In Roman timekeeping, a day was divided into periods according to the available technology. Initially, the day was divided into two parts: the ante meridiem (before noon) and the post meridiem (after noon). With the advent of the sundial circa 263 BC, the period of the natural day from sunrise to sunset was divided into twelve hours.^[1]

An hour was defined as one twelfth of the daytime, or the time elapsed between sunset and sunrise. Since the duration varied with the seasons, this also meant that the length of the hour changed. Winter days being shorter, the hours were correspondingly shorter and longer in summer.^[1] At Mediterranean latitude, one hour was about 45 minutes at the winter solstice, and 75 minutes at summer solstice.^[2]

The Romans understood that as well as varying by season, the length of daytime depended on latitude.

The Romans used various ancient timekeeping devices. Sundials were set up in public places,^[2] beginning after one was imported from Sicily in 263 BC.^[6] Sundials were used to calibrate water clocks.^[7] The disadvantage of sundials, or shadow clocks, was that they worked only in sunshine and had to be recalibrated depending on the latitude and season.^[8]

• The Roman day starting at dawn survives today in the Spanish word siesta, literally the sixth hour of the day (sexta hora).^[9]
• The daytime canonical hours of the Catholic Church take their names from the Roman clock: the prime, terce, sext and none occur during the first (prīma) = 6 am, third (tertia) = 9 am, sixth (sexta) = 12 pm, and ninth (nōna) = 3 pm, hours of the day.
• The English term noon is also derived from the ninth hour. This was a period of prayer initially held at three in the afternoon but eventually moved back to midday for unknown reasons.^[10] The change of meaning was complete by around 1300.^[11]
• The terms a.m. and p.m. are still used in the 12-hour clock, as opposed to the 24-hour clock.

• Ancient Greek calendars
• Egyptian calendar
• Roman calendar
• Relative hour