The slowing of Earth’s rotation varies irregularly, and so the number of leap seconds by which UTC must be retarded to keep it in epoch with UT1 cannot be predicted years in advance. To remedy this discrepancy, UTC is kept within 0.9 second of UT1 by adding a leap second to UTC as needed the last minute of December or June is made to contain 61 seconds. Because of the irregular slowing of Earth’s rate of rotation by tidal friction and other forces, there is now about one more (atomic clock-derived) second in a solar year than there are UT1 seconds. UTC is thus kept within an exact number of seconds of International Atomic Time and is also kept within 0.9 second of the solar time denoted UT1 ( see Universal Time). UTC serves to accommodate the timekeeping differences that arise between atomic time (which is derived from atomic clocks) and solar time (which is derived from astronomical measurements of Earth’s rotation on its axis relative to the Sun).
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