What is a leap second?
The Earth’s rotation around its own axis is not regular, as sometimes it slows down and sometimes speeds up due to various factors, including moon’s gravitational forces and earthquakes. As a result, astronomical time (UT1) gradually falls out of sync with atomic time (UTC). When the difference between UTC and UT1 approaches 0.9 seconds, a leap second is added to UTC through atomic clocks worldwide to match both times.
