The day is a unit of time measurement. Before precision time measurement became available, the day was measured based on the apparent motion of the sun in the earth's sky. A day was either the entire period between sunrise and sunset, or one full cycle of the sun's motion, starting variously at sunset, sunrise, midnight, or noon (mid-day). With the development of precision astronomical instruments and time measurement, it was discovered that the length of the solar day was not entirely fixed. Eventually, this lead to the standardization of the civil (legal) day as being equal to the mean solar day.
In astronomy, each planet has two different sorts of days, the solar day and the sidereal day. The solar day is the average time between two transits of the sun past a given meridian. The sidereal day is the average time between transits of "fixed" stars. Due to the motion of planets around the sun, there will be either one more or one less sidereal day in a year than there are solar days. The length of the sidereal day on Earth is slightly more than 23 hours, 56 minutes, and there are 366.242 sidereal days in a year.
The length of the mean solar day has increased by roughly 2 milliseconds since it was exactly 86,400 seconds of atomic time in 1820. That is, the length of the mean solar day is at present about 86,400.002 seconds instead of exactly 86,400 seconds. Over the course of one year, the difference accumulates to almost one second, which is compensated by the insertion of a leap second into the scale of UTC with a current regularity of a little less than once per year. Other factors also affect the Earth, some in unpredictable ways, so that it is necessary to monitor the Earth's rotation continuously.
In order to keep the cumulative difference in UT1-UTC less than 0.9 seconds, a leap second is added to the atomic time to decrease the difference between the two. This leap second can be either positive or negative depending on the Earth's rotation. Since the first leap second in 1972, all leap seconds have been positive and there were 23 leap seconds in the 34 years to January, 2006. This pattern reflects the general slowing trend of the Earth due to tidal braking.
- Leap Seconds. U.S. Naval Observatory. Retrieved on 2 March 2007.