Ole Roemer and the Observatoire

For the "Invisible Universe" conference, Paris Observatory was location of a welcoming ceremony Sunday and a farewell cocktail Friday. The grounds contain a line marking the Prime Meridian. For many years navigators disagreed whether to measure longitude from here or Greenwich. Eventually Britain's mastery of the seas led the world to adopt the Greenwich Observatory as Prime Meridian.

For centuries scientists disagreed whether light had a finite speed at all. Aristotle and even Kepler believed that light travelled instantaneously. This was once a reasonable assumption, for light travels so fast that to most observers it seems instantaneous. Galileo suggested stationing observers on distant hilltops with lanterns to determine if there was a time delay. This would have been difficult for Galileo, because a good clock had not been invented!

Young astronomer Ole Roemer came to work at this observatory in 1672, working for Dominic Cassini. At the time there was an anomaly in observations of the Galilean moon Io. The time when the moon emerged from behind Jupiter appeared strangely delayed. Cassini ordered his staff to make more precise observations to resolve the puzzle. Roemer realized that, if light had a finite speed it would take time to cross Io's orbit, delaying the moon's apparent emergence. Using data from this observatory, Roemer was first to measure the speed of light.

Having made a great discovery, Roemer was unable to convince his elders. Cassini was a distinguished astronomer in his own right, but believed that light travelled instantaneously. Finally in 1675 Roemer was bold enough to present the results on his own. He also predicted that on November 9, 1676 Io would appear at 5:35:45 rather than 5:25:45 as astronomers had calculated. Though Roemer's prediction was correct, Cassini and others who followed him insisted that there was no speed of light. It was 50 years before other experiments proved that light had a finite speed.

As scientists once believed that light travelled instantaneously, today they say that its speed is finite but fixed. This was once a reasonable assumption, for channge in c is so slow that to most observers it seems constant. Observations of high-redshift supernovae indicate that light has slowed over time. This result is corroborated by measurements of solar luminosity and light signals reflected from Earth's moon. We will not have to wait 50 years to prove that the speed of light is slowing.