David Nightingale: Ole Roemer (1644 – 1710)

Jul 8, 2018

A diagram of Jupiter (B) eclipsing its moon Io (DC) as viewed from different points in earth's orbit around the sun.
Credit Ole Rømer

When I first heard about Ole Roemer I fleetingly thought the speaker was being overly familiar – but not so. Ole is a Norse and Viking name. For example, there is the Norwegian-born Ole Evinrude, famous manufacturer of outboard engines for boats.

This essay is about a Danish man born in 1644, just after Galileo died, and who became a student in the University of Copenhagen. Copenhagen was once a little fishing village, across the water from Sweden, (the two countries are now connected by a bridge) but is now the capital of Denmark – although with a population still less than a million. So why is Ole Roemer so highly thought of today?

Well, in those years, people generally believed that light propagated instantaneously. Hmm; it does, doesn’t it? Galileo himself had even tried to measure it with a friend, using lanterns on opposite sides of a valley, but with no success. He too had concluded that the speed of light was either very fast or perhaps infinite.

At this point in our story remember that Galileo, with his self-made telescope had clearly seen 4 of Jupiter’s moons. But to Ole, any one of these moons was like a cosmic clock. The fastest moving one was little Io, orbiting exactly every 1.77 days.

After Ole Roemer graduated he went to France, where Louis XIV employed him as a tutor for the Dauphin, or heir-apparent to the French crown, and in his spare time he was able to watch Galileo’s little moons as they passed in front of Jupiter in their very regular way. He watched the circuits or orbits of little Io over some years, each circuit taking, as we said 1.77 days, and he could predict when the eclipses would occur, even years later. But he noticed that the tiny planet was sometimes early, sometimes late. How could that be?

He suspected that this delay or lateness was because light was taking a finite time to travel an extra distance. What extra distance? Well, as we ourselves go round the sun, we may typically (for example) observe Jupiter when we are closest to it, and 6 months later when we are farthest away. That extra distance is just the diameter of the Earth’s orbit around the sun, which Ole knew, from Galileo’s and Newton’s work, although not very accurately.

So the key idea was that the eclipses were running late when we were furthest from the sun. The eclipses or circuits or orbits still took 1.77 days, but they weren’t occurring when expected – at worst the lateness he noticed was about 20 minutes.  (Today we know that the delay is nearer 17 minutes.)

So Ole said that the speed of light had to be just the extra distance of our orbital diameter divided by his 22 mins.

He returned home when he was about 37, and his estimate of the speed of light prompted his elevation to Professor of Astronomy.

This, by the way, is actually not all he did. For a while he was Copenhagen’s chief of police. He also developed a simple temperature scale for the range between ice and steam.

Thus Ole Roemer was the very first to give us a value for the speed of light. His value was somewhat less than what we know now, and in ensuing centuries many other (and more accurate) methods were devised.

Today, it’s something students can do quite easily in a physics lab.

David Nightingale is Professor of Emeritus of Physics at the State University of New York at New Paltz. His latest book is A Kitchen Course in Electricity and Magnetism, published by Springer, New York.

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