Leap Second

About Leap Second

• It is used as a measure to combat the long-term slowdown in the Earth’s rotation which is caused by the constant melting and refreezing of ice caps.
• It is added every now and then to Coordinated Universal Time (UTC) in order to synchronize a clock worldwide with the Earth’s ever slowing rotation.
• The system of leap seconds was introduced in the early 1970s. So far, 27 positive leap seconds have been added.
• UTC consists of a time scale that combines the output of more than 300 highly precise Atomic clocks worldwide. Atomic clocks are very accurate and are stable within 1 second over a period of millions of years.
• On the other hand, the Astronomical Time known as Universal Time (UT1) refers to the Earth’s rotation around its own axis and determines the length of a day.
• Reason for addition: The Earth’s rotation around its own axis is not regular, as sometimes it speeds up and sometimes it slows down, due to various factors including the moon’s gravitational Earth-braking forces that often results in ocean tides.
• As a result, Astronomical Time (UT1) gradually falls out of synch with Atomic time (UTC), and as and when the difference between UTC and UT1 approaches 0.9 seconds, a “Leap Second” is added to UTC through Atomic clocks worldwide.
• A leap second is normally inserted either on June 30 or December 31.

What is Negative Leap second?

• It is a second that is subtracted from our clocks to keep them in sync with the Earth’s rotation.
• Till date no negative leap second was introduced because, in the last few decades the Earth’s rotation has generally been a bit slow
• The International Earth Rotation and Reference Systems Service (IERS) monitors the Earth’s rotation, and takes decisions on when to add or subtract a leap second.
• Since Earth is spinning faster than usual recently, timekeepers had thought of using negative leap seconds for the first time.

In other words, they thought of subtracting leap seconds from our clocks to synchronise them with Earth’s rotation.

Q1) What is Greenwich Mean Time (GMT)?

It is the mean (average) solar time at the Greenwich Meridian or Prime Meridian, 0 degrees longitude. The time displayed by the Shepherd Gate Clock at the Royal Observatory in Greenwich, London, is always GMT.

Source: Melting ice sheets may postpone need for ‘negative leap second

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