| Reference | Hours (hr) | Minutes (min) |
|---|---|---|
| A blink of an eye | 0.0000277778 hr | 0.00166667 min |
| One heartbeat at rest | 0.000230556 hr | 0.0138333 min |
| Half of a football match | 0.75 hr | 45 min |
| A full day | 24 hr | 1440 min |
The hour is a unit of time equal to 60 minutes, or 3600 seconds. Its symbol is h, though hr is common in English-language informal use. Twenty-four hours make a day, and the unit is accepted for use with the SI without belonging to it.
Its length was once variable. Ancient Egyptian and later Greco-Roman practice divided daylight into twelve parts and darkness into twelve more, which meant an hour of daylight in summer was considerably longer than one in winter. These seasonal or temporal hours persisted in Europe into the medieval period and were still used in monastic timekeeping long after mechanical clocks appeared. Equal hours won out because a clock cannot easily be made to run at a rate that changes with the season.
Twenty-four is the count because twelve is the count. Egyptian astronomers used a set of thirty-six decan stars, of which twelve were visible across a night, and twelve was already a convenient number in a duodecimal counting tradition that divides cleanly by two, three, four and six. Doubling it to cover the day gave twenty-four, and no serious attempt to change it has succeeded since.
The hour organises modern working life more than any other unit. Wages, shifts, opening times, flight durations, parking charges and legal limits on driving are all expressed in hours, and the hour is the base of the kilowatt-hour, which is how virtually all electricity is billed. Timetables are built on hour boundaries, and time zones are defined as whole-hour offsets from Coordinated Universal Time, with a handful of exceptions offset by thirty or forty-five minutes.
In science the hour appears less often than the second but survives wherever a process runs long enough to make seconds unwieldy: reaction times in chemistry, dosing intervals in medicine, and half-lives in the range where hours are the natural scale.
Geography is divided by it. The Earth turns fifteen degrees of longitude in an hour, so the globe was cut into twenty-four zones an hour apart, a scheme adopted at the International Meridian Conference of 1884 after railways made local solar time unworkable. The tidiness is partial: several countries keep offsets of half an hour or forty-five minutes, and China spans five geographic zones on a single one. The hour also lends its name to a unit of energy. A kilowatt-hour is the energy delivered by a kilowatt for an hour, 3.6 megajoules, and it is the quantity on every electricity bill in the world.
One hour equals 60 minutes, 3600 seconds, or about 0.0416667 days.
The minute is a unit of time equal to 60 seconds, or one sixtieth of an hour. Its symbol is min. It is not an SI unit but is accepted for use with the SI, and it is one of the very few sexagesimal survivals in ordinary modern life.
The division of the hour into sixty parts comes from Babylonian astronomy by way of Greek and Islamic scholarship. Sixty is a highly composite number, divisible by 1, 2, 3, 4, 5, 6, 10, 12, 15, 20, 30 and 60, which means halves, thirds, quarters, fifths and sixths of an hour are all whole numbers of minutes. A decimal hour of a hundred parts would give clean halves, quarters and fifths but no exact third, and a third of an hour is a quantity people actually use.
For centuries the minute was a theoretical division rather than a measured one, because no clock could show it. Mechanical clocks with only an hour hand were normal until the seventeenth century, and it was the pendulum, developed by Christiaan Huygens from Galileo's observation of isochronism, that first made minute-level accuracy ordinary. The minute hand followed the pendulum, not the other way round.
The French Republic tried to abolish it. The decimal time introduced in 1794 divided the day into ten hours of a hundred minutes, each of a hundred seconds. It was mandatory for less than two years, failed because every clock in the country would have needed replacing, and remains the clearest demonstration that metric reform succeeds only where the existing units are genuinely inconvenient.
Modern use is universal and unremarkable. Appointments, cooking times, journey durations, sports periods and billing increments are all in minutes, and the unit sits at a natural human scale: long enough to be a meaningful span of activity, short enough to be planned around without further subdivision.
Its sixtieth-part logic is shared with the circle, and the two meet in navigation. A minute of arc of latitude is one nautical mile, and a ship covering one nautical mile in one minute of time is travelling at sixty knots, which is why an old rule of thumb converts between distance and time on a chart without arithmetic. Music divides time the same way. Tempo is given in beats per minute, from a funeral march near 60 to dance music at 120 or more, and the metronome that Maelzel patented in 1815 fixed that convention in print. In both fields the minute is short enough to feel and long enough to count.
One minute equals 60 seconds, one sixtieth of an hour, or about 0.000694444 days.