Naturalist's Almanac

On June 30 some years a leap second is added to the most precise clocks the human species has yet succeeded in devising. Why? Think of leap year, when we add an extra day to keep our human calendars aligned with the solar year. A leap second involves adding an extra second — or possibly subtracting one — to keep our human clocks aligned with the solar day.

Neither of these fine-tunings was necessary during the earliest period of our evolution when we, like the plants and animals around us, responded directly to the sun. The problems arose when our ancestors began thinking it was important to tell, measure, and keep time.

Early sundials were good at telling time, and hourglasses could measure short periods of it, but neither kept track of its continuous passage. So scientists invented clocks. While calendars required only that the number days agree with the solar year, clocks introduced smaller time units: hours, minutes, and seconds. And as the time units became smaller, precision became more important.

Clocks have been refined over the years, resulting in today's extremely precise atomic clocks. The problem now is that these atomic clocks are too precise. The Earth wobbles and fluctuates as it rotates on its axis and is in fact slowing down. So atomic time can differ from the Earth's rotation time by what can accumulate toward a second.

Because we can't adjust the Earth's rotation, we have to adjust our atomic clocks. A group of extremely attentive observers working at the International Earth Rotation Service decide exactly when we need to add — or subtract — a leap second.

Since June 30, 1972, we’ve added 22 leap seconds, nine of them to June 30 and thirteen of them to December 31. A June 30 leap second is added right after what we in Vermont experience as 7:59 p.m. Eastern Daylight Time. At the Greenwich Observatory it is 23:59:59, then 23:59:60, then 00:00:00 of July 1. A December 31 leap second works the same way, only it’s a tad more exciting because it happens on New Year’s Eve.

June and December are both such busy months for me that I don’t really have time to celebrate a leap second. But I’m grateful to the Earth for making my clocks give me those extra seconds every once in a while to do with what I will.

MORE INFORMATION

National Institute of Standards and Technology
http://www.boulder.nist.gov/timefreq/pubs/bulletin/leapsecond.htm

This link will take you to the National Institute of Standards and Technology’s Physics Lab in Boulder, Colorado. Their Time and Frequency Division is the keeper of official time for the United States. Their explanation of the leap second is short and sweet. It includes a list of the leap seconds that have been added since 1972 as well as a notice about whether or not one is expected soon. Once you’re at this site, you might want to check out their link to CURRENT TIME, where you will learn the exact time in your time zone to within two seconds, and their FREQUENTLY ASKED QUESTIONS section just to see if they’ve answered any questions you’ve been asking.

One-Man Web Site
http://www.leapsecond.com

This is a one-man Web site full of information about time. His home page includes several interesting links: <http://www.leapsecond.com/notes/whyls.htm> takes you to his brief discussion of leap seconds. <http://www.leapsecond.com/java/nixie.htm> takes you to a clock that’s counting down to the next leap second. And <http://www.leapsecond.com/java/gpsclock.htm> takes you to a page that uses your computer’s time to generate five different kinds of time. This page also offers numerous links to other Web sites that have information about leap seconds.

U.S. Naval Observatory's Time Service
http://tycho.usno.navy.mil/leapsec.html

The U.S. Naval Observatory’s Time Service Department offers the most detailed and authoritative discussion of leap seconds.