Hour (Gr. wpa; Lat. hora), a measure of time equal to 1/24 of a mean solar day, or this proportion of the period between sunrise and sunrise at the time of the equinoxes. Thus applied, it becomes a definite measure; but as employed by the ancients to designate 1/12 of the natural day, it was an indefinite period, varying with the times of rising and setting of the sun, times which continually changed with the season, and between increasing extremes as the observations were made in higher and higher latitudes. Even in the latitude of Rome, the length of the hour on June 25 was about 1/12 part of 15 hours 6 minutes, as now reckoned, and on December 23 it was only 1/12 part of 8 hours 54 minutes. At the two equinoxes only would the hour agree with its present measure. Hours thus divided were known as " temporary hours," in reference to their constant change of length. When the day was thus first divided is unknown. Herodotus states that the Greeks obtained the practice from the Babylonians. Wilkinson, however, says that " there is reason to believe that the day and night were divided, each into 12 hours, by the Egyptians, some centuries before that idea could have been imparted to the Greeks from Babylon." The division of the night as well as the day into 12 equal parts was not practised by the Romans until the time of the Punic wars, and the use of equinoctial hours was not adopted till toward the end of the 4th century of our era; the first calendar known to have been made after this system is the Calendarium Rusticum Farnesianum. Hours are now reckoned in common practice in two series of 12 each, from midnight to midday, and from this to midnight, which corresponds to the supposed divisions of the ancient Egyptians. Astronomers count 24 hours from one midday to the next; and the Italians 24 hours from one sunset to the next, changing the commencement of the day with the season.

The Chinese divide the day into 12 hours, one of their hours being equal to two of ours. They reckon from an hour (of our time) before midnight.. In the use of clocks in the 11th century it was the duty of the sacristans of the churches to regulate the horologia each morning.