Roman timekeeping

With the introduction of the Greek sundial to Rome from the Samnites circa 293 BC, the period of the natural day from sunrise to sunset was divided into twelve hours.

[6] It was divided into the following parts: The natural day (dies naturalis) ran from sunrise to sunset.

[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.

[11] 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.

This was a period of prayer initially held at three in the afternoon but eventually moved back to midday for unknown reasons.