Leap Second June 30th
UNH-IOL Staff - June 27, 2012
Been feeling unsteady on your feet lately? Take heart, you’re not sick. The earth is slowing down. The rotating guys whose job it is to keep the earth rotating (like those featured in this Monster.com commercial Guy with Big Legs on YouTube: http://www.youtube.com/watch?v=RBZmLzv9NKQ) have been slacking, so now at midnight every night, each point on the earth’s surface is arriving at its proper location a half second late. In effect, when our clocks say it’s midnight, astronomers say the middle of the night has not quite arrived. To compensate for this deteriorating state of affairs, the folks at (where else) the Earth Rotation Service (a.k.a. International Earth Rotation and Reference Systems Service) have decided that it will be easier to have everyone add a second to their clocks than to push the earth around a little faster.
Therefore, for the first time in 3½ years, a leap second will be added to Coordinated Universal Time (UTC) this Saturday night, June 30th, at midnight.
Technically, what will happen is that two different time standards will be synchronized. The most common standard—UTC—is based on seconds defined by atomic clocks. Another standard—UT1—is based on the earth’s rotation. Since one earth rotation does not take an exact multiple of atomic-clock seconds, adding a second every few years will prevent UTC midnight from creeping away from astronomical midnight and eventually occurring during daylight hours thousands of years from now.
We in the IOL’s IEEE 1588 Precision Time Protocol (PTP) Consortium are interested in this event because PTP has a special header field just for leap seconds. Two clocks can be synchronizing with each other for years without using the leap second bit, but then on “leap second day” master clocks will set that bit to true from noon to midnight, telling slave clocks to prepare.
You may find it interesting to stay up this Saturday night and watch your computer clock. If it behaves properly, instead of displaying the normal sequence 23:59:59, 00:00:00 it will display 23:59:59, 23:59:60, 00:00:00. The ISO 8601 time format standard specifies that minutes will always be in the range 0-59, but it allows seconds to be in the range 0-60 for just this special case.
On a unix computer you can monitor the system time with this command:
$ watch -n 1 “date +%s; date”
From a web browser on any computer you may find it interesting to watch here:
Don’t believe people when they tell you last Wednesday (summer solstice) was the longest “day” of 2012. The longest day will be this Saturday. Just think, you get to sleep in an extra second.
Jeff Laird, Research and Development