Day length, or length of day, or length of daytime, is the time each day from the moment
the upper limb of the sun's disk appears above the horizon during sunrise to the moment when
the upper limb disappears below the horizon during sunset.
Because of the diffusion and refraction of sunlight by the atmosphere, there is actually
daylight even when the sun is slightly below the horizon.
The period when it is still somewhat light even though the sun is below the horizon is
called twilight.
Description In general, the length of a day varies throughout
the year, and depends upon latitude.
This variation is caused by the tilt of the Earth's axis of rotation with respect to the
ecliptic plane of the Earth around the sun.
At the solstice occurring about June 20–22, the north pole is tilted toward the sun, and
therefore the northern hemisphere has days ranging in duration from just over 12 hours
in the southern portion of the Tropic of Cancer to 24 hours in the Arctic Circle, while the
southern hemisphere has days ranging in duration from just under 12 hours in the northern portion
of the Tropic of Capricorn to zero in the Antarctic Circle.
At the equinox occurring about September 22–23, the poles are neither tilted toward nor away
from the sun, and the duration of a day is generally about 12 hours all over the Earth.
At the solstice occurring about December 20–22, the south pole is tilted toward the sun, and
therefore the southern hemisphere has days ranging in duration from just over 12 hours
in the northern portion of the Tropic of Capricorn to 24 hours in the Antarctic Circle, whereas
the northern hemisphere has days ranging in duration from just under 12 hours in the southern
portion of the Tropic of Cancer to zero in the Arctic Circle.
At the equinox occurring about March 19–21, the poles are again aligned so that the duration
of a day is generally about 12 hours all over the Earth.
In each hemisphere, the higher the latitude, the shorter the day during winter.
Between winter and summer solstice, the day's duration increases, and the rate of increase
is larger the higher the latitude.
A fast increase of day length is what allows a very short day on winter solstice at 60
degrees latitude to reach about 12 hours by the spring equinox, while a slower increase
is required for a much longer day on winter solstice at 20 degrees latitude to reach 12
hours by the spring equinox.
The rate of change of day duration is generally fastest at the equinoxes, although at high
latitudes the change is similar for several weeks before and after the equinoxes.
The rate of change of day duration at each solstice is zero as the change goes from positive
to negative, or vice versa.
Some interesting facts are as follows: On the Equator, the duration of daylight is
not exactly 12 hours all the year round, but rather — because of atmospheric refraction
and the size of the Sun — exceeds 12 hours by about 7 minutes each day;
Because the sun is north of the equator for almost 4 days more than half the year, because
of the eccentricity of Earth's orbit, the duration of the average day at a given latitude
in the northern hemisphere exceeds the duration of the average day at the same latitude in
the southern hemisphere by a few minutes; During a few days around the equinoxes—about
March 19–22 and September 21–24—both poles experience simultaneously 24 hours of
daytime, mainly because of atmospheric refraction.
Each pole has only one sunrise and one sunset per year, around the time of the equinoxes.
Each pole’s sunrise is nearly coincident with the other's sunset, with minor differences
mainly resulting from atmospheric refraction.
Alternative definition
More conveniently, atmospheric refraction is ignored and the center of the sun is often
used in place of the upper limb for computing a day's duration.
When sunrise and sunset do occur, the day duration can be computed as 2ωo/15°, where
ωo is the sunset hour angle in degrees given by the sunset equation.
When sunrise and sunset do not occur during the course of a day, the day duration is either
0 or 24 hours.
Increasing day length The earth is constantly losing angular velocity
and rotational energy through a process called tidal acceleration, which leads to a slow
lengthening of the day.
A century ago, the average day was about 1.7 milliseconds shorter than today, while in
the late Neoproterozoic about 620 million years ago a day had only about 21.9±0.4 hours.
See also Fluctuations in the length of day
Photoperiodism Postglacial rebound
Solar day Sun declination
Sunrise equation Nepali Method References
External links Compute precise times of rising and setting
of the Sun, Moon and planets.
Sun, Moon and Earth Applet, by Jurgen Giesen Formulae to calculate day length, by Herbert
Glarner Calculate sun rise/set, by U.S. Naval Observatory
Datasets on earth rotation and alignment, IERS, International Earth Rotation and Reference
Systems Service Sun chart generator with Daylight Sunrise
Sunset table values Sunrise, sunset, daylight in a graph