Guest Author - Violette DeSantis

This July a notice was sent out by a group called the International Earth Rotation and Reference System Service (IERS) to announce the addition of a leap second. IERS is located in Paris, France and it is a group of astronomers, engineers and mathematicians who decide if the earth’s rotation on its axis has slowed down enough for us to need another second added to the world’s clocks.

Twice a year data is studied to see if the leap second should be added. In July they decide if it is necessary to add a second to the last second in December before the start of the New Year. In January they consider if a leap second should be added to the last second in June.

A second is added or taken away to keep the world’s clocks in sync with the Earth’s rotation. Since the program began in 1972, they’ve never had to take a second away. They have had to add 23 seconds though, one each to the following months and years.

June 30, 1972
December 31, 1972
December 31, 1973
December 31, 1974
December 31, 1975
December 31, 1976
December 31, 1977
December 31, 1978
December 31, 1979
June 30, 1981
June 30, 1982
June 30, 1983
June 30, 1985
December 31, 1987
December 31, 1989
December 31, 1990
June 30, 1992
June 30, 1993
June 30, 1994
December 31, 1995
June 30, 1997
December 31, 1998
December 31, 2005
December 31, 2008

When the second, known as a positive leap second, is added in, it looks like 23:59:60 or 11:59:60 PM.

So what makes the Earth slow down each year? Friction of the tides which are caused by the motions of the Sun, Moon and Earth. The Sun and Moon have an effect on the tides based on their location to each other and the Earth. When the Sun and Moon are opposite each other a Spring Tide occurs, where the Earth has maximum and low tides. When the Sun, Earth and Moon are at right angles minimum high and low tides occur called Neap Tides. An easy way to remember the difference of the two is that Spring Tide is during the full moon and the Neap Tides are during the half-moon.

All of this causes the Earth to slow down about 2 milliseconds a day putting our time difference every 500 days to 0.9 seconds. The leap second was created to make sure the time difference does not exceed this number.

Adding the leap second to the end of 2008 makes it the longest year since 1992 since both these years are leap years with an added leap second.