Editing Day (section)

=== Apparent and mean solar day ===
[[File:EpicEarth-Globespin-tilt-23.4.gif|thumb|Earth's rotation imaged by [[Deep Space Climate Observatory]], showing axis tilt]]
Several definitions of this universal human concept are used according to context, need, and convenience. Besides the day of 24 hours (86,400 seconds), the word ''day'' is used for several different spans of time based on the rotation of the Earth around its axis. An important one is the ''solar day'', the time it takes for the Sun to return to its culmination point (its highest point in the sky). Due to an orbit's eccentricity, the Sun resides in one of the orbit's [[Focus (geometry)|foci]] instead of the middle. Consequently, due to [[Kepler's laws of planetary motion#Second law|Kepler's second law]], the planet travels at different speeds at various positions in its orbit, and thus a solar day is not the same length of time throughout the orbital year. Because the Earth moves along an [[Semi-major and semi-minor axes#Semi-major and semi-minor axes of the planets|eccentric orbit]] around the Sun while the Earth spins on an inclined axis, this period can be up to 7.9 seconds more than (or less than) 24 hours. In recent decades, the average length of a solar day on Earth has been about 86,400.002 seconds<ref>{{cite web|title=Earth Orientation Parameters|url=http://hpiers.obspm.fr/eoppc/eop/eopc04/eopc04.62-now|publisher=International Earth Rotation and Reference Systems Service|archive-url=https://web.archive.org/web/20150426160146/http://hpiers.obspm.fr/eoppc/eop/eopc04/eopc04.62-now|archive-date=April 26, 2015|url-status=live}}</ref> (24.000 000 6 hours). There are currently about 365.2421875 solar days in one mean [[tropical year]].

Ancient custom has a new day starting at either the rising or setting of the Sun on the local horizon (Italian reckoning, for example, being 24 hours from sunset, old style).<ref>L. Holford-Stevens, ''The History of Time'' (Oxford 2005) p. 6</ref> The exact moment of, and the interval between, two sunrises or sunsets depends on the geographical position ([[longitude]] and latitude, as well as altitude), and the time of [[year]] (as indicated by ancient hemispherical [[sundial]]s).

A more constant day can be defined by the Sun passing through the local [[meridian (geography)|meridian]], which happens at local [[noon]] (upper [[culmination]]) or [[midnight]] (lower culmination). The exact moment is dependent on the geographical longitude, and to a lesser extent on the time of the year. The length of such a day is nearly constant (24 hours ± 30 seconds). This is the time as indicated by modern sundials.

A further improvement defines a fictitious mean Sun that moves with constant speed along the [[celestial equator]]; the speed is the same as the average speed of the real Sun, but this removes the variation over a year as the Earth moves along its orbit around the Sun (due to both its velocity and its axial tilt).

In terms of Earth's rotation, the average day length is about 360.9856°. A day lasts for more than 360° of rotation because of the Earth's revolution around the Sun.  With a full year being slightly more than 360 days, the Earth's daily orbit around the Sun is slightly less than 1°, so the day is slightly less than 361° of rotation.

Elsewhere in the [[Solar System]] or other parts of the [[universe]], a day is a full rotation of other large [[astronomical object]]s with respect to its star.<ref>{{Cite web |title=day |url=https://www.britannica.com/science/day |access-date=2022-08-17 |website=[[Encyclopædia Britannica]] |language=en |archive-date=2022-07-10 |archive-url=https://web.archive.org/web/20220710214718/https://www.britannica.com/science/day |url-status=live }}</ref>

==== Civil day ====
For civil purposes, a common clock time is typically defined for an entire region based on the local mean solar time at a central meridian. Such [[time zone|'' time zones'']] began to be adopted about the middle of the 19th century when [[Rail transport|railroads]] with regularly occurring schedules came into use, with most major countries having adopted them by 1929. As of 2015, throughout the world, 40 such zones are now in use: the central zone, from which all others are defined as offsets, is known as [[UTC+00:00|UTC+00]], which uses [[Coordinated Universal Time|Coordinated Universal Time (UTC)]].

The most common convention starts the civil day at [[midnight]]: this is near the time of the [[Culmination|lower culmination]] of the Sun on the central meridian of the time zone. Such a day may be called a [[calendar day]].

A day is commonly divided into 24 hours, with each hour being made up of 60 minutes, and each minute composed of 60 seconds.