Slipping in a Second

I have good news for all us summer-starved Alaskans who wish the month of June could be longer: for 1994, June has been extended--by one second.

I wish I could claim this boon came by popular demand, but it's merely a decision from the temporal powers and had nothing to do with democratic desires. Instead, it's the recognition of a certain amount of wear and tear on Earth's rotation.

When you think about it, circling around a stellar body while rotating about one's axis should be a fairly stressful experience. As Earth makes its annual orbit around the sun, it's smacking into all matter of celestial debris, from atoms of hydrogen to chunks of asteroid. It's dragging along on this yearly traverse a honking great satellite---our moon is uncommonly large in relation to its planet when compared to the satellites of the other planets of this solar system. And Earth has a great deal of stuff sloshing about, from the oceans and atmosphere of its surface layers to its fluid core. Various contributors of friction, gravitational tugs and pulls---these can add up to plenty of causes for occasional minor adjustments in the clock.

Given enough time, the adjustments would be more than minor. Overall, Earth has slowed its spin. When dinosaurs ruled, they did so during days only about 22 hours and 45 minutes long.

"Our clocks must be adjusted to stay in pace with Earth's rotation if we want to continue to see the sun in the daytime hours," according to astronomer Bill Klepczynski of the U.S. Naval Observatory's Time Service in Washington, D.C. Klepczynski was quoted in the June issue of Natural History magazine, which is where I learned about the additional second for June.

Accepting the length of a day as right around 24 hours is good enough for most human purposes, but it is too sloppy for precise navigation. Astronomers became keepers of official time because they observed the passage of the stars overhead, a passage that marked the daily turn of Earth exactly. In the early days of the nation, the Naval Observatory had the job of adjusting the chronometer of every ship in the fleet before it sailed for the high seas.

Better instruments permitted greater precision, and the discrepancies between the clock's measurements and Earth's rotation became more apparent. Sometimes better technologies demanded greater precision as well.

"In electronic navigation, a time error of a millionth of a second can produce a position error of about a quarter of a mile," Klepczynski said.

It's easy to imagine that scale of error putting a ship on a reef, but it could also put a jet plane off a runway or a guided missile off target.

So, in our efforts to keep more accurate time, humans have outgrown the abilities of the pendulum clock and even the vibrating quartz crystal. We now count the regular oscillations of atoms. Since 1967, the definitive second has been internationally accepted as exactly 9,192,631,771 oscillations of a cesium atom. Exactly. Not a single oscillation more or less, no matter how tempting rounding it off might be.

And that is precisely the amount of extra time June 1994 has been granted by decision of the International Earth Rotation Service in Paris. The second will be officially inserted between the last second of June 30 and the first second of July 1, Universal Time. Because Universal Time, or UT, was formerly known as Greenwich Mean Time, or GMT, that must be taken as 23:59:59 at the longitude of the Greenwich Observatory in England.

Alaska is nine hours behind the Greenwich meridian, so we should take that extra second during the afternoon here. Pause for a second---precisely a second---just before 3 p.m. on the last day of June this year, and enjoy the gift of time.