The day is not exactly 24 hours
How many last night? Depends on the circumstances. Day is the period of time during which Earth completes one rotation around its axis. And it is never exactly twenty-four hours.
Amazingly, this figure could fluctuate in either direction by as much as fifty seconds! This is because the speed of Earth's rotation changes all the time — because of the friction caused by SYNOPTIC situations, high tides/low tides and geological events.
In an average year, day and fraction of a second shorter than twenty-four hours.
When these discrepancies were revealed using atomic clocks, it was decided to redefine the second as a fixed proportion of the solar day, or, millionstar-forty thousand.
New second came into use in 1967 and is defined as "the time interval equal to 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the atom of cesium-133 in the absence of perturbation by external fields". Rather not say — just a bit too dreary to pronounce it all at the end of a long day.
A new definition of the second means that the solar day is gradually shifted towards nuclear. As a result, scientists had to enter into the atomic year of the so-called "leap second" (or "second coordinate"), to agree a nuclear year with solar.
The last time the "leap second" (seventh from the time when in 1972 he established the coordinated Universal time — UTC) has been added 31 Dec 2005, as directed by the International service of estimating the rotation parameters and the Ground coordinates based at the Paris Observatory.
The good news for astronomers and those of us who love, when the clock is ticking in step with the movement of the Earth around the Sun, but a headache for computer programs and the equipment that is on space satellites.
The idea of entering a "leap second" has met strong resistance from the International telecommunications Union, which even made an official proposal to abolish it by December 2007.
There is, of course, a compromise: wait until the difference between coordinated Universal time (UTC) and the Average time across Greenwich (GMT) reaches exactly one hour (after 400 years) and then have to put everything in order. In the meantime, the debate over what constitutes "real" time continues.
Source: http://mirfactov.com/v-sutkah-ne-byivaet-rovno-24-chasa/