Researchers are turning time

Networked radio telescopes make it possible to correct the world time

Worldwide network: Depicted are the participating stations, each of which observes the Earth's rotation live. The radio telescopes are located in Wettzell (Germany), Ny Ålesund (Norway) and Tsukuba (Japan). In Bonn the measurements are analyzed on a special computer. © IGG
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The official world time is based, among other things, on the changing earth rotation over time. An international collaboration of scientists has now measured this almost in real time for the first time. By networking their telescopes worldwide, researchers can observe and correct the deviation from standard time.


The earth rotates slower every year. This is partly because the tides slow down our planet every day. This creates a conflict between two versions of time: the astronomical time UT1, which is based on the Earth's rotation, and the physical atomic time, which is determined by cesium atomic clocks on Earth. To coordinate both time versions, the world standard time UTC was introduced. It combines both times and gives since 1956 consistently, what time it is. But so that the two time versions do not always continue to diverge, since 1972 the world standard time has been rotated forward by one second every few years to approximate the astronomical time - the so-called leap second. When this leap second is due depends on the Earth's rotation, which therefore needs to be measured accurately, otherwise the difference in time chaos ends.

One second instead of ten days

An international collaboration of researchers from Germany, Japan and Norway can now use radio telescopes to measure the Earth's rotation to an accuracy of about three millimeters and thus determine, almost in real time, how much the two time versions differ. The scientists from the Institute of Geodesy and Geoinformation of the University of Bonn, as well as the Max Planck Institute for Radio Astronomy (MPIfR) and the Federal Agency for Cartography and Geodesy, in collaboration with Japanese and Norwegian researchers, can observe worldwide how the Earth's rotation is changing. Thus, the researchers determine relatively quickly how much the time definitions differ.

That was very different in the past. "In the Gregorian calendar reform in 1582, a whole ten days disappeared at once. At that time on Thursday, October 4th, Friday, October 15th followed, "says Axel Nothnagel from the University of Bonn, project manager and research group:" Today we are already achieving the accuracy of leap seconds every few years to correct the daylength be introduced. "message

Composite of three telescopes

To be able to measure so accurately, three different radio telescopes in Germany, Norway and Japan simultaneously observe the sky for about an hour. The data then flow through fiber-optic cables to the Max Planck Institute for Radio Astronomy in Bonn, covering more than 17, 000 kilometers. The amount of data sent corresponds to 13 fully written DVDs per telescope, each measuring half an hour. "It may seem like little at first, but every week we get over 50 gigabytes of the three telescope stations online, " says Walter Alef, a researcher in the radiointerferometry research group headed by Anton Zensus at the MPIfR.

Deviation redetermined every week

The latest measurements will be used worldwide. That's why they are accessible to all major scientific and commercial institutions on a web server in Paris. From now on, the experiment will provide steady data so that the International VLBI Service for Geodesy & Astrometry (IVS) can regularly determine the Earth's rotation - just hours after the telescopes have looked at the sky. "With this now-new observation we are making an important scientific contribution to the global determination of UT1", says Arno M skens from the University of Bonn.

(MPG, 27.08.2007 - NPO)