Noon (also midday or noon time) is 12 o'clock in the daytime, as opposed to midnight. The term 12 p.m. (often written as 12pm) is sometimes used for noon but this may be considered to be ambiguous and, therefore, technically incorrect.
Solar noon is when the Sun transits the local celestial meridian; this is when the sun is at its highest altitude in the sky, at 12 noon apparent solar time. The local or clock time of solar noon depends on the longitude and date.
In many cultures in the Northern Hemisphere, noon had ancient geographic associations with the direction "south" (as did midnight with "north" in some cultures). Remnants of the noon = south association are preserved in the words for noon in French (Midi) and Italian (Mezzogiorno), both of which also refer to the southern parts of the respective countries. Modern Polish, Belarusian, Ukrainian, and Serbian go a step farther, with the words for noon (po?udnie, ????????, ???????, ?o??e - literally "half-day") also meaning "south" and the words for "midnight" (pó?noc, ??????, ??????, ?o?o? - literally "half-night", as with English mid(dle) meaning "half") also meaning "north".
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Etymology
The word noon is derived from Latin nona hora, the ninth hour of the day, and is related to the liturgical term none. The Roman and Western European medieval monastic day began at 6:00 a.m. (06:00) at the equinox by modern timekeeping, so the ninth hour started at what is now 3:00 p.m. (15:00) at the equinox. In English, the meaning of the word shifted to midday and the time gradually moved back to 12:00 local time (that is, not taking into account the modern invention of time zones). The change began in the 12th century and was fixed by the 14th century.
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Solar noon
Solar noon (informally high noon) is the moment when the Sun contacts the observer's meridian, reaching its highest position above the horizon on that day ("Sun transit time"). This is also the origin of the terms ante meridiem (a.m.) and post meridiem (p.m.), as noted below. The Sun is directly overhead at solar noon at the Equator on the equinoxes, at the Tropic of Cancer (latitude 23°26?13.1? N) on the June solstice and at the Tropic of Capricorn (23°26?13.1? S) on the December solstice. In the Northern Hemisphere, north of the Tropic of Cancer, the Sun is due south of the observer at solar noon; in the Southern Hemisphere, south of the Tropic of Capricorn, it is due north.
The elapsed time from the local solar noon of one day to the next is exactly 24 hours on only four instances in any given year. This occurs when the effects of Earth's obliquity of ecliptic and its orbital speed around the Sun offset each other. These four days for the current epoch are centered on 11 February, 13 May, 25 July, and 3 November.
It occurs at only one particular line of longitude each event. This line varies year to year, since Earth's true year is not an integer number of days. This event time and location also varies due to Earth's orbit being gravitationally perturbed by the planets.
These four 24-hour days occur in both hemispheres simultaneously. The precise UTC times for these four days also mark when the opposite line of longitude, 180° away, experiences precisely 24 hours from local midnight to local midnight the next day. Thus, four varying great circles of longitude define from year to year when a 24-hour day (noon to noon or midnight to midnight) occurs.
The two longest time spans from noon to noon occur twice each year, around 20 June (24 hours plus 13 seconds) and 21 December (24 hours plus 30 seconds).
The shortest time spans occur twice each year, around 25 March (24 hours minus 18 seconds) and 13 September (24 hours minus 22 seconds).
Nomenclature
Noon is commonly indicated by 12 p.m., and midnight by 12 a.m. While some argue that such usage is "improper" based on the original Latin meanings (a.m. stood for ante meridiem and p.m. for post meridiem, meaning "before midday" and "after midday" respectively), the usage of 12 p.m. to refer to noon is conventional. An earlier standard of indicating noon as "12M" or "12m" (for "meridies"), which was at one time specified in the U.S. GPO Government Style Manual, has fallen into relative obscurity; the current edition of the GPO makes no mention of it.
However, some still consider the use of 12 a.m. and 12 p.m. confusing and seek a less ambiguous phrasing. Common alternative methods of representing these times are:
- to use a 24-hour clock (00:00 and 12:00, 24:00; but never 24:01)
- to use "12 noon" or "12 midnight" (though "12 midnight" may still present ambiguity regarding the specific date)
- to specify midnight as between two successive days or dates (as in "midnight Saturday/Sunday" or "midnight December 14/15")
- to avoid those specific times and to use "11:59 p.m." or "12:01 a.m." instead. (This is common in the travel industry to avoid confusion to passengers' schedules, especially train and plane schedules.)
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