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InRoman timekeeping, a day was divided into periods according to the available technology. Initially, the day was divided into two parts: theante meridiem (before noon) and thepost meridiem (after noon). With the introduction of theGreeksundial to Rome from theSamnites circa 293 BC, the period of thenatural day from sunrise to sunset was divided into twelvehours.[1][2][3]
An hour was defined as one twelfth of thedaytime, 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] AtMediterranean latitude, one hour was about 45 minutes at thewinter solstice, and 75 minutes atsummer solstice.[4]
The Romans understood that as well as varying by season, thelength of daytime depended onlatitude.
The civil day (dies civilis) ran from midnight (media nox) to midnight.[5] The date of birth of children was given as this period.[6]
It was divided into the following parts:
The natural day (dies naturalis) ran from sunrise to sunset.[6]
The hours were numbered from one to twelve ashora prima,hora secunda,hora tertia, etc. To indicate that it is a day or night hour, Romans used expressions such as for exampleprima diei hora (first hour of the day), andprima noctis hora (first hour of the night).[7]
The Romans used variousancient timekeeping devices. According toPliny,Sundials, or shadow clocks, were first introduced to Rome when a Greek sundial captured from the Samnites was set up publicly around 293-290 BC.,[2] with another early known example being imported fromSicily in 263 BC.[8] Despite rapidly gaining popularity soon after their introduction, it wouldn't be until 164 BC that the first sundial specifically designed for the city of Rome was constructed.[2] The main disadvantage of sundials were that they worked only in sunshine and had to be recalibrated depending on thelatitude and season.[9][4] For this reason, they were often used as a method to calibratewater clocks, which could always tell the time, even on cloudy days and at night.[10]
The Roman day starting at dawn survives today in the Spanish wordsiesta, literally the sixth hour of the day (sexta hora).[11]
The daytimecanonical hours of theCatholic Church take their names from the Roman clock: theprime,terce,sext andnone 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 termnoon is also derived from the ninth hour. This was a period of prayer initially held at three in the afternoon but eventually moved back tomidday for unknown reasons.[12] The change of meaning was complete by around 1300.[13]
The terms a.m. and p.m. are still used in the12-hour clock, as opposed to the24-hour clock.
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