| Reference | Days (d) | Minutes (min) |
|---|---|---|
| A blink of an eye | 0.00000115741 d | 0.00166667 min |
| One heartbeat at rest | 0.00000960648 d | 0.0138333 min |
| Half of a football match | 0.03125 d | 45 min |
| A full day | 1 d | 1440 min |
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.
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.