| Reference | Hours (hr) | Days (d) |
|---|---|---|
| A blink of an eye | 0.0000277778 hr | 0.00000115741 d |
| One heartbeat at rest | 0.000230556 hr | 0.00000960648 d |
| Half of a football match | 0.75 hr | 0.03125 d |
| A full day | 24 hr | 1 d |
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 day is a unit of time equal to 24 hours, or 86,400 seconds. Its symbol is d. It is accepted for use with the SI without being an SI unit, and it is the only common time unit that corresponds directly to a physical cycle people can observe without instruments.
There is more than one kind of day, and the difference matters in astronomy. The solar day is the time between successive noons, when the Sun returns to the same position in the sky. The sidereal day is the time the Earth takes to rotate once relative to the distant stars, and it is shorter by about three minutes and fifty-six seconds. The gap exists because the Earth also moves along its orbit, so it must turn slightly more than a full rotation to bring the Sun back overhead.
The solar day is not constant either. The Earth's orbit is elliptical and its axis is tilted, so the interval between noons varies through the year by up to about thirty seconds either way. The mean solar day averages this out, and the equation of time describes the difference, which is why a sundial can run a quarter of an hour ahead of or behind a clock depending on the season.
The civil day of exactly 86,400 seconds is a convention rather than a measurement. The Earth's rotation is gradually slowing through tidal friction with the Moon, at a rate that lengthens the day by roughly two milliseconds per century, and it also varies irregularly with atmospheric and core motions. Leap seconds have been inserted since 1972 to keep clock time within a second of solar time, and the practice is due to be discontinued by 2035 in favour of allowing a larger drift.
Beyond timekeeping the unit organises everything periodic in ordinary life: employment and rental contracts, interest calculations, drug dosing schedules, weather statistics, shipping and delivery estimates, and the whole apparatus of calendars. Biology has its own version in the circadian rhythm, an internal cycle of very nearly twenty-four hours that persists even in the absence of daylight cues.
Astronomy uses the Julian day, a continuous count of days since a fixed epoch in 4713 BC, precisely because calendars are irregular and a simple running total is easier to compute with.
One day equals 24 hours, 1440 minutes, or 86,400 seconds.