Marking Time

In Quasars by Brian Koberlein2 Comments

Radio astronomy is incredibly precise. This is particularly true when they’re used in combination through a process known as Very Long Baseline Interferometry (VLBI). It is so precise that by observing quasars we can measure not only changes in Earth’s rotation, but also tectonic drift between radio telescopes.

The way VLBI works is by measuring how long it takes for fluctuations in a quasar’s light to reach different radio telescopes. Since light travels at a constant rate, the difference in arrival time can be used to determine the difference in distance. Using an array of telescopes you can determine not only the distance between the telescopes, but the precise location of the signal. VBLI can determine the distance between antennas within millimeters, and the position of a radio source to within a fraction of a milliarcsecond.

Because quasars are so distant, they can be treated as fixed points of reference in space. This makes them useful for determining the motion of objects relative to them, such as the precise motion of Saturn, or the Magellanic clouds. But because they are fixed points in the sky, any shift in the location or timing of quasars must necessarily be due to a change in the position of the telescopes themselves, or the rotation of the Earth.

What’s amazing about this technique is that it uses objects billions of light years away to measure millimeter shifts in position on the Earth.


  1. Well, that’s pretty nifty. Awfully considerate of the distant black holes to be nice and active for us humans to calibrate our fancy radio dishes with. If I was a giant space moth, I’d have radio dish eyes.

  2. Fascinating! A relative happened to note this in a family xmas letter 45 yrs ago: “One of the strangest and newest ways to measure distances on earth uses a pair of antennas recording signals from quasars. With a few hours of recording some of the smart scientists at JPL and MIT are getting surveys to within two or three inches. Differences in elevation are measured just as accurately as distances. For this research I am making small changes in some of our antennas, and also putting together a portable station, using an old 30-foot diameter antenna that the US Army gave us. With a portable station we can run up and down California taking a lot of measurements; then a few months later go back to the same spots and see how much they have moved. If strains are developing we may be able to predict where earthquakes are likely. Later we will be into all kinds of geological measurements: continental drifts, islands rising, mountains tipping, for example…”

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