Title: The Future of Time: UTC and the Leap Second
Authors: David Finkleman, Steve Allen, John H. Seago, Rob Seaman, P. Kenneth Seidelmann
First Author’s Institution: Center for Space Standards and Innovation
What does a day in the life of a human mean? Many philosophers have spent lifetimes pondering this question and the ontological threads that branch from it. There is not enough time to go into the conversation here, but we can still reflect on how the quantitative nature of the day has changed over the course of human history.
Since antiquity, the concept of a day has perhaps always been the most fundamental, measured by the time it takes the sun to transit the meridian, set, rise, and return to the meridian. The month was a longer period of time that relied upon the phases of the Moon (perhaps we should call it a moonth…). The year measures the time it takes the Earth to orbit around the sun, or–if you weren’t yet a card-carrying member of the heliocentric school of thought–the time it took for the sun to return to the same place against the constellations. A favorite device of sci-fi writers is to create extra-terrestrial societies with disparate years, months, and days that differ from those of Earth (and which would be entirely realistic). None of these units of time depend explicitly on the other, yet we use them to segment our lives into units of time, and because they do not fit perfectly into each other, the remainder of the division is the domain of leap years.
The units we use to measure time have until recently been always derived from the movements of the heavens, which we have considered immutable because we have never had the accuracy of measurement to notice otherwise. Starting with the Babylonians, we divided our days into neatly divisible blocks of hours, minutes, and then seconds. The advent of extremely accurate atomic clocks that rely upon the hyperfine quantum transitions of Cesium-133 atoms have challenged our traditional notions of time.
Now might be a convenient time to begin watching the accompanying World Wide Telescope tour below. If you are reading on an RSS reader, go here: http://youtu.be/K66CzkPq4sU
If we use a sundial to measure the time of day, we would be measuring the apparent solar time (which is not uniform throughout the year). Once you’ve watched the tour, consider checking out this neat interactive demonstration of analemmas by Wolfram.
However, if we were going to stretch this idea of the apparent solar day to be consistent over the year, we would reach the current idea of the mean solar day, which is our normal concept of the day as measured by our wristwatches.
Astronomers have long used the concept of a sidereal day, which is the time it takes for the Earth to make one rotation against the background stars. As seen in the tour, because the Earth moves in its orbit around the sun, a sidereal day is only 23 hours and 56 minutes long.
The onset of atomic timekeeping has created a situation that is untenable and unsustainable. Clocks are so accurate that we can now detect the slowing of the Earth’s rotation. Using these clocks, we can define a uniform time tied to atomic (and presumably constant) processes rather than to dynamic processes like the Earth’s orbit around the sun or the Earth’s rotation, which we know can easily change over long periods of… time.
In their article in the July-August 2011 issue of American Scientist (posted to the arXiv), Finkleman et al discuss the nature, measurement, and future of our concept of time. Because the accuracy of atomic clocks make the drift in the Earth’s rotation period apparent (due to the Moon and lunar tides) corrections are needed if we wish our clocks to be synchronized with the motions of the sun. It wouldn’t make much sense if 11pm actually corresponded to what we now consider noon. Thus, since 1972, leap seconds have been occasionally added to keep these measurements synchronized.
As one can imagine, adding these leap seconds to every clock in the world is not straightforward. One would hope that a normal person wouldn’t mind if their wristwatch were only a second off (in fact, most of us might wish for that much accuracy…) but our GPS satellites and communication networks care very much. Sub-second accuracy is essential to their functionality. So how do we all synchronize these clocks? Not every timekeeping device is capable of just inserting a leap second (11:59:60 pm) on command.
An organization affiliated with the United Nations, the Radiocommunication Sector of the International Telecommunications Union (ITU-R), has proposed that we eliminate the difficulty of leap seconds all together–let the clocks diverge from the heavens!
Tweaking calendars is not a new practice conjured up for the atomic age. Institutions like churches, governments, and emperors have changed their calendars many times to account for errors in time keeping. Regularly we join the likes of the Pope Gregory XIII, Sweden, and Julius Caesar!
The World Wide Telescope (WWT) tour was created by Katherine Rosenfeld and Ian Czekala. Microsoft’s telescope software lets you interactively explore the universe. We’ve also entered our WWT tour in the Summer WWT Tour contest. If you’d like to give it a shot, you can make your own tour too.