In Roman timekeeping, a day was divided into periods according to the technology available at the time. Initially the day was divided into two parts: the ante meridiem and the post meridiem, before noon and afternoon, respectively. With the advent of the sundial in about 263 BC, the period from sunrise to sunset was divided into 12 hours.
The civil day ran from midnight to midnight. The date of birth of children was given as this period, as is the case today. It was divided into the following parts: 1. Media nox, 2. Mediae noctis inclinatio, 3. Gallicinium, 4. Conticinium, 5. Diluculum, 6. Mane, 7. Antemeridianum tempus, 8. Meridies, 9. Tempus pomeridianum, 10. Solis occasus, 11. Vespera, 12. Crepusculum, 13. Prima fax, 14. Concubia nox, 15. Intempesta nox, 16. Inclinatio ad mediam noctem.
Natural day
The natural day ran from sunrise to sunset. The hours were numbered from one to 12 as follows: hora prima, hora secunda, hora tertia, etc. To indicate that it is a day or night hour Romans used expressions such as for example prima diei hora, and prima noctis hora.
The Romans divided the night into four watches,, following the Greek practice. "In the fourth watch of night" meant just before dawn. The four night watches were called prima vigilia, secunda vigilia, tertia vigilia, and quarta vigilia, and the intervals of time were tracked using water clocks.
Time keeping devices
The Romans used various ancient timekeeping devices. The sundial was imported from Sicily in 263BC and they were set up in public places. Sundials were used to calibrate water clocks. 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.
Legacy
The Roman day starting at dawn survives today in the Spanish word siesta, literally the sixth hour of the day.
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 - 6am, third - 9am, sixth - 12pm, and ninth = 3pm, hours of the day.
The English term noon is also derived from the ninth hour. This was a period of prayer initially held at 3 in the afternoon but eventually moved back to midday for unknown reasons. The change of meaning was complete by around 1300.
The terms a.m. and p.m. are still used by people using the 12-hour clock, as opposed to the 24-hour clock.
