What Do Timekeepers Do?

Listen to radio waves; drink café au lait.

On New Year’s Eve at 6:59:59 p.m. ET, an “international consortium of timekeepers”will add one second to the world’s clock. How do you get to be an official timekeeper?

Earn a Ph.D. in astronomy and move to France. Tweaks to the official clock are announced by the Earth Orientation Center, a Paris-based subunit of the International Earth Rotation and Reference Systems Service. The IERS was established in 1987 by two professional associations comprising thousands of astronomers and geodesists (people who measure the Earth and its movements) around the world. It has no dedicated staff or payroll, and it exists merely as a group of government agencies, universities, and foundations that have agreed to share data on the position of celestial bodies and ensure that our clocks are consistent with the Earth’s rotation. Duties are divided among the member institutions: As the parent institution of the EOC, the Paris Observatory is responsible for deciding when to adjust the world’s clocks. The task of data collection is shared among other facilities around the world.

So if you wanted official control over adding a leap second, you would have to convince the Paris Observatory board of directors to make you the director of the EOC. Even then, you’d have little discretion in the matter—the decision to push the second hand is automatically triggered when the world’s clocks fall behind the Earth’s actual rotational speed by more than 0.9 seconds. (All the director does is send out the official memo.) If you’re interested in the day-to-day work of monitoring the Earth’s rotation, you’d do better to seek employment at the U.S. Naval Observatory or other IERS member institutions where the data is actually collected. If you had the right credentials—e.g., a degree in astronomy or geodesy, with a focus on the behavior and orientation of the Earth—you might get hired for the job.

Timekeepers calculate precise and universal clock values with an array of radio telescopes located in Hawaii, South Africa, Brazil, Australia, and other locations, and focused on distant galaxies, called quasars. * Every day, astronomers at each telescope fill a series of hard drives with exact data on the radio signal from those quasars and ship them via common carrier to the other IERS institutions. Each institution compares the signals recorded at all the telescope sites and uses the differences to compute the speed of the Earth’s rotation. Their calculations normally agree to within a few microseconds.

The system requires the close cooperation of scientists around the globe. Astronomers formed the first transnational society to observe polar motion in 1895, called the International Latitude Service. In 1919, the International Time Bureau was established in Paris and became responsible for adjusting a universal clock. (Until then, those decisions were left up to individual countries.) The bureau retained this authority until it was replaced by the IERS in the 1980s.

Got a question about today’s news? Ask the Explainer.

Explainer thanks Dennis McCarthy of the U.S. Naval Observatory. Thanks also to Laurie Gabik for asking the question.

Correction, March 3, 2009: This article mistakenly described quasars generically as objects.( Return to the corrected sentence.)