Day length


Day length
Day length as a function of latitude and the day of the year

Day length, or length of day, or length of daytime, refers to 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. Due to 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.

Contents

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 (either north or south) 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 (again, either north or south) 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 — due to atmospheric refraction and the size of the Sun — exceeds 12 hours by about 7 minutes each day;
  • During a few days around the equinoxes—about March 19–22 and September 21–24—both poles experience simultaneously 24 hours of daytime, due mainly to 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 due mainly to atmospheric refraction.

Alternate definition

Day length, sunset and sunrise in Madrid (40º25') during 2011.

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.

To considerable accuracy, all the points at the same latitude on the same calendar date can be considered to have the same day length. The contour plot in the figure is computed using the sunrise equation.

With this definition the following features can be observed:

  • When the northern polar cap of certain range has duration of daytime of 24 hours, the southern polar cap of the same range has duration of nighttime of 24 hours, and vice versa.

Historical variation of day length because of tidal acceleration

The day length was less in the past. About 320 million years ago there were 400 days in the year.

See Historical evidence of tidal acceleration

See also

External links


Wikimedia Foundation. 2010.

Look at other dictionaries:

  • day-length — dayˈ length noun (botany) The number of hours of daylight in a day, a trigger for flowering in some plants • • • Main Entry: ↑day …   Useful english dictionary

  • day|length — «DAY LEHNGKTH, LEHNGTH», noun. = photoperiod: (Cf. ↑photoperiod) »... the onset of winter with its low temperature and shorter daylength (New Scientist) …   Useful english dictionary

  • day length — the duration of the light period in a given 24 hours. Day length or daylength may be manipulated for aquaculture purposes such as early smoltification and breeding …   Dictionary of ichthyology

  • day-neutral plant — noun (botany) A plant in which flowering is not sensitive to day length • • • Main Entry: ↑day …   Useful english dictionary

  • Day — For other uses, see Day (disambiguation). Water, Rabbit, and Deer: three of the 20 day symbols in the Aztec calendar, from the Aztec calendar stone. A day is a unit of time, commonly defined as an interval equal to 24 hours …   Wikipedia

  • Day neutral plant — A day neutral plant does not require a specific day length to flower. It is neither a short day plant nor a long day plant. Day neutral plants flower after attaining an overall developmental stage or age, or in response to alternative… …   Wikipedia

  • Day & Age — Studio album by The Killers Released November 18, 2008 …   Wikipedia

  • Day 'n' Nite — Single by Kid Cudi from the album Man on the Moon: The End of Day Rel …   Wikipedia

  • Day of Mourning (album) — Day of Mourning Studio album by Despised Icon Released Septemb …   Wikipedia

  • day — /day/, n. 1. the interval of light between two successive nights; the time between sunrise and sunset: Since there was no artificial illumination, all activities had to be carried on during the day. 2. the light of day; daylight: The owl sleeps… …   Universalium


Share the article and excerpts

Direct link
Do a right-click on the link above
and select “Copy Link”

We are using cookies for the best presentation of our site. Continuing to use this site, you agree with this.