The wordnoon is derived from Latinnona hora, the ninthcanonical hour of the day, in reference to theWestern Christian liturgical term Nones, (number nine), one of theseven fixed prayer times in traditionalChristian denominations. TheRoman and Western European medieval monastic day began at 6:00 a.m. (06:00) at theequinox by modern timekeeping, so the ninth hour ended at what is now 3:00 p.m. (15:00) at the equinox.[2] In English, the meaning of the word shifted[why?] tomidday 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.[3]
Solar noon, also known as thelocal apparent solar noon andSun transit time (informallyhigh noon),[4] is the moment when the Sun contacts the observer's meridian (culmination ormeridian transit), reaching its highest position above the horizon on that day and casting the shortest shadow. This is also the origin of the termsante meridiem (a.m.) andpost meridiem (p.m.), as noted below. The Sun is directly overhead at solar noon at theEquator on theequinoxes, at theTropic of Cancer (latitude 23°26′09.3″ N) on theJune solstice and at theTropic of Capricorn (23°26′09.3″ S) on theDecember solstice. In theNorthern Hemisphere, north of the Tropic of Cancer, the Sun is due south of the observer at solar noon; in theSouthern Hemisphere, south of the Tropic of Capricorn, it is due north.
When the Sun contacts the observer's meridian at the observer'szenith, it is perceived to be directly overhead and noshadows are cast. This occurs at Earth'ssubsolar point, a point which moves around thetropics throughout the year.
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'sobliquity of ecliptic and itsorbital speed around the Sun offset each other. These four days for thecurrent epoch are centered on 11 February, 13 May, 26 July, and 3 November. It occurs at only one particular line of longitude in each instance. This line varies year to year, since Earth's trueyear is not an integer number of days. This event time and location also varies due toEarth's orbit being gravitationally perturbed by the planets. These four 24-hourdays occur in both hemispheres simultaneously. The preciseCoordinated Universal Times for these four days also mark when the opposite line of longitude, 180° away, experiences precisely 24 hours from localmidnight to local midnight the next day. Thus, four varyinggreat 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).
For the same reasons, solar noon and "clock noon" are usually not the same. Theequation of time shows that the reading of a clock at solar noon will be higher or lower than 12:00 by as much as 16 minutes. Additionally, due to the political nature of time zones, as well as the application ofdaylight saving time, it can be off by more than an hour.
In the US, noon is commonly indicated by 12 p.m., and midnight by 12 a.m. While some argue that such usage is "improper"[5] based on the Latin meaning (a.m. stands forante meridiem and p.m. forpost meridiem, meaning "before midday" and "after midday" respectively), digital clocks are unable to display anything else, and an arbitrary decision must be made. An earlier standard of indicating noon as "12M" or "12m" (for "meridies"), which was specified in theU.S. GPO Government Style Manual,[6] has fallen into relative obscurity; the current edition of the GPO makes no mention of it.[7][8][nb 1] However, due to the lack of an international standard, the use of "12 a.m." and "12 p.m." can be confusing. 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.)
