Leap Seconds: A Brief Introduction

Leap Seconds
Screenshot of the UTC clock from time.gov during the leap second on December 31, 2016. In the U.S., the leap second took place at 18:59:60 local time on the East Coast, at 15:59:60 local time on the West Coast, and at 13:59:60 local time in Hawaii.

Unlike leap years, which require adding an extra day to the calendar every few years to keep the calendar year synchronized with the astronomical year or seasonal year, leap seconds are not utilized on a regular schedule because variations in the length of the day are not entirely predictable.

Leap seconds are a one-second adjustment that is occasionally applied to Coordinated Universal Time to accommodate the difference between precise time measured by atomic clocks and imprecise observed solar time. The imprecise observed solar time does vary because of irregularities and long-term slowdown in the Earth’s rotation.  It is worth noting that over time, the day lengths will increase, but at a slow rate of an amount close two thousandths of a second per century. This means that atomic clocks would be not perfectly aligned with the current time by a very short amount over a period of several thousand years.

Insertion of each UTC leap second is usually decided about six months in advance by the International Earth Rotation and Reference Systems Service (IERS), to ensure that the difference between readings will never exceed 0.9 seconds.

This practice has proved disruptive, particularly in the twenty-first century and especially in services that depend on precise time stamping or time-critical process control. The relevant international standards body has been debating whether or not to continue the practice, with an increasing number of nations supporting its abolition. Moreover, the practice of inserting leap seconds is under review by the relevant international standards body to determine if it can be eliminated or changed.



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