| Reference | Days (d) | Deciseconds (ds) |
|---|---|---|
| A blink of an eye | 0.00000115741 d | 1 ds |
| One heartbeat at rest | 0.00000960648 d | 8.3 ds |
| Half of a football match | 0.03125 d | 27000 ds |
| A full day | 1 d | 864000 ds |
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 decisecond is a unit of time equal to one tenth of a second. Its symbol is ds. Like the centisecond it is a name that is almost never used, while the quantity it names turns up constantly under other descriptions.
A tenth of a second is roughly the shortest interval a person can consciously register as a duration rather than an instant. Voluntary reaction to a sound takes about one and a half deciseconds, and to a light rather longer. The blink of an eye lasts between one and four deciseconds. These figures set the boundary between what feels immediate and what feels delayed, and interface designers work to keep responses inside it.
Sport uses the quantity where the hundredth would be false precision. Long-distance running, road cycling, cross-country skiing and marathon swimming publish results to the tenth, because over an hour or more the differences between competitors are large and the measurement uncertainties from course length and timing points make finer figures unjustifiable. Field events measured in distance rather than time follow a parallel logic with the centimetre.
Speech occupies the same scale. An average spoken syllable lasts around two deciseconds, and the pause that listeners hear as the end of a sentence is roughly five. Speech recognition systems segment audio into frames of about one tenth to one quarter of a decisecond, and the natural rhythm they are trying to capture sits an order of magnitude above that.
Engineering meets the tenth of a second in relay timing, in the response of mechanical safety interlocks, and in the specification of how quickly a residual current device must disconnect a faulty circuit. Music sits here too: a metronome at one hundred and twenty beats per minute puts five deciseconds between beats.
The deci prefix is among the least used in the SI. It survives in the decibel and the decilitre and almost nowhere else, so a tenth of a second is nearly always written and spoken as exactly that.
Photography meets the same interval from the other side. A shutter open for a tenth of a second gathers plenty of light but records any camera shake as blur, which is why the old rule was never to hand-hold a lens slower than the reciprocal of its focal length. Cinema and video sit just below: at twenty-four frames a second a tenth of a second is a little over two frames, and at fifty it is five, so a tenth is the shortest event an ordinary recording can show as motion rather than as a single frame. Instrument logging often samples at this rate for the same reason, fast enough to catch a change and slow enough to keep the file small.
One decisecond equals 0.1 seconds, 10 centiseconds, or 100 milliseconds.