Day length
Underground length can - based on the whole earth - the exact duration of a solar day be understood, or - in relation to a particular geographical location - the respective duration of a clear day.
Duration of a solar day
The exact duration of a true solar day is measured, for example from a peak of the true sun until the next true solar time high. This period of time can take different lengths and different 24 hours by up to 30 seconds from the mean. For because of the different speeds of the earth when run on its eccentric orbit around the sun varies the true solar day during the year and will run roughly towards shorter depending on the solstices ( solstices ), the equinoxes ( the equinoxes ) toward longer. Additional differences may arise as a result of Earth's rotation with different speeds.
The indicated deviations change slightly in the course of years.
Currently, the Earth rotates once in about 23 h 56 min 4.10 s to himself; this rotation period for a self- rotation of 360 ° is also called a sidereal day (English " stellar day" ). Since the Earth takes around 366 self- rotations in prograde direction during one year run of around 365 days of sunshine, there is a difference from an average of about 4 minutes between solar day and the sidereal day. The average length of the true solar day is called the mean solar day and is the basis for the mean time (LMT ).
Duration of the bright day
The duration of the bright or clear day is measured from sunrise to sunset for a particular location on Earth and referred fachsprachlich as diurnal arc or LOD (of "length of day" ). It depends on the season and the latitude of the site. In Central Europe about this period of daylight varies seasonally depending from 8 hours in the winter and 16 hours in the summer, so for about 8 hours.
The time from the rising of the sun to its highest level - that is half the duration of a true solar day or a half day length in true local time (LAT ) - can be determined by the following formula ( in hours h):
With φ = latitude of the location, δ = declination of the sun, vacillating depending on the season between approximately 23.5 ° North ( early summer ) and 23.5 ° South ( beginning of winter ).
If one of the angles φ and δ is zero, the product of the tangent values zero and since arccos 0 = 90 ° is true, then half the length of the day is found to be 90 ° / 15 ° = 6 hours. Half the diurnal arc under these conditions is therefore 6 hours, and the whole 12 hours. Twice a year the sun has this declination δ = 0, namely the astronomical beginning of spring and early autumn. The sun then goes around the earth exactly in the east and sets in the west and the day lasts as long as the night; such a date is therefore also called day and night are equally or equinox, outside the polar regions is the length of the day then 12 hours.
The day length than the duration of the clear day changes over the course of a year, not even pace from day to day, but most near the equinoxes in spring and autumn. Then lengthened or shortened in one place the following day at 50 ° latitude by almost four and a half minutes. In contrast, the day lengths are around the time of the solstices or solstices in summer and in winter only slightly different from each other (see figures).
General
The possible hours of sunshine throughout the year are considered for all regions of the world roughly about the same sum. An exception to high mountains and deep valleys dar. Thus, the sunrise is at one standing high mountain (eg Kilimanjaro ) much earlier and the sunset significantly later than in the area. Deep valleys, however, may get in the winter in general no direct sunlight.
If one takes into account that because of the elliptical orbit of the earth around the sun, the duration of the seasons varies and currently on the northern hemisphere, the period between early spring and early autumn almost 8 days ( time unit d) lasts longer than the period between the beginning of fall and early spring, then the average day length in about a quarter of an hour different for the two hemispheres: north about 12 hours and 15 minutes, and south about 11 hours and 45 minutes.