While observing Jupiter’s satellites from the Paris Observatory, intending to base an international time standard on their eclipses, he noticed that the times of eclipses disagreed with predictions by G. D. Cassini, varying by 22 minutes over a six-month period. Römer deduced that light took longer to reach Earth when Jupiter was farther away. In 1676 he calculated the speed of light from his timings, using the contemporary value of the astronomical unit, finding it to be about 200 000 km/s. This is two-thirds of the true value, but was the first demonstration that light had a finite speed. He also built the first transit instrument.