Noon (or midday) is 12

o'clock 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 ...

in the

daytime Daytime as observed on Earth is the period of the day during which a given location experiences natural illumination from direct sunlight. Daytime occurs when the Sun appears above the local horizon, that is, anywhere on the globe's hemis ...

. It is written as 12 noon, 12:00 m. (for meridiem, literally 12:00 noon), 12 p.m. (for post meridiem, literally "after noon"), 12 pm, or 12:00 (using a 24-hour clock) or 1200 ( military time). Solar noon is the time when the Sun 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 A list of highest points typically contains the name, elevation, and location of the highest point in each of a set of geographical regions. Such a list is important in the sport of highpointing. A partial list of highpoint lists is below: World ...

in the sky, at 12 noon apparent solar time and can be observed using a

sundial A sundial is a horological device that tells the time of day (referred to as civil time in modern usage) when direct sunlight shines by the apparent position of the Sun in the sky. In the narrowest sense of the word, it consists of a flat ...

. The local or clock time of solar noon depends on the longitude 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 In the practice of Christianity, canonical hours mark the divisions of the day in terms of fixed times of prayer at regular intervals. A book of hours, chiefly a breviary, normally contains a version of, or selection from, such prayers. In t ...

of the day, in reference to the Western Christian 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 E ...

, one of the seven fixed prayer times in traditional

Christian denomination A Christian denomination is a distinct religious body within Christianity that comprises all church congregations of the same kind, identifiable by traits such as a name, particular history, organization, leadership, theological doctrine, worsh ...

s. 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

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 In observational astronomy, culmination is the passage of a celestial object (such as the Sun, the Moon, a planet, a star, constellation or a deep-sky object) across the observer's local meridian. These events were also known as meridian transits ...

'' 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 The equator is a circle of latitude, about in circumference, that divides Earth into the Northern and Southern hemispheres. It is an imaginary line located at 0 degrees latitude, halfway between the North and South poles. The term can als ...

on the equinoxes, at the Tropic of Cancer ( latitude N) on the June solstice and at the Tropic of Capricorn ( S) on the

December solstice The December solstice, also known as the southern solstice, is the solstice that occurs each December – typically on 21 December, but may vary by one day in either direction according to the Gregorian calendar. In the Northern Hemisphere, the ...

. In the

Northern Hemisphere The Northern Hemisphere is the half of Earth that is north of the Equator. For other planets in the Solar System, north is defined as being in the same celestial hemisphere relative to the invariable plane of the solar system as Earth's Nort ...

, 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 in each instance. 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 Earth orbits the Sun at an average distance of 149.60 million km (92.96 million mi) in a counterclockwise direction as viewed from above the Northern Hemisphere. One complete orbit takes days (1 sidereal year), during which time Earth ...

being gravitationally perturbed by the planets. These four 24-hour

day A day is the time period of a full rotation of the Earth with respect to the Sun. On average, this is 24 hours, 1440 minutes, or 86,400 seconds. In everyday life, the word "day" often refers to a solar day, which is the length between two so ...

s 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 Midnight is the transition time from one day to the next – the moment when the date changes, on the local official clock time for any particular jurisdiction. By clock time, midnight is the opposite of noon, differing from it by 12 hours. ...

to local midnight the next day. Thus, four varying

great circle In mathematics, a great circle or orthodrome is the circular intersection of a sphere and a plane passing through the sphere's center point. Any arc of a great circle is a geodesic of the sphere, so that great circles in spherical geomet ...

s 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 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, 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