Solar day

A solar day is the current length of a day in which the sun crosses the meridian exactly at noon and nearly reaches its daily peak. Measured the time between two consecutive lower meridian passages for a particular terrestrial observation, which may differ by up to about 30 seconds in 24 hours.

Definition

The solar day is the true local WOZ ( true solar time ) of a site measured; this is also the time that displays a simple sundial and can be read on the Sun. A solar day begins around 0 clock WOZ at the moment of the lower meridian passage of the sun and lasts until the next lower meridian passage (24 clock WOZ ). In the upper passage of the sun through the meridian 12 clock WOZ or true noon. For an observation point on the northern hemisphere of the earth, the sun is then precisely to the south. In most cases, corresponds to this time not exactly the upper culmination and the maximum daily position of the sun for a few seconds reached sooner or later. The sun's altitude at noon varies with the angle of declination during the year by about ± 23.4 ° ( obliquity ) to a value which depends in each case on the latitude B of the site ( 90 ° B), for example, on the 50th degree of latitude between 63 4 ° and 16.6 ° at the solstices.

The time interval between two successive meridian passages duration varies, because the sun moves at different speeds to its apparent annual path as it describes the second Kepler law. The differences compared with the fastest movement in January while passing through the perihelion ( the Periapsiszeit Earth ) add up. The differences from the mean over time allowing the equation to calculate the mean local time MOZ ( mean solar time ).

The duration of the solar day can thus vary from 24 hours in the year by approximately ± 30 seconds.

Mean solar

A mean solar day is the average mean true solar day, averaged over a number of years. It corresponds to the period of the run of a fictitious mean sun, whose daily path in the sky with a uniform speed runs ( on a circular path ). For a " eclipsed sun " for the uniform rotation of the earth and a " equatorial sun " for the uniform rotation of the earth must be constructed. The mean time MOZ ( mean solar time ) is calculated on the basis of the mean solar day, which is the constant value in the above correction of the equation of time. A mean solar day lasts 24 hours or 24 · 60 · 60 = 86400 Sun seconds.

Is the duration of the mean solar day used to form a measure of time for a time pattern, - based on any current day - the same shall apply in each case, so the metered length of time is referred to as civil day. This forms the basis for one day in the usual calendar calculations. About the mentioned decomposition into 86,400 equal parts he originally was the benchmark for the second or then the indication was for the factor in the definition of the SI unit of time. The SI second self but is now no longer defined on the Middle sunny day, but as the atomic second and represented using atomic clocks.

The civil day is different in today's calendar calculations only by the calendar of that in the latter case the duration is sometimes changed by leap seconds. Therefore, individual calendar days may be longer or shorter than the bourgeois day. Therefore discrete adjustments to the current average actual values ​​of the length of day is possible. Since this varies not only during the round, but also because of the non-constant rotational period of the earth and increases due to the long term effects of decelerating. These adjustments will apply to the Universal Time ( Coordinated Universal Time UTC) and synchronize it with the Universal Time ( UT1 universal solar time ) except for a small difference ( DUT 1 ).