Last weekend, a rare alignment of the Earth, Jupiter and a group of distant stars gave physicists and astronomers a rare opportunity to directly measure the speed of gravity.

As telescopes all over the world watched, Jupiter moved briefly in front of a collection of stars, bending their light as it passed.

As light passes by a very large object, it gets bent by the object’s gravitational field, making a star appear in a slightly different position in the night sky.

Einstein’s theory of general relativity predicts that gravity should exert its effects at the speed of light. By very carefully measuring the positions of the stars as Jupiter moves in front of them, astronomers should be able to infer the speed of gravity.

But Jupiter’s gravitational field bends light from the stars by only a few billionths of a degree as the planet passes in front of them, which means the measurements had to be very precise.

“It’s a fairly straightforward experiment,” said Prof. Charles Dyer of the department of astronomy and astrophysics at the University of Toronto.

“It will require very careful handling of the data to see the effect at all, and to know that any deviations which might appear are not caused by [errors].”

The results of the measurements will be published in a few months.