Reckoning / Sky & sea / Kinds of time
Sky & sea · time
Kinds of time
“Noon” used to mean the moment the Sun crossed your meridian. That moment drifts through the year, differs from town to town, and is not the moment a star returns to the same place. Each of those facts gave navigators and astronomers a different clock.
Now, at longitude
Device clock: . Formulas: NOAA solar calculator (Sun) and USNO sidereal time (sidereal time).
The equation of time
A sundial and a clock agree on only four days a year. The difference, the equation of time, comes from two effects: the Earth moves faster near perihelion in early January, and the Sun’s path is tilted to the equator, so equal steps along it are not equal steps of right ascension.
Plot the Sun’s declination against the equation of time and you get the analemma, the figure eight traced by the Sun if you photograph it at the same clock time every few days for a year. Its top and bottom are the solstices; its width is the equation of time.
The build checks this code against the published worked examples for 1992 October 13 (equation of time +13 m 42.7 s) and finds this year’s extremes, about −14.2 min in February and +16.4 min in early November, with four zero crossings.
Time zones at sea
Ships keep zone time: the ocean is divided into 15° strips centred on multiples of 15°, each one hour from the next. The zone description is the number of hours you add to zone time to get UTC, so it is negative east of Greenwich. Each zone also has a letter; J is not used, and Z, for Greenwich, gave “Zulu time”.
| Letter | Zone description | Central meridian | Strip |
|---|---|---|---|
| M | −12 | 180°E | 172.5°E – 180° |
| L | −11 | 165°E | 157.5°E – 172.5°E |
| K | −10 | 150°E | 142.5°E – 157.5°E |
| I | −9 | 135°E | 127.5°E – 142.5°E |
| H | −8 | 120°E | 112.5°E – 127.5°E |
| G | −7 | 105°E | 97.5°E – 112.5°E |
| F | −6 | 90°E | 82.5°E – 97.5°E |
| E | −5 | 75°E | 67.5°E – 82.5°E |
| D | −4 | 60°E | 52.5°E – 67.5°E |
| C | −3 | 45°E | 37.5°E – 52.5°E |
| B | −2 | 30°E | 22.5°E – 37.5°E |
| A | −1 | 15°E | 7.5°E – 22.5°E |
| Z | 0 | 0° | 7.5°W – 7.5°E |
| N | +1 | 15°W | 7.5°W – 22.5°W |
| O | +2 | 30°W | 22.5°W – 37.5°W |
| P | +3 | 45°W | 37.5°W – 52.5°W |
| Q | +4 | 60°W | 52.5°W – 67.5°W |
| R | +5 | 75°W | 67.5°W – 82.5°W |
| S | +6 | 90°W | 82.5°W – 97.5°W |
| T | +7 | 105°W | 97.5°W – 112.5°W |
| U | +8 | 120°W | 112.5°W – 127.5°W |
| V | +9 | 135°W | 127.5°W – 142.5°W |
| W | +10 | 150°W | 142.5°W – 157.5°W |
| X | +11 | 165°W | 157.5°W – 172.5°W |
| Y | +12 | 180°W | 180° – 172.5°W |
Zone 12 is split by the date line: the half west of 180° (−12, M) is a day ahead of the half east of it (+12, Y). Land time zones follow borders and politics instead.
Which clock is which
| Time | Kept by | Day length |
|---|---|---|
| Apparent solar time | the real Sun; a sundial. Noon is when the Sun crosses your meridian. | differs from 24 h by up to about half a minute |
| Local mean time | an imaginary Sun moving evenly along the equator. Apparent time = mean time + equation of time. | 24 h exactly |
| Zone time | local mean time of the zone’s central meridian. | 24 h |
| UTC | atomic clocks, kept within 0.9 s of the Earth’s rotation (UT1) by leap seconds. | 86 400 SI seconds (86 401 with a leap second) |
| Sidereal time | the stars: the hour angle of the March equinox. A star rises 3 min 56 s earlier each day by the clock. | 23 h 56 min 4.09 s |
Until 1925 astronomers began the day at noon, so that a night’s observations kept one date. Greenwich Mean Time was the world’s reference until UTC took over; leap seconds, added since 1972, are to stop by 2035 (CGPM 2022, Res. 4), when UTC will be allowed to drift further from the Earth’s rotation.