| Reference | Days (d) | Microseconds (µs) |
|---|---|---|
| A blink of an eye | 0.00000115741 d | 100000 µs |
| One heartbeat at rest | 0.00000960648 d | 830000 µs |
| Half of a football match | 0.03125 d | 2.7 × 109 µs |
| A full day | 1 d | 8.64 × 1010 µs |
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 microsecond is a unit of time equal to one millionth of a second, or one thousandth of a millisecond. Its symbol is µs, written with the Greek letter mu, and the form us appears where that character is inconvenient to type.
It sits below the threshold of human experience entirely. Nothing a person does or perceives happens on this scale, so the unit belongs wholly to instruments. Light travels about three hundred metres in a microsecond, roughly the length of three football pitches, which sets a hard floor on how fast any signal can cross a room, a circuit board or a continent.
Satellite navigation depends on measuring it accurately. A receiver works out its position from the arrival times of signals from several satellites, and an error of one microsecond in that timing translates into a position error of about three hundred metres. This is why the satellites carry atomic clocks and why the system corrects for relativistic effects: the clocks in orbit run measurably faster than clocks on the ground, by about thirty-eight microseconds a day, and without that correction navigation would drift by kilometres within hours.
Electronics works comfortably at this scale. The switching time of a power transistor, the pulse width in radar, the interval between samples in high-speed data acquisition and the response of an analogue-to-digital converter are all quoted in microseconds. A microcontroller running at one megahertz completes one instruction cycle per microsecond, which makes the unit the natural currency of embedded timing.
Physics and chemistry use it for processes that are fast but not extreme. Muons produced in the upper atmosphere have a mean lifetime of about 2.2 microseconds, and the fact that they reach the ground at all is a direct experimental demonstration of time dilation. Some fluorescence decays, gas-phase reactions and shock-wave phenomena also occupy this range.
Audio engineering encounters it in digital sampling. At the standard rate of 44,100 samples per second, one sample lasts about 22.7 microseconds, and the timing jitter that degrades converter performance is measured in fractions of that.
One microsecond equals 0.000001 seconds, 0.001 milliseconds, or 1000 nanoseconds.