Tag Archives: Leap Second

The Days Keep Getting Longer … Literally

Keeping track of time is never easy without an accurate clock and so people have come up with a number of different folk methods to keep themselves on pace. One of the most common techniques is to introduce a multi-syllable word as you count seconds so that you don’t count too fast. The most familiar phrase is probably something like “A thousand one, a thousand two …” but there are several others. My Dad actually had a teacher in school that used the phrase “steam engine” and I’ve heard others use words like “Mississippi” or even “alligator.” Basically, any four or five syllable phrase will serve as a good placeholder. Whatever phrase you favor, be prepared to dust it off tonight as the world is officially given an extra leap second at the end of the day.

The reason for this extra second is rather complicated. A normal “day” was officially defined back in 1967 as 86,400 seconds in the International System of Units (SI) and it is tracked by a very precise atomic clock. This is the Universal Coordinated Time (UTC) that we all know and love. The actual solar day is pretty much the same length but not quite. There are several different events that can speed up or slow down the Earth’s rotation by a few thousandths of a second. These events can include earthquakes, changes in the jet stream, the tidal pull of the Moon, the position of the Earth in its orbit, fluid motion at the Earth’s core, and the gradual slowing of the Earth’s rotation.

Whenever these forces cause the solar day and the UTC to get too far out of whack, the Sub-bureau for Rapid Service and Predictions of Earth Orientation Parameters of the International Earth Rotation and Reference System Service — let me pause here while I catch my breath — calls for a leap second. This manifests itself as an additional second tacked on to the normal clock reading around midnight (11:59:59 –> 11:59:60 –> 12:00:00). This whole process is essentially designed to keep the sun directly above you at high noon.

Pretty cool, eh?

What’s really interesting about this issue is that, in the long run, it doesn’t really matter because the Earth’s rotation is slowing by a few fractions of a second each year and the standard Earth day continues to get longer. This is one of those weird facts that kind of blew my mind when I first heard it. I guess I had read too many science fiction stories where the hero hops into his time machine and goes back to some ridiculously precise date like 10:24 AM on Tuesday, August 13, 250,000,000 B.C. In reality, our current concept of days and dates are firmly based on the Earth’s current circumstances. Back in dinosaur times, the typical Earth day was an hour or two shorter and there were an extra 10-20 days in the year (the length of the year was the same overall). When the Earth was really young, days were only six hours long and there were over 1,000 of them per year.

In order to visualize this, I found a paper online which provided me with a model that estimates the length of a day and the number of days per year for any time period. I’m not sure how official these calculations are but they do appear to correlate with data obtained from fossil corals and radiometric dating methods. I’ve included information on each geologic period from Wikipedia, so use with the appropriate amount of caution.

Anyway, enjoy your leap second! One steam engine …

Update: