Sometimes it takes a tiny yardstick to measure a big distance.

Determining precisely how far away objects in the sky are ranks among the biggest problems facing astronomers. But a group of scientists using the Chandra X-ray Observatory in Earth orbit have come up with a clever new way of using the very small waves of X-rays to measure distance.

By looking at how X-ray radiation from objects like neutron stars and black holes is scattered by interstellar dust, the researchers can much more precisely measure the distances to objects in distant parts of the universe.

When radiation like visible light or X-rays passes through clouds of dust or water vapor, the source appears to have a halo around it due to some of the light being reflected off the particles in the cloud. Since this reflected light then has a longer way to travel, it therefore is slightly delayed. That means the halo of light around a traffic light seen through fog is actually arriving at your eyes fractions of a second later than the light that is not bent by obstructions.

"When the light switches from red to green (or vice versa), the halo around the light is also slightly delayed," said Peter Predehl of the Max Planck Institute, Garching, Germany, the lead author on a report to be published in the European journal Astronomy and Astrophysics.

"No one would use this delay for determining the distance to the traffic light, of course [the delay is only a few billionths of a second]. But if the 'traffic light' is 30,000 light-years away, the delay is on the order of 15 minutes. Using the excellent and unprecedented resolution of the Chandra observatory, we can distinguish between light that was 30,000 years on its way and other light that needed only a few minutes more."

The "traffic light" the scientists used is an X-ray source -- probably a neutron star or black hole that is in a stable orbit around a companion star. By watching how the X-rays it emits are delayed during different times in its orbit, scientists were able to determine the difference between when it was closer and further from the Earth. X-rays are a particularly good type of radiation to use for this kind of measurement, since their high energy means they arent deflected very much when they hit intervening particles of dust. That means the X-ray halo is delayed by a very small amount, which gives the scientists a very close approximation of the distance to the target.

This technique was actually proposed some years ago, but it was only when the highly sensitive Chandra Observatory came online that it was able to be tested.