Noon (or midday) is 12 o'clock in the daytime. It is written as 12 noon, 12:00 m. (for meridiem, literally 12:00 noon), 12 p.m. (for

post meridiem The 12-hour clock is a time convention in which the 24 hours of the day are divided into two periods: a.m. (from Latin , translating to "before midday") and p.m. (from Latin , translating to "after midday"). For different opinions on represent ...

, literally "after noon"), 12 pm, or 12:00 (using a 24-hour clock) or 1200 (

military time The modern 24-hour clock, popularly referred to in the United States as military time, is the convention of timekeeping in which the day runs from midnight to midnight and is divided into 24 hours. This is indicated by the hours (and minutes) pass ...

). Solar noon is the

time Time is the continued sequence of existence and events that occurs in an apparently irreversible succession from the past, through the present, into the future. It is a component quantity of various measurements used to sequence events, ...

when the

Sun The Sun is the star at the center of the Solar System. It is a nearly perfect ball of hot plasma, heated to incandescence by nuclear fusion reactions in its core. The Sun radiates this energy mainly as light, ultraviolet, and infrared radi ...

appears to

contact Contact may refer to: Interaction Physical interaction * Contact (geology), a common geological feature * Contact lens or contact, a lens placed on the eye * Contact sport, a sport in which players make contact with other players or objects * ...

the local celestial

meridian Meridian or a meridian line (from Latin ''meridies'' via Old French ''meridiane'', meaning “midday”) may refer to Science * Meridian (astronomy), imaginary circle in a plane perpendicular to the planes of the celestial equator and horizon * ...

. This is when the Sun reaches its apparent highest point in the sky, at 12 noon

apparent solar time Solar time is a calculation of the passage of time based on the position of the Sun in the sky. The fundamental unit of solar time is the day, based on the synodic rotation period. Two types of solar time are apparent solar time (sundial ti ...

and can be observed using a sundial. The local or clock time of solar noon depends on the

longitude Longitude (, ) is a geographic coordinate that specifies the east– west position of a point on the surface of the Earth, or another celestial body. It is an angular measurement, usually expressed in degrees and denoted by the Greek lette ...

and date, with Daylight Savings Time tending to place solar noon closer to 1:00pm.

Etymology

The word ''noon'' is derived from Latin ''nona hora'', the ninth canonical hour of the day, in reference to the

Western Christian Western Christianity is one of two sub-divisions of Christianity (Eastern Christianity being the other). Western Christianity is composed of the Latin Church and Western Protestantism, together with their offshoots such as the Old Catholic ...

liturgical term

none None may refer to: *Zero, the mathematical concept of the quantity "none" * Empty set, the mathematical concept of the collection of things represented by "none" *''none'', an indefinite pronoun in the English language Music * ''None'' (Meshuggah ...

, one of the seven fixed prayer times in traditional Christian denominations. The Roman and Western European medieval monastic day began at 6:00 a.m. (06:00) at the

equinox A solar equinox is a moment in time when the Sun crosses the Earth's equator, which is to say, appears directly above the equator, rather than north or south of the equator. On the day of the equinox, the Sun appears to rise "due east" and se ...

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

Solar noon, also known as the local apparent solar noon and Sun transit time (informally high noon), is the moment when the Sun contacts the observer's meridian ('' culmination'' or ''meridian transit''), reaching its highest position above the horizon on that day and casting the shortest shadow. 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

es, at the

Tropic of Cancer The Tropic of Cancer, which is also referred to as the Northern Tropic, is the most northerly circle of latitude on Earth at which the Sun can be directly overhead. This occurs on the June solstice, when the Northern Hemisphere is tilted tow ...

(

latitude In geography, latitude is a coordinate that specifies the north– south position of a point on the surface of the Earth or another celestial body. Latitude is given as an angle that ranges from –90° at the south pole to 90° at the north pol ...

N) on the

June solstice The June solstice is the solstice on Earth that occurs annually between 20 and 22 June according to the Gregorian calendar. In the Northern Hemisphere, the June solstice is the summer solstice (the day with the longest period of daylight), whi ...

and at the

Tropic of Capricorn The Tropic of Capricorn (or the Southern Tropic) is the circle of latitude that contains the subsolar point at the December (or southern) solstice. It is thus the southernmost latitude where the Sun can be seen directly overhead. It also reac ...

( 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 In gravitationally bound systems, the orbital speed of an astronomical body or object (e.g. planet, moon, artificial satellite, spacecraft, or star) is the speed at which it orbits around either the barycenter or, if one body is much more mas ...

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 in each instance. This line varies year to year, since Earth's true

year A year or annus is the orbital period of a planetary body, for example, the Earth, moving in its orbit around the Sun. Due to the Earth's axial tilt, the course of a year sees the passing of the seasons, marked by change in weather, the hou ...

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). For the same reasons, solar noon and "clock noon" are usually not the same. The

equation of time In mathematics, an equation is a formula that expresses the equality of two expressions, by connecting them with the equals sign . The word ''equation'' and its cognates in other languages may have subtly different meanings; for example, in F ...

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 of

daylight saving time Daylight saving time (DST), also referred to as daylight savings time or simply daylight time (United States, Canada, and Australia), and summer time (United Kingdom, European Union, and others), is the practice of advancing clocks (typicall ...

, it can be off by more than an hour.

Nomenclature

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" based on the Latin meaning (a.m. stands for ''ante meridiem'' and p.m. for ''post 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 the ''U.S. GPO Government Style Manual'', has fallen into relative obscurity; the current edition of the GPO makes no mention of it.The 29th edition of the ''U.S. GPO Government Printing Office Style Manual (2000)'' section 12.9 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.)

Notes

References

External links