Initially, only the day was divided into 12 seasonal hours and the night into 3 or 4 night watches.By the Hellenistic period the night was also divided into 12 hours. The day-and-night (νυχθήμερον) was probably first divided into twenty-four hours by Hipparchus of Nicaea. The Greek astronomer Andronicus of Cyrrhus oversaw the construction of a horologion called the Tower of the Winds in Athens during the first century BCE. This structure tracked a 24-hour day using both sundials and mechanical hour indicators.The canonical hours were introduced to early Christianity  from Second Temple Judaism. By AD 60, the Didache recommends disciples to pray the Lords Prayer three times a day; this practice found its way into the canonical hours as well. By the second and third centuries, such Church Fathers as Clement of Alexandria, Origen, and Tertullian wrote of the practice of Morning and Evening Prayer, and of the prayers at the third, sixth and ninth hours. In the early church, during the night before every feast, a vigil was kept. The word "Vigils", at first applied to the Night Office, comes from a Latin source, namely the Vigiliae or nocturnal watches or guards of the soldiers. The night from six oclock in the evening to six oclock in the morning was divided into four watches or vigils of three hours each, the first, the second, the third, and the fourth vigil.The Horae were originally personifications of seasonal aspects of nature, not of the time of day.
Vatican II ordered their reformation for the Catholic Church in 1963, though they continue to be observed in the Orthodox churches. When mechanical clocks began to be used to show hours of daylight or nighttime, their period needed to be changed every morning and evening (for example, by changing the length of their pendula). The use of 24 hours for the entire day meant hours varied much less and the clocks needed to be adjusted only a few times a month. Modernity
The minor irregularities of the apparent solar day were smoothed by measuring time using the mean solar day, using the Suns movement along the celestial equator rather than along the ecliptic. The irregularities of this time system were so minor that most clocks reckoning such hours did not need adjustment. However, scientific measurements eventually became precise enough to note the effect of tidal deceleration of the Earth by the Moon, which gradually lengthens the Earths days. During the French Revolution, a general decimalisation of measures was enacted, including decimal time between 1793 and 1795. Under its provisions, the French hour (French: heure) was 1⁄10 of the day and divided formally into 100 decimal minutes (minute décimale) and informally into 10 tenths (décime). This hour was only briefly in official use, being repealed by the same 1795 legislation that first established the metric system. The metric system bases its measurements of time upon the second, defined since 1952 in terms of the Earths rotation in AD 1900.