Noon

Noon is commonly understood as 12 o'clock in the daytime. However, there is a scientific distinction between clock time and solar noon. Solar noon, also known as local apparent solar noon or Sun transit time, is a specific astronomical moment. It occurs when the Sun contacts the local celestial meridian. This is the point when the Sun reaches its highest apparent position in the sky for that day. At this exact moment, the Sun casts the shortest shadow of the day. This astronomical event is the basis for our terms ante meridiem (a.m.) and post meridiem (p.m.).

The mechanism of solar noon depends on the observer's position on Earth. When the Sun contacts the observer's meridian, it is said to culminate. At the subsolar point, the Sun is at the observer's zenith, meaning it is directly overhead. At this position, no shadows are cast at all. The subsolar point moves around the tropics throughout the year. At the Equator, the Sun is directly overhead during the equinoxes. At the Tropic of Cancer, the Sun is overhead during the June solstice. At the Tropic of Capricorn, this occurs during the December solstice.

An observer's hemisphere also determines the Sun's direction at solar noon. In the Northern Hemisphere, north of the Tropic of Cancer, the Sun is due south. In the Southern Hemisphere, south of the Tropic of Capricorn, the Sun is due north. The timing of solar noon is not constant. The elapsed time from one solar noon to the next is exactly 24 hours on only four instances each year. These days occur when the Earth's orbital speed and its obliquity of ecliptic offset each other. For the current epoch, these four days center on 11 February, 13 May, 26 July, and 3 November.

These four 24-hour days occur in both hemispheres at the same time. They happen at only one specific line of longitude during each instance. This line changes every year because the Earth's true year is not an integer number of days. The timing and location also shift because Earth's orbit is gravitationally perturbed by other planets. The precise Coordinated Universal Times for these days also mark when the opposite longitude, 180 degrees away, experiences exactly 24 hours from midnight to midnight. These four varying great circles of longitude define when a 24-hour day occurs.

The history of the word "noon" reveals a shift in how humans track time. The term is derived from the Latin "nona hora," meaning the ninth hour. This refers to the Western Christian liturgical term Nones. In medieval monastic traditions, the day began at 6:00 a.m. at the equinox. Under that system, the ninth hour ended at 3:00 p.m. The meaning of the word began to shift toward midday in the 12th century. This change was fully fixed by the 14th century.

Solar noon and clock noon are rarely the same due to several factors. The equation of time shows that a clock tracking mean solar time can differ from solar noon by as much as 16 minutes. Furthermore, political decisions regarding time zones can create large gaps. Daylight Saving Time often places solar noon closer to 1:00 p.m. The length of the solar day also fluctuates. The two longest solar days occur around 20 June and 21 December. These last 24 hours plus 13 seconds and 24 hours plus 30 seconds, respectively. The shortest spans occur around 25 March and 13 September. These are 24 hours minus 18 seconds and 24 hours minus 22 seconds.

Because of these complexities, different naming conventions exist to avoid confusion. In the United States, noon is commonly labeled 12 p.m. Some argue this is improper because p.m. means "after midday." An older standard used "12M" or "12m" for "meridies," but this is now obscure. To ensure clarity, many people use a 24-hour clock, marking noon as 12:00. The travel industry often uses 11:59 p.m. or 12:01 a.m. to avoid scheduling errors. These methods help people navigate the difference between the Sun's movement and our artificial clocks.