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    Question ID:   1756         Current Version: 1
Question: What is a leap second?
Category: Science > Physics
Keywords: Coordinated Universal Time, UTC, definition, define, time, leap, second
Type: what
Rating:(0 ratings)    Views: 1068    Discussions: 0   In Watch Lists: 1  

 
    Answer:
A leap second is a second added to Coordinated Universal Time to make it agree with astronomical time to within 0.9 seconds. UTC is an atomic time scale, based on the performance of atomic clocks. Astronomical time is based on the rate of rotation of the Earth. Since atomic clocks are more stable than the rate at which the Earth rotates, leap seconds are needed to keep the two time scales in agreement.

The first leap second occurred on June 30, 1972. There have been a total of 18 leap seconds to 1994. This means that leap seconds occur at a rate of slightly less than one per year. Although it is possible to have a negative leap second (a second removed from UTC), so far, all leap seconds have been positive (a second has been added to UTC). Based on what we know about the Earth's rotation, it is unlikely that we will have a negative leap second in the foreseeable future.

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    More Readings :
[QID:1755]    How is the second defined?  
[QID:1758]    What is an atomic clock?  
[QID:1760]    How long are a nanosecond, a picosecond or a femtosecond?  
[QID:1768]    What is the origin of hours, minutes and seconds?  
[QID:1757]    Was the year 2000 a leap year?  
[QID:1763]    How is Coordinated Universal Time (UTC) currently calculated?  
[QID:1765]    How does Global Positioning System (GPS) time differ from Coordinated Universal Time (UTC)?  
[QID:1754]    Is Coordinated Universal Time or UTC the same thing as Greenwich Mean Time?  
[QID:1764]    What time does NIST distribute through its time and frequency services?  
[QID:1293]    What is the 2nd largest whale?  
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