What is a leap second?

Answer 1

The earth's rotation is slowing down. Yes, it's true; tidal forces between the Earth and the Moon are slowing down the rotation of the Earth.

It is by a tiny fraction of a second per day, but after a few years, it gets to be measurable. (Not noticeable, but with the super-accurate atomic clocks these days, our clocks are more accurate than the Earth is! ) In a few hundred million years, the day will be 25 hours long. So every few years, in consultation with other time-keeping agencies around the world, a "leap-second" is added to December 31, making the minute from 11:59 PM to 12:00 AM last 61 seconds rather than 60 seconds. The primary purpose of this is to keep the navigational beacons and GPS satellites accurate.

It's time to do this, so today will last one second longer than yesterday did. Tomorrow the day will be back to the normal length. Enjoy your extra second!

AlternativeThe continuing slowing down of the Earth is a part of the leap second picture, but it is not nearly the most important part at present. The main reason for leap seconds is because of how the "second" used by "atomic clocks" was defined.

Please see the related questions, particularly the one that starts "If we need to add...".

The adding of leap seconds does represent a cumulative slowing down of the Earth over time, but not in a simple way. It is a slightly tricky idea, but you will understand once you check the linked questions.

Also, see "related links" below for nice explanations from good sources.

The first part of the second link is an especially clear explanation.

Answer 2

It hasn't been determined yet. The last leap second was a positive leap second on December 31, 2008. No leap second is scheduled for December 31, 2009 and no determination has been made yet for leap seconds in 2010.