Day
This daily cycle drives circadian rhythms in many organisms, which are vital to many life processes.
A collection of sequential days is organized into calendars as dates, almost always into weeks, months and years.
In recent decades, the average length of a solar day on Earth has been about 86,400.002 seconds[2] (24.000 000 6 hours).
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).
[3] 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).
The exact moment is dependent on the geographical longitude, and to a lesser extent on the time of the year.
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.
In 1967–68, during the 13th CGPM (Resolution 1),[14] the International Bureau of Weights and Measures (BIPM) redefined a second as "the duration of 9,192,631,770 periods of the radiation corresponding to the transition between two hyperfine levels of the ground state of the caesium-133 atom".
Other than the two-millisecond variation from tidal deceleration, other factors minutely affect the day's length, which creates an irregularity in the placement of leap seconds.
Discovered by paleontologist John W. Wells, the day lengths of geological periods have been estimated by measuring sedimentation rings in coral fossils,[22][20] due to some biological systems being affected by the tide.
[20] Arbab attributed this to the change of water volume present affecting Earth's rotation.
Humans, with their cultural norms and scientific knowledge, have employed several different conceptions of the day's boundaries.
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.
The International Meridian Conference of 1884 resolved That the Conference expresses the hope that as soon as may be practicable the astronomical and nautical days will be arranged everywhere to begin at midnight.In ancient Egypt the day was reckoned from sunrise to sunrise.
The parts of the day do not have set times; they can vary by lifestyle or hours of daylight in a given place.
[25] Daytime is the part of the day during which sunlight directly reaches the ground, assuming that there are no obstacles.
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.
This period of time sees human's highest body temperature,[30] an increase of traffic collisions,[31] and a decrease of productivity.
[39] Light pollution during night can impact human and animal life, for example by disrupting sleep.
