Noon (also known as noontime or midday) is 12 o'clock in the

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

. It is written as 12 noon, 12:00 m. (for '' meridiem'', literally 12:00 midday), 12 p.m. (for ''post meridiem'', literally "after midday"), 12 pm, or 12:00 (using a 24-hour clock) or 1200 ( military time). Solar noon is the

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

when the Sun appears to contact the local celestial meridian. This is when the Sun reaches its apparent highest point in the sky, at 12 noon apparent solar time and can be observed using a

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

. The local or clock time of solar noon depends on the date,

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 lett ...

, and

time zone A time zone is an area which observes a uniform standard time for legal, Commerce, commercial and social purposes. Time zones tend to follow the boundaries between Country, countries and their Administrative division, subdivisions instead of ...

, with

Daylight Saving Time Daylight saving time (DST), also referred to as daylight savings time, daylight time (Daylight saving time in the United States, United States and Daylight saving time in Canada, Canada), or summer time (British Summer Time, United Kingdom, ...

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 prayer times#Christianity, fixed times of prayer at regular intervals. A book of hours, chiefly a breviary, normally contains a version of, or sel ...

of the day, in reference to the Western Christian liturgical term Nones (liturgy), (number nine), one of the seven fixed prayer times in traditional

Christian denomination A Christian denomination is a distinct Religion, religious body within Christianity that comprises all Church (congregation), church congregations of the same kind, identifiable by traits such as a name, particular history, organization, leadersh ...

s. 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 appears directly above the equator, rather than to its north or south. On the day of the equinox, the Sun appears to rise directly east and set directly west. This occurs twice each year, arou ...

by modern timekeeping, so the ninth hour ended 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 timethat 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 are also known as meridian tran ...

'' 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 the circle of latitude that divides Earth into the Northern Hemisphere, Northern and Southern Hemisphere, Southern Hemispheres of Earth, hemispheres. It is an imaginary line located at 0 degrees latitude, about in circumferen ...

on the

es, at the

Tropic of Cancer The Tropic of Cancer, also known as the Northern Tropic, is the Earth's northernmost circle of latitude where the Sun can be seen directly overhead. This occurs on the June solstice, when the Northern Hemisphere is tilted toward the Sun ...

(

latitude In geography, latitude is a geographic coordinate system, geographic 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 t ...

N) on the June solstice 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 reach ...

( S) on the December solstice. 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 by humans as being in the same celestial sphere, celestial hemisphere relative to the invariable plane of the Solar ...

, 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. When the Sun contacts the observer's meridian at the observer's

zenith The zenith (, ) is the imaginary point on the celestial sphere directly "above" a particular location. "Above" means in the vertical direction (Vertical and horizontal, plumb line) opposite to the gravity direction at that location (nadir). The z ...

, it is perceived to be directly overhead and no shadows are cast. This occurs at Earth's subsolar point, a point which moves around the

tropics The tropics are the regions of Earth surrounding the equator, where the sun may shine directly overhead. This contrasts with the temperate or polar regions of Earth, where the Sun can never be directly overhead. This is because of Earth's ax ...

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'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 (the combined center of mass) or ...

around the Sun offset each other. These four days for the current 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 true

year A year is a unit of time based on how long it takes the Earth to orbit the Sun. In scientific use, the tropical year (approximately 365 Synodic day, solar days, 5 hours, 48 minutes, 45 seconds) and the sidereal year (about 20 minutes longer) ...

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 astronomical unit, average distance of , or 8.317 light-second, light-minutes, in a retrograde and prograde motion, counterclockwise direction as viewed from above the Northern Hemisphere. One complete orbit takes & ...

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

day A day is the time rotation period, period of a full Earth's rotation, rotation of the Earth with respect to the Sun. On average, this is 24 hours (86,400 seconds). As a day passes at a given location it experiences morning, afternoon, evening, ...

s occur in both hemispheres simultaneously. The precise

Coordinated Universal Time Coordinated Universal Time (UTC) is the primary time standard globally used to regulate clocks and time. It establishes a reference for the current time, forming the basis for civil time and time zones. UTC facilitates international communicat ...

s 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. Discussion Any arc of a great circle is a geodesic of the sphere, so that great circles in spher ...

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 The equation of time describes the discrepancy between two kinds of solar time. The two times that differ are the apparent solar time, which directly tracks the diurnal motion of the Sun, and mean solar time, which tracks a theoretical mean Sun ...

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, daylight time (Daylight saving time in the United States, United States and Daylight saving time in Canada, Canada), or summer time (British Summer Time, United Kingdom, ...

, 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