One of the most important physical quantities is the time. Life on Earth is closely connected with the periodic movement of the Sun across the sky, therefore, since ancient times, the expense of time and definition of time units associated with this movement. One of these units — a solar day is the interval between two successive passages of the Sun across the highest point above the horizon (between the two afternoon). For measuring long intervals of time, a year is one revolution of the Earth around the Sun. For measuring small time periods the day was divided into 24 hours, the hours 60 minutes, one minute — 60 seconds. Thus, second is 1/86 400 part of the sun day.

For a long time astronomical observations were the only means of precise measurements of time. With the invention of hours people had the opportunity to reproduce the unit of time. As the development of the hours grew their accuracy; this allowed us to determine, what is the daily rotation of the Earth is not quite uniform and the length of the day varies slightly. Therefore, to establish the units of time were used the average annual solar day, moreover, for certainty was chosen 1900 G., as it turned out, the length of the year is reduced by about half a second per century. So, was taken, what is second 1/86 400 share of average solar day 1900 g.

This definition of the standard second, which in physics is the basic unit of time, uncomfortable, since this standard cannot accurately reproduce. The development of nuclear physics allowed to establish the new benchmark seconds.