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A day is a unit of time. In common usage, it is an interval equal to 24 hours.[1] It also can mean the consecutive period of time during which the Sun is above the horizon, also known as daytime. The period of time during which the Earth completes one rotation with respect to the Sun is called a solar day.[2][3]

Several definitions of this universal human concept are used according to context, need and convenience. In 1967, the second was redefined in terms of the wavelength of light, and was designated the SI base unit of time. The unit of measurement "day", redefined in 1967 as 86 400 SI seconds and symbolized d, is not an SI unit, but is accepted for use with SI.[1] A civil day is usually also 86 400 seconds, plus or minus a possible leap second in Coordinated Universal Time (UTC), and, in some locations, occasionally plus or minus an hour when changing from or to daylight saving time. The word day may also refer to a day of the week or to a calendar date, as in answer to the question "On which day?" The life patterns of humans and many other species are related to Earth's solar day and the day-night cycle (see circadian rhythms).

In recent decades the average length of a solar day on Earth has been about 86 400.002 seconds[4] (24.000 000 6 hours) and there are about 365.242 2 solar days in one mean tropical year. 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. A day, understood as the span of time it takes for the Earth to make one entire rotation[5] with respect to the celestial background or a distant star (assumed to be fixed), is called a stellar day. This period of rotation is about 4 minutes less than 24 hours (23 hours 56 minutes and 4.1 seconds) and there are about 366.242 2 stellar days in one mean tropical year (one stellar day more than the number of solar days). Mainly due to tidal effects, the Earth's rotational period is not constant, resulting in further minor variations for both solar days and stellar "days". Other planets and moons also have stellar and solar days.



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 the Earth orbits the Sun elliptically as the Earth spins on an inclined axis, this period can be up to 7.9 seconds more than (or less than) 24 hours. On average over the year this day is equivalent to 24 hours (86 400 seconds).


Ancient custom has a new day start at either the rising or setting of the Sun on the local horizon (Italian reckoning, for example[citation needed]). The exact moment of, and the interval between, two sunrises or sunsets depends on the geographical position (longitude as well as latitude), and the time of year (as indicated by ancient hemispherical sundials).



The Earth's day has increased in length over time. This phenomenon is due to tides raised by the Moon which slow Earth's rotation. Because of the way the second is defined, the mean length of a day is now about 86 400.002 seconds, and is increasing by about 1.7 milliseconds per century (an average over the last 2 700 years). (See tidal acceleration for details.) The length of a day circa 620 million years ago has been estimated from rhythmites (alternating layers in sandstone) as having been about 21.9 hours. The length of day for the Earth before the moon was created is still unknown.[citation needed]


The term comes from the Old English dæg, with its cognates such as dagur in Icelandic, Tag in German, and dag in Norwegian, Danish, Swedish and Dutch. All of them from the Indo-European root dyau which explains the similarity with Latin dies though the word is known to come from the Germanic branch. As of October 17, 2015, day is the 205th most common word in US English,[6] and the 210th most common in UK English.[6]

International System of Units (SI)

A day, symbol d, is defined as 86 400 seconds. The Second is the base unit of time in SI units.

A day according to Coordinated Universal Time (UCT) can include a negative or positive leap second, and can therefore have a length of either 86 399 or 86 401 seconds.

In 1967–68, during the 13th CGPM (Resolution 1),[7] the International Bureau of Weights and Measures (BIPM) redefined a second as


This makes the SI-based day last exactly 794 243 384 928 000 of those periods.

Decimal and metric time

In the 19th century, an idea circulated to make a decimal fraction (​110 000 or ​1100 000) of an astronomical day the base unit of time. This was an afterglow of the short-lived movement toward a decimalisation of timekeeping and the calendar, which had been given up already due to its difficulty in transitioning from traditional, more familiar units. The most successful alternative is the centiday, equal to 14.4 minutes (864 seconds), being not only a shorter multiple of an hour (0.24 vs 2.4) but also closer to the SI multiple kilosecond (1 000 seconds) and equal to the traditional Chinese unit, ke.


A day of exactly 86 400 SI seconds is the base unit of time in astronomy (the second is not preferred).[9]




The word refers to various similarly defined ideas, such as:

  • 24 hours (exactly)
  • The period of light when the Sun is above the local horizon (that is, the time period from sunrise to sunset)
  • The full day covering both the dark and light periods, beginning from the start of the dark period or from a point near the middle of the dark period
  • A full dark and light period, sometimes called a nychthemeron in English, from the Greek for night-day[citation needed]
  • The time period from 06:00–18:00 (6:00 am – 6:00 pm) or 21:00 (9:00 pm) or some other fixed clock period overlapping or offset from other time periods such as "morning", "evening", or "night".
  • The time period from first-light "dawn" to last-light "twilight".

Civil day

For civil purposes, a common clock time is typically defined for an entire region based on the local mean solar time at some 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.


Leap seconds

In order to keep the civil day aligned with the apparent movement of the Sun, positive or negative leap seconds may be inserted from time to time. Therefore, although typically 86 400 SI seconds in duration, a civil day can be either 86 401 or 86 399 SI seconds long on such a day.

Leap seconds are announced in advance by the International Earth Rotation and Reference Systems Service (IERS), which measures the Earth's rotation and determines whether a leap second is necessary. Leap seconds occur only at the end of a UTC-calculated month, and have only ever been inserted at the end of June 30 or December 31.[citation needed]

Boundaries of the day

Sun and Moon, Hartmann Schedel's Nuremberg Chronicle, 1493

For most diurnal animals, the day naturally begins at dawn and ends at sunset. Humans, with their cultural norms and scientific knowledge, have employed several different conceptions of the day's boundaries. The Jewish day begins at either sunset or nightfall (when three second-magnitude stars appear). Medieval Europe also followed this tradition, known as Florentine reckoning: in this system, a reference like "two hours into the day" meant two hours after sunset and thus times during the evening need to be shifted back one calendar day in modern reckoning. Days such as Christmas Eve, Halloween, and the Eve of Saint Agnes are remnants of the older pattern when holidays began during the prior evening. Common convention in modern times is for the civil day to begin at midnight, i.e. 00:00, and last a full 24 hours until 24:00 (i.e. 00:00 of the next day). Prior to 1926, Turkey had two time systems: Turkish (counting the hours from sunset) and French (counting the hours from midnight).

In ancient Egypt, the day was reckoned from sunrise to sunrise. Muslims fast from sunrise to sunset each day during the month of Ramadan. The "Damascus Document", copies of which were also found among the Dead Sea scrolls, states regarding the observance of the Sabbath that "No one is to do any work on Friday from the moment that the Sun's disk stands distant from the horizon by the length of its own diameter," presumably indicating that the monastic community responsible for producing this work counted the day as ending shortly before the Sun had begun to set.[citation needed]

In many cultures, nights are named after the previous day. For example,"Friday night" usually means the entire night between Friday and Saturday. This difference from the civil day often leads to confusion. Events starting at midnight are often announced as occurring the day before. TV-guides tend to list nightly programs at the previous day, although programming a VCR requires the strict logic of starting the new day at 00:00 (to further confuse the issue, VCRs set to the 12-hour clock notation will label this "12:00 AM"). Expressions like "today", "yesterday" and "tomorrow" become ambiguous during the night. Because Jews and Muslims begin their days at nightfall, "Saturday" (Arabic word) night, for example, is what most people would call Friday night.


24 hours vs daytime


See also


  1. 1.0 1.1 BIPM (2014) [2006]. "Non-SI units accepted for use with the SI, and units based on fundamental constants". SI Brochure (8th ed.).<templatestyles src="Module:Citation/CS1/styles.css"></templatestyles>
  2. Weisstein, Eric W. (2007). "Solar Day". Retrieved 2011-05-31.<templatestyles src="Module:Citation/CS1/styles.css"></templatestyles>
  4. The average over the last 50 years is about 86 400.002. The yearly average over that period has ranged between about 86 400 and 86 400.003, while the length of individual days has varied between about 86 399.999 and 86 400.004 seconds. See this graph: Deviation of day length from SI day.svg (data from "EARTH ORIENTATION PARAMETERS". International Earth Rotation and Reference Systems Service. Archived from the original on April 26, 2015. Unknown parameter |deadurl= ignored (help)<templatestyles src="Module:Citation/CS1/styles.css"></templatestyles>).
  5. Some authors caution against identifying "day" with rotation period. For example: Courtney Seligman. "Rotation Period and Day Length". Retrieved 2011-06-03. A Cautionary Note: Because the rotation period of the Earth is almost the same as the length of its day, we sometimes get a bit sloppy in discussing the rotation of the sky, and say that the stars rotate around us once each day. In a similar way, it is not unusual for careless people to mix up the rotation period of a planet with the length of its day, or vice versa.<templatestyles src="Module:Citation/CS1/styles.css"></templatestyles>
  6. 6.0 6.1 "English Words". Oxford Dictionaries Online (ODO). Oxford University Press. Retrieved 2015-10-17.<templatestyles src="Module:Citation/CS1/styles.css"></templatestyles>
  7. "SI Unit of Time (Second)". Resolution 1 of the 13th CGPM (1967/68). Bureau International des Poids et Mesures (BIPM). Retrieved 2015-10-17.<templatestyles src="Module:Citation/CS1/styles.css"></templatestyles>
  8. "Unit of Time (Second)". SI Brochure: The International System of Units (SI) (8 ed.). Bureau International des Poids et Mesures (BIPM). 2014 [2006]. Retrieved 2015-10-17.<templatestyles src="Module:Citation/CS1/styles.css"></templatestyles>
  10. "singladura - Definición". Retrieved 2011-06-03.<templatestyles src="Module:Citation/CS1/styles.css"></templatestyles>