Days of Future Passed

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 solar noons or times the Sun reaches the highest point. The word "day" may also refer to ''daytime'', a time period when the location receives direct and indirect sunlight. On Earth, as a location passes through its day, it experiences morning, noon, afternoon, evening, and night. The effect of a day is vital to many life processes, which is called the circadian rhythm.

A collection of sequential days is organized into calendars as dates, almost always into weeks, months and years. Most calendars' arrangement of dates use either or both the Sun with its four seasons (solar calendar) or the Moon's phasing (lunar calendar). The start of a day is commonly accepted as roughly the time of the middle of the night or midnight, written as 00:00 or 12:00 am in 24- or 12-hour clocks, respectively. Because the time of midnight varies between locations, time zones are set up to facilitate the use of a uniform standard time. Midnight is not the only convention used to determine the start of a new day. Other defining moments have been used throughout history, and some are used even today, such as with the Jewish religious calendar, which counts days from sunset to sunset, so the Jewish Sabbath begins at sundown on Friday. Astronomers also have a convention where their day begins at high noon. This way, all of their observations throughout a single night are recorded as happening on the same day. This method removes ambiguity of a particular observation happening on a calendar day, eliminating the need to further determine which night it happened on. Because when using midnight as the start of day, each calendar day is associated with two separate night periods.

In specific applications, the definition of a day is slightly modified, such as in the ISQ day (exactly 86,400 seconds) used for computers and standards keeping, local mean time accounting of the Earth's natural fluctuation of a solar day, and stellar day and sidereal day (using the celestial sphere) used for astronomy. In most countries outside of the tropics, daylight saving time is practiced, and each year there will be one 23-hour civil day and one 25-hour civil day. Due to slight variations in the rotation of the Earth, there are rare times when a leap second will get inserted at the end of a UTC day, and so while almost all days have a duration of 86,400 seconds, there are these exceptional cases of a day with 86,401 seconds (in the half-century spanning 1972 through 2022, there have been a total of 27 leap seconds that have been inserted, so roughly once every other year).

Etymology

The term comes from the Old English term ''dæġ'' (), with its cognates such as ''dagur'' in Icelandic, ''Tag'' in German, and ''dag'' in Norwegian, Danish, Swedish and Dutch – all stemming from a Proto-Germanic root ''*dagaz''.Mallory, James P.; Adams, Douglas Q. (2006). ''The Oxford Introduction to Proto-Indo-European and the Proto-Indo-European World''. Oxford, UK: Oxford University Press. p. 124. . , ''day'' is the 205th most common word in US English, and the 210th most common in UK English.

Definitions

Apparent and mean solar day

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'', defined as the time it takes for the Sun to return to its culmination point (its highest point in the sky). Because celestial orbits are not perfectly circular, and thus objects travel at different speeds at various positions in their orbit, a solar day is not the same length of time throughout the orbital year. Because the Earth moves along an 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 (24.000 000 6 hours) and there are currently about 365.2421875 solar days in one mean tropical year.

Ancient custom has a new day start at either the rising or setting of the Sun on the local horizon (Italian reckoning, for example, being 24 hours from sunset, oldstyle). 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 sundials).

A more constant day can be defined by the Sun passing through the local 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 objects with respect to its star.

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 zones'' began to be adopted about the middle of the 19th century when 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, which uses Coordinated Universal Time (UTC).

The most common convention starts the civil day at midnight: this is near the time of the lower culmination of the Sun on the central meridian of the time zone. Such a day may be referred to as a calendar day