of the season and actual length of the period of daylight. In the second century BC, the astronomer Hipparchus found such an arrangement was useless for his purposes. He, therefore, created a 24-hour system, based on the duration of hours at an equinox - that is, when day and night are the same length. (Non-astronomers continued to use the 12-hour system for over 1000 years, however.) Later, in the second century AD, Ptolemy - another Egyptian scientist - divided these 24 equinoctial hours into 60 minutes, based on a system originally devised by the Babylonians.