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Daytime#Length

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+#REDIRECT [[Daytime#Length]] {{R from merge}}
-{{about|duration of sunlight in a day|the rotation of the Earth around its own axis|Earth's rotation#Rotation_period|the length of a mean solar day compared to a uniform time scale|ΔT}}
-{{Refimprove|date=January 2013}}
-{{Merge to|Daytime (astronomy)|date=October 2013}}
-[[Image:Hours of daylight vs latitude vs day of year cmglee.svg|thumb|'''Day length''' as a [[function (mathematics)|function]] of [[latitude]] and the day of the year ]]
-'''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 [[plane of the ecliptic|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 solstice|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 — 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 [[equinox]]es—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 [[equinox]]es.  Each pole’s sunrise is nearly coincident with the other's sunset, with minor differences mainly resulting from atmospheric refraction.
-==Alternative definition==
-[[File:Duración del día Madrid 2011.png|thumb|250px|Day length, sunset and sunrise in Madrid (40º25') during 2011.]]
-[[File:WorldSunshine.ogg|thumb|250px|Sunshine at 12:00 UTC during a year]]
-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 2ω<sub>o</sub>/15°, where ω<sub>o</sub> is the [[sunset]] [[hour angle]] in degrees (°) given by the [[sunrise equation|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 [[millisecond]]s shorter than today,<ref>[[Dennis McCarthy (scientist)|McCarthy, D.D.]] & Seidelmann, P.K. ''TIME: From Earth Rotation to Atomic Physics.'' Weinheim: Wiley-VCH. (2009). pp. 88&ndash;89</ref> while in the late [[Neoproterozoic]] about 620 million years ago a day had only about 21.9±0.4 hours.<ref>{{Cite journal | doi = 10.1029/1999RG900016 | last1 = Williams | first1 = George E. | year = 2000 | title = Geological constraints on the Precambrian history of Earth's rotation and the Moon's orbit | bibcode = 2000RvGeo..38...37W | journal = Reviews of Geophysics | volume = 38 | issue = 1| pages = 37–60 }}</ref>
-==See also==
-*[[Fluctuations in the length of day]]
-*[[Photoperiodism]] (for effects on living organisms)
-*[[Postglacial rebound]]
-*[[Solar day]]
-*[[Position of the Sun|Sun declination]]
-*[[Sunrise equation]] Nepali Method
-==References==
-{{reflist}}
-==External links==
-{{commonscat}}
-* [http://www.astroid.eu/ Compute precise times of rising and setting of the Sun, Moon and planets.]
-* [http://www.jgiesen.de/SME/ Sun, Moon and Earth Applet], by Jurgen Giesen
-* [http://herbert.gandraxa.com/length_of_day.aspx Formulae to calculate day length], by Herbert Glarner
-* [http://www.usno.navy.mil/USNO/astronomical-applications/data-services/rs-one-year-us Calculate sun rise/set], by U.S. Naval Observatory
-* [http://www.iers.org/ Datasets on earth rotation and alignment], IERS, International Earth Rotation and Reference Systems Service (formerly: International Earth Rotation Service)
-* [http://www.sunearthtools.com/dp/tools/pos_sun.php Sun chart generator with Daylight Sunrise Sunset table values]
-* [http://ptaff.ca/soleil/?lang=en_CA Sunrise, sunset, daylight in a graph]
-[[Category:Solar phenomena]]
-[[Category:Earth phenomena]]