Hour (h)
The hour is 3,600 seconds or 60 minutes, and it is the unit that structures daily human life more than any other. We think of our days in terms of hours: an eight-hour workday, a two-hour film, a three-hour drive. The division of the day into 24 hours is so deeply embedded in human culture that it is the one system of measurement that has never seriously been proposed for decimalization. Clocks, calendars, timetables, contracts, and medical prescriptions all organize time in hours. The hour is accepted for use with the International System of Units and appears alongside seconds in virtually every domain of applied science and engineering.
Definition
One hour equals exactly 3,600 seconds or exactly 60 minutes. One day contains exactly 24 hours. One week contains exactly 168 hours. One year of 365 days contains exactly 8,760 hours.
History
The division of the day into 24 hours originates in ancient Egypt, where astronomers divided the night sky into 12 star-based time segments and divided the daylight period into 12 equal parts. These 24 divisions were adopted by Greek astronomers and spread through the Roman world. However, for most of antiquity and the Middle Ages, the hour was not fixed at 60 minutes. Instead, the 12 daylight hours and 12 nighttime hours varied in length with the seasons: summer daylight hours were longer, winter ones shorter. These were called seasonal or temporal hours. Only with the spread of mechanical clocks in fourteenth-century Europe did the equal-length, fixed 60-minute hour become standard. The mechanical clock made it practical to divide the hour uniformly and eventually led to the 60-minute convention becoming universal.
Common Uses
The hour is the primary unit of labor measurement in most economies. Employment contracts specify hourly wages, annual salaries are divided by hours worked, and time-tracking systems record project time in hours and minutes. Flight durations are always given in hours: a transatlantic flight takes about 7 to 9 hours, a flight from London to Sydney about 22 hours. In engineering, specifications for equipment operation use hours: a machine rated for 50,000 operating hours, a battery lasting 10 hours. In medicine, drug dosing schedules use hours: take every 4 hours, or every 6 to 8 hours as needed. In astronomy, right ascension — the celestial equivalent of longitude — is measured in hours, with 24 hours representing the full 360-degree circle of the sky.
Did You Know? Facts About Hour
- The 24-hour day is one of the oldest surviving systems of measurement, tracing back roughly 3,500 years to ancient Egypt. Despite multiple historical attempts to decimalize time — most famously during the French Revolution, when a 10-hour day was briefly mandated — the 24-hour hour has proven completely resistant to replacement.
- The French Revolutionary decimal time system, introduced in 1793, divided the day into 10 decimal hours, each of 100 decimal minutes, each of 100 decimal seconds. The experiment was so unpopular that it was abandoned within two years, though decimal clocks from this period survive in French museums.
- The word hour comes from the Latin hora, itself from the Greek hora, meaning season, time of day, or period. The same root gives us horoscope, literally a look at the hour of birth.
- In astronomy, one hour of right ascension corresponds to 15 degrees of arc, because the sky completes a full 360-degree rotation in 24 hours. Star catalogs and telescope pointing systems use hours, minutes, and seconds of right ascension as standard coordinates.
- The record for the longest continuously operated manned aircraft flight is 64 days, 22 hours, 19 minutes, and 5 seconds, set in 1958 by Robert Timm and John Cook over Las Vegas. The plane was refueled in the air by truck.