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 A sundial is a horological device that tells the time of day (referred to as civil time in modern usage) when direct sunlight shines by the apparent position of the Sun in the sky. In the narrowest sense of the word, it consists of a flat ...

circa 263 BC, the period of the natural day from sunrise to sunset was divided into twelve

hours An hour (symbol: h; also abbreviated hr) is a unit of time conventionally reckoned as of a day and scientifically reckoned between 3,599 and 3,601 seconds, depending on the speed of Earth's rotation. There are 60 minutes in an hour, and 24 hou ...

Variation

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 vice versa in summer. At Mediterranean latitude, one hour was about 45 minutes at the winter solstice, and 75 minutes at

summer solstice The summer solstice, also called the estival solstice or midsummer, occurs when one of Earth's poles has its maximum tilt toward the Sun. It happens twice yearly, once in each hemisphere ( Northern and Southern). For that hemisphere, the summer s ...

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

latitude In geography, latitude is a coordinate that specifies the north–south position of a point on the surface of the Earth or another celestial body. Latitude is given as an angle that ranges from –90° at the south pole to 90° at the north pole ...

Subdivision of the day and night

Ancient Roman time keeping hora vigilia equinox solstice

Civil day

The civil day (''dies civilis'') ran from midnight (''media nox'') to midnight. The date of birth of children was given as this period. It was divided into the following parts: # ''Media nox'' (midnight) # ''Mediae noctis inclinatio'' (the middle of the night) # ''Gallicinium'' (cock crowing) # ''Conticinium'' (cock stops crowing) # ''Diluculum'' (dawn) # ''Mane'' (morning) # ''Antemeridianum tempus'' (forenoon) # ''Meridies'' (mid-day) # ''Tempus pomeridianum'' (afternoon) # ''Solis occasus'' (sunset) # ''Vespera'' (evening) # ''Crepusculum'' (twilight) # ''Prima fax'' (lighting of candles) # ''Concubia nox'' (bed-time) # ''Intempesta nox'' (far into the night) # ''Inclinatio ad mediam noctem'' (approaching midnight)

Natural day

The natural day (''dies naturalis'') ran from sunrise to sunset. The hours were numbered from one to twelve as ''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'' (first hour of the day), and ''prima noctis hora'' (first hour of the night).

Timekeeping devices

The Romans used various ancient timekeeping devices. The

was imported from Sicily in 263 BC and they were set up in public places. Sundials were used to calibrate

water clock A water clock or clepsydra (; ; ) is a timepiece by which time is measured by the regulated flow of liquid into (inflow type) or out from (outflow type) a vessel, and where the amount is then measured. Water clocks are one of the oldest time-m ...

s. 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 A ''siesta'' (from Spanish, pronounced and meaning "nap") is a short nap taken in the early afternoon, often after the midday meal. Such a period of sleep is a common tradition in some countries, particularly those in warm-weather zones. The ...

'', literally the sixth hour of the day (''sexta hora''). *The daytime canonical hours of the Catholic Church take their names from the Roman clock: the

prime A prime number (or a prime) is a natural number greater than 1 that is not a product of two smaller natural numbers. A natural number greater than 1 that is not prime is called a composite number. For example, 5 is prime because the only ways ...

terce Terce is a canonical hour of the Divine Office. It consists mainly of psalms and is held around 9 a.m. Its name comes from Latin and refers to the third hour of the day after dawn. With Sext, None and Compline it belongs to the so-called "Litt ...

sext Sext, or Sixth Hour, is a canonical hour of the Divine Office of almost all the traditional Christian liturgies. It consists mainly of psalms and is held around noon. Its name comes from Latin and refers to the sixth hour of the day after dawn. W ...

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. The change of meaning was complete by around 1300. *The terms a.m. and p.m. are still used in the

12-hour clock The 12-hour clock is a time convention in which the 24 hours of the day are divided into two periods: a.m. (from Latin , translating to "before midday") and p.m. (from Latin , translating to "after midday"). For different opinions on represent ...

, as opposed to the

24-hour clock The modern 24-hour clock, popularly referred to in the United States as military time, is the convention of timekeeping in which the day runs from midnight to midnight and is divided into 24 hours. This is indicated by the hours (and minutes) pass ...

References

{{Commons category, Ancient Roman time keeping Timekeeping Time measurement systems Ancient Roman units of measurement