The Egyptians divided the day into twelve hours and the night into twelve hours. By the increase (or the decrease) of the daylight or the darkness of night, the number of hours was increased or decreased. The idea of dividing the complete day into 24 hours was inherited from the Ancient Egyptians, while the idea of equal hours came from the Babylonians. In the second century BC, Hipparchos divided the day and the night together into 24 hours. They were 24 unequal or seasonal hours consisting of 12 diurnal hours with a specific length, and 12 nocturnal hours of a different length. Sundials and night clocks were organized in such a way as to indicate the actual hours throughout the whole year.

Aristarchus of Samos (280 BC) invented a type of sundial, which had an unequal concave form that was different from other sundials, and was hemispherical, with a marker (gnomon) that was suitable for the hemispherical shape. The gnomon showed the direction and height of the sun because its shadow fell on the lines traced on the concave bowl. Aristarchus stated that the sun was the center of the universe, and then calculated that the universe expands infinitely. Thus, the angle of the stars is not relevant to seeing them, despite the width of the earth's orbit round the sun. Aristarchus did not hesitate for one moment in accepting such an apparently irrational hypothesis. However, it requires considerable imagination to visualize the challenge of such a hypothesis.