Additionally, the atmosphere refracts sunlight in such a way that some of it reaches the ground even when the Sun is below the horizon by about 34 minutes of arc. So the first light reaches the ground when the centre of the Sun is still below the horizon by about 50 minutes of arc. Two effects make days on average longer than nights. A day, as opposed to night, is commonly defined as the period during which sunlight directly reaches the ground, assuming that there are no local obstacles. The difference in time depends on the angle at which the Sun rises and sets , but amounts to almost seven minutes at least. The Sun is not a point, but has an apparent size of about 32 minutes of arc
The day has several definitions
7 milliseconds per century . The Earth’s day has increased in length over time. See tidal acceleration for details. The original length of one day, when the Earth was new about 4. 002 seconds, and is increasing by about 1. Because of the way the second is defined, the mean length of a day is now about 86,400. 9 hours 620 million years ago as recorded by rhythmites . It was 21. This phenomenon is due to tides raised by the Moon which slow Earth’s rotation. 5 billion years ago, was about six hours as determined by computer simulation
Ancient custom has a new day start at either the rising or setting of the Sun on the local horizon The exact moment of, and the interval between, two sunrises or two sunsets depends on the geographical position , and the time of year. This is the time as indicated by ancient hemispherical sundials
A day of exactly 86,400 SI seconds is the fundamental unit of time in astronomy
The word refers to various relatedly defined ideas, including the following:
A further improvement defines a fictitious mean Sun that moves with constant speed along the celestial 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 moves along its orbit around the Sun
A day contains 86,400 SI seconds. Each second is currently defined as
A day is a unit of time equivalent to 24 hours and approximately the duration of a single rotation of planet Earth relative to the Sun. It is not an SI unit but it is accepted for use with SI. The term comes from the Old English dæg. The word is also used to mean daytime, the period of daylight experienced once per day and alternating with night
This is the time as indicated by modern sundials. The exact moment is dependent on the geographical longitude, and to a lesser extent on the time of the year. A more constant day can be defined by the Sun passing through the local meridian, which happens at local noon or midnight . The length of such a day is nearly constant
… the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium-133 atom
This was an afterglow of the decimal time used with the French Republican Calendar, which had already been given up. In the 19th century it had also been suggested to make a decimal fraction of an astronomic day the base unit of time
For a given planet, there are two types of day defined in astronomy:
The word day is used for several different units of time based on the rotation of the Earth around its axis. The reason for this apparent motion is the rotation of the Earth around its axis, as well as the revolution of the Earth in its orbit around the Sun. The most important one follows the apparent motion of the Sun across the sky