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Noon (also midday or noon time) is usually defined as 12 o'clock in the daytime. The term midday is also used colloquially to refer to a range of time, often 11 a.m. to 1 p.m. Solar noon is 12 o'clock apparent solar time, or around 12 – 1 p.m. local time, depending on daylight saving time and the moment when the sun crosses the meridian. Solar noon is when the sun is at its highest elevation in the sky. The 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 and Ukrainian go a step farther, with the words for noon (południe, південь – literally "half-day") also meaning "south" and the words for "midnight" (północ, північ – literally "half-night", as with English mid(dle) meaning "half") also meaning "north".
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.
Solar noon is the moment when the sun transits the celestial meridian – roughly the time when it is highest 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′ 16″ N) on the June solstice, and at the Tropic of Capricorn (23° 26′ 16″ S) on the December solstice.
|This section possibly contains original research. (June 2011)|
With 12-hour time notation, most authorities[who?] recommend avoiding confusion by using "noon", "12 noon", or "12:00 noon".
Digital clocks and computers commonly display 12 p.m. for noon. While that phrase may be used practically, it helps to understand that any particular time is actually an instant. The "p.m." shown on clock displays refers to the 12-hour period following the instant of noon, not to the instant itself.
While computers and digital clocks display "12:00 a.m." and "12:00 p.m.", these notations provide no clear and unambiguous way to distinguish between midnight and noon. It is technically improper to use "a.m." and "p.m." when referring to 12:00. The abbreviation a.m. stands for the Latin ante meridiem ("before the meridian") and p.m. stands for post meridiem ("after the meridian"), with the meridian ("middle of day") being 12:00 noon. For this reason, neither abbreviation is correct for noon or midnight. The length of the error is determined by the smallest unit of time: 12:00:01 p.m. would be correctly noted, as would even 12:00:00.00001 p.m. The antiquated use of 12m or 12M for 12noon is rarely seen today, where the "M" or "m" were abbreviations of the Latin "meridies."
The most common ways to represent these times are:
Earlier editions of the U.S. GPO Government Style Manual correctly specified 12:00 M for noon, being the M in am and pm, with PM being midnight, because midnight is the twelfth hour after noon, the very end of the day, before the next day starts.