Hour
Non-SI unit accepted for use · quantity: time
For two thousand years the hour was a floating measure. An Egyptian scribe divided the daylight into twelve parts, and in July each of those parts came out longer than in December — the day itself was longer. Nobody found this awkward: the hour was a share of the day, not a fixed length.
01 · Definition
One hour is 3600 seconds exactly, that is sixty minutes. The unit is not part of the SI but is accepted for use with it: no timetable, no labour code and no tariff can do without it.
The twenty-four parts of the day are an Egyptian inheritance: ten parts of daylight, two of twilight and twelve of night by the decans. The twelve came not from convenience of counting but from observation: over the course of a night twelve reference star groups rose above the horizon, and the night was marked out by them just as the day was marked out by the gnomon’s shadow.
That system lived naturally as long as time was told by the sun and by water: the dial was simply marked for the season and the clepsydra filled differently. The mechanical tower clocks of the 14th century could not do this — the gears ran at a constant rate — and towns adopted equal hours because that is how the mechanism worked. Time stopped being a share of the day and became a convention.
Five hundred years later the same logic produced time zones: a railway cannot live by the local noon of every station. Today the hour is counted from UTC, the scale of the atomic clocks, and is entirely cut off from the Earth’s rotation — days lengthen and shorten by milliseconds, while the hour stays exactly three thousand six hundred seconds.
Set the latitude and the day of the year: the length of daylight is worked out from the Sun’s declination, and then day and night are each divided into twelve parts, as was done before mechanical clocks. On the left is the Sun’s arc above the horizon and the gnomon’s shadow; on the right two rows of twelve divisions — the hours of day against the hours of night.
The Earth turns fifteen degrees an hour, so longitude translates straight into time. The problem was solved not by geometry but by clockmaking: to find longitude at sea one had to carry the time of the home port, and a marine chronometer accurate to seconds was needed for that.
02 · Conversion
Enter a time — the sheet will convert it
The hour is convenient as a base for compound units: the kilowatt-hour counts energy, the man-hour counts labour, the kilometre per hour counts speed. In each case the number comes out of a size a person can hold in mind, which is exactly why the hour survived the metric reform.
The year in the table is taken as a mean one — 365.25 days, 8766 hours. A common year has 8760, a leap year 8784, and this difference is exactly why calendar periods are not reduced to the second.
The kilowatt-hour is the best-known compound unit with the hour in it: three million six hundred thousand joules. The meter in the hallway counts them because a joule is too small for domestic scale — an ordinary flat gets through some three hundred kilowatt-hours a month, which in joules would be a number with eleven digits.
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03 · Orders of magnitude
from a minute to a working lifetime04 · Measuring instruments
What hours were measured with
The gnomon’s shadow shows local solar time. The divisions on the plate are uneven, and with temporal hours one and the same plate was good only for its own season — for another one it had to be re-marked.
The clepsydra worked at night and in cloudy weather. The Egyptian vessel was made conical so that the level fell evenly as the water ran out, and the divisions on the wall were still marked for the season: twelve parts of the night, whatever its length.
A weight, a verge escapement and a bar with two small masses swinging back and forth. The accuracy was hopeless — up to a quarter of an hour a day — but the swing did not depend on the season, and that was enough for the hour to become equal.
Today the hour is counted from Coordinated Universal Time, kept by hundreds of atomic clocks around the world. The hour itself has no standard: the second does, and the hour is obtained by multiplying by three thousand six hundred.
05 · Writing rules
h in lower case, kW·h with a middle dot
A capital H means the henry. Compound units are written with a multiplication sign: kW·h, not kWh and not “kwh”. Between the number and the symbol there is a space, and there is no full stop after the symbol.
There are thirty-eight time zones on the planet, not twenty-four: Nepal is offset by 5:45, India and Iran by half an hour, and the Chatham Islands by 12:45. The reason is not astronomy but politics — a country chooses its offset itself.
06 · Neighbouring units
The hour stands above the minute and below the day, and the whole trio rests on factors of sixty and twenty-four. Compound units — kW·h, man·h, km/h — keep the hour in modern engineering far more firmly than any timetable does.
1/60 of an hour
3600 s
3.6 MJ
The sidereal hour is ten seconds shorter than the ordinary one, because the sidereal day is four minutes shorter than the solar day: over a year the Earth makes one extra turn relative to the stars.
07 · Historical section
Hours that came in different lengths
Daylight was divided into twelve parts in any season. In Cairo a summer hour lasted seventy minutes, a winter one about fifty, and this was not considered an error — the hour was meant to be a share of the day.
Four hours is a shift aboard ship, six watches to the day. Bells were struck every half hour by the sandglass, and eight bells meant the watch was over. The system outlived the sailing ship and survives in naval routine to this day.
A tenth of the day, a hundred and forty-four ordinary minutes. The one part of the metric reform that did not take root anywhere: clockmakers were making dials with two rings, and the public went on reading the ordinary one.
An hour of the sidereal day, ten seconds shorter than the ordinary one. Observatories kept separate clocks on this scale: a star returns to the meridian by sidereal time, not by solar.
The hour became constant not because of science but because of a toothed wheel
Before the 14th century the hour was a share of daylight, and its length floated with the season: in middle latitudes a summer hour ran to seventy-five minutes and a winter one to forty-five. A sundial coped with this easily — the plate was simply marked for the season.
Towns adopted equal hours simply because that is how the mechanism worked, and time turned from a natural quantity into an agreed one. Nobody debated it: the tower clock struck, and the town lived by its striking.
Catalogue · units of measurement
A passport for every quantity
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