Day

A day is approximately the period of time during which the Earth completes one rotation around its axis.[1] A solar day is the length of time which elapses between the Sun reaching its highest point in the sky two consecutive times.[2]

In 1960, the second was redefined in terms of the orbital motion of the Earth in year 1900, and was designated the SI base unit of time. The unit of measurement "day", was redefined as 86,400 SI seconds and symbolized d. In 1967, the second and so the day were redefined by atomic electron transition.[3] A civil day is usually 86,400 seconds, plus or minus a possible leap second in Coordinated Universal Time (UTC), and occasionally plus or minus an hour in those locations that change from or to daylight saving time.[2][1]

Day can be defined as each of the twenty-four-hour periods, reckoned from one midnight to the next, into which a week, month, or year is divided, and corresponding to a rotation of the earth on its axis.[4] However its use depends on its context, for example when people say 'day and night', 'day' will have a different meaning. It will mean the interval of light between two successive nights; the time between sunrise and sunset,[5] in this instance 'day' will mean time of light between one night and the next.[6] However, in order to be clear when using 'day' in that sense, "daytime" should be used to distinguish it from "day" referring to a 24-hour period;[7] this is since daytime usually always means 'the time of the day between sunrise and sunset.[8] 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 (circadian rhythms) of humans and many other species are related to Earth's solar day and the day-night cycle.

Daytime image of the bay of Naples, Italy

Introduction

Dagr, the Norse god of the day, rides his horse in this 19th-century painting by Peter Nicolai Arbo.

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 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. In recent decades, the average length of a solar day on Earth has been about 86 400.002 seconds[9] (24.000 000 6 hours) and there are 365.242199074 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).[10] 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).

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).

Stellar day

A day, understood as the span of time it takes for the Earth to make one entire rotation[11] 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 366.2422 stellar days in one mean tropical year (one stellar day more than the number of solar days). Other planets and moons have stellar and solar days of different lengths from Earth's.

Daytime

A day, in the sense of daytime that is distinguished from night time, is commonly defined as the period during which sunlight directly reaches the ground, assuming that there are no local obstacles. The length of daytime averages slightly more than half of the 24-hour day. Two effects make daytime on average longer than nights. The Sun is not a point, but has an apparent size of about 32 minutes of arc. Additionally, the atmosphere refracts sunlight in such a way that some of it reaches the ground even when the Sun is below the horizon by about 34 minutes of arc. So the first light reaches the ground when the centre of the Sun is still below the horizon by about 50 minutes of arc.[12] Thus, daytime is on average around 7 minutes longer than 12 hours.[13]