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Different definitions of the day are based on the apparent motion of the Sun across the sky (solar day). The reason for this apparent motion is the rotation of the Earth around its axis, as well as the revolution of the Earth in an orbit around the Sun.
Ancient custom has a new day start at either the rise or set of the Sun on the local horizon. The exact moment, and the interval between two sunrises or two sunsets, depends on the geographical position ( longitude as well as latitude), and the time of year.
A more constant day can be defined by the Sun passing through the local meridian, which happens at local noon (upper culmination) or midnight (lower culmination). The exact moment is dependent on the geographical longitude, and to a lesser extent on the time of the year. The length of a such a day is nearly constant. This is the time as indicated by sundials.
A further improvement defines a fictituous mean Sun that moves with constant speed over the equator; the speed is the same as the average speed of the real Sun, but this removes the variation over a year as the Earth runs its orbit around the Sun.
The earth has over time had an increasingly longer day. The original length of one day, when the earth was new, is actually closer to 23 hours. This phenomenon is due to the moon which slows the Earth's rotation slowly over time.
For civil purposes, since the middle of the 19th century when railroads with regular schedules came into use, a common clock time has been defined for an entire region based on the mean local solar time at some central meridian. For the whole world, about 30 such time zones are defined. The main one is "world time" or UTC (Coordinated Universal Time).
The present common convention has the civil day start at midnight, which is near the time of the lower culmination of the mean Sun on the central meridian of the time zone. A day is commonly divided into 24 hours of 60 minutes of 60 seconds each.
When taking leap secondA leap second is a one- second adjustment to civil time in order to keep it close to the mean solar time. Civil clock time is based on "Coordinated Universal Time" (UTC), which is maintained by extremely precise atomic clocks. In contrast, the rotation ofs into account, a civil clock day is 86400 or 86401 SIThe International System of Units (symbol: SI (for the French phrase Systeme International d'Unites , is the most widely used system of units. It is used for everyday commerce in virtually every country of the world except the United States, and it is uni secondThis article is about the unit of time. See second (disambiguation) for other uses The second (symbol s is a unit for time, and one of seven SI base units. It is defined as the duration of 9,192,631,770 periods of the radiation corresponding to the transis long (or theoretically 86399 s on occasion, which has never happened).
In astronomy also the sidereal dayprograde planet like the Earth, the sidereal day is shorter than the solar day. At time 1, the sun and a certain distant star are both overhead. At time 2, the planet has rotated 360° and the distant star is overhead again (1→2 one sidereal day). is used; it is ca. 3 minutes 56 seconds shorter than the solar day, and close to the actual rotation period of the Earth.