When I was growing up in the dim and distant twentieth century spin-off was one of the most frequently used buzz words in the public discussion of science and technology; a spin-off being an unintended and unexpected positive product of scientific or technological research. Politicians would use the term to justify high levels of expenditure on political prestige projects claiming that the voters/taxpayers would benefit through the spin-offs from the research. The example that was almost always quoted by the media was that the non-stick coating for frying pans was a spin-off from the space programme. This is, like many popular stories in the history of science and technology, actually a myth but that is not the subject of this post.

Now spin-offs are not a modern phenomenon but have been turning up ever since humans first began hammering bits of stone to make tools and as the title of this post suggests, the first successful scientific determination of the speed of light was actually a spin-off from a project to find a more accurate way to determine longitude.

The speed of light had been a problem since at least the beginning of the ancient Greek study of optics. The Greeks themselves were split into two camps on the subject. Some like Empedocles, on whose shoulders the beginnings of much of Greek science rests, thought that the speed of light was finite arguing that light was something in motion and therefore required time to travel. Heron of Alexandria, a representative of the geometrical school of optics, thought that the transmission of light was spontaneous, the speed thus infinite, because extremely distant objects such as stars appear instantly when we open our eyes. Down through the ages those writing on optics took one side or the other in the argument. In the seventeenth century two of the most important optical experts Kepler and Descartes both argued for an infinite speed of light . Galileo and Isaac Beeckman, who both thought the speed of light was finite, proposed, and may have carried out, experiments to try and determine the speed of light but were of course defeated by its actually extremely high velocity and their very, very primitive timing devices. The actual solution came from the astronomers and it was Galileo who unwittingly set the ball rolling. [Modified 26.09.2013 I done screwed up again! See comments]

In 1610 Galileo and Simon Marius both discovered the four largest, or Galilean, Moons of Jupiter, Io, Europa, Callisto and Ganymede. The orbits of the four are all relatively short and they disappear and reappear from behind Jupiter in a complex but regular dance. Galileo realised that if one could determine the orbits accurately enough then one could use these disappearances and reappearances (eclipses of the moons) as an astronomical clock in order to determine longitude. One would need to create an accurate table of the time of the eclipses for a given prime meridian then in order to determine the longitude of a given point the cartographer-astronomer only needs to determine the local time of the occurrence of one of the eclipses look in his tables to calculate the time difference and thus the longitude difference to his prime meridian. Having thought up this, actually quite brilliant, idea Galileo, never one to pass up a chance to shine and at the same time earn a fast buck, tried to sell it first to Spain and then to Holland; an interesting combination as the two countries were at war with each other at the time. Both sales pitches failed and Galileo never actually produced the necessary tables. Fast-forward about fifty years to Paris.

Ensconced in the new observatory in Paris and equipped with far superior telescopes to those of Galileo, Europe’s star astronomer, Giovanni Domenico (Jean-Dominique) Cassini took up the task abandoned by Galileo and produced the necessary tables to a high enough degree of accuracy to enable the French astronomer-cartographers to accurately determine longitude. (I should point out that this method is impractical at sea as the accurate telescopic observation of the moons of Jupiter on a rolling ship is well-nigh impossible). This is a pan European story. We started in Germany and Northern Italy then moving on to Paris we now take a short diversion to Denmark to meet Ole Rømer.

Ole Rømer was born in Århus on the 25th September 1644. In 1662 he started studying at the University of Copenhagen under the mathematician and physician Rasmus Bartholin. In 1671 the French astronomer-cartographer Jean Picard went to Demark to accurately re-measure the latitude and longitude of Tycho Brahe’s observatory on the island of Hven, using the moons of Jupiter, in order to better integrate Tycho’s observation into those made by the observatory in Paris. Picard took Rømer with him to Hven as an assistant. Much impressed by the young Dane Picard offered to take him back with him to Paris. Given the chance of working at the world’s leading centre for astronomical research at that time, Rømer didn’t hesitate, packed his bags and was soon installed as an assistant to Cassini at the Paris Observatory.

A problem had turned up in the eclipse tables for the moons of Jupiter and Rømer took part in the observation programme to try and determine where the error lay. His observations showed that the period between the eclipse of Io got shorter as Earth got closer to Jupiter and longer as Earth moved away. Over a period of eight years Rømer observed and accurately calculated the delay in the eclipse time, which were in fact due to the finite speed of light and the differences in the distance that the light from Io must travel depending on the relative positions of Earth and Jupiter. On the assumption that this was indeed the cause and that the speed of light was finite Christiaan Huygens calculated it from Rømer’s figures producing the first ever scientific calculation of the speed of light. The figure at about 200 000 km per sec is too low and was not universally accepted as many still believed that the speed of light was infinite. The matter was finally settled as James Bradley discovered stellar aberration in the 1720’s and used it to calculate a more accurate figure.

Rømer returned to Copenhagen in 1681 as professor of astronomy at the university, where he made further minor contribution to the sciences. However he’ll always be chiefly remembered as the man who first determined that the speed of light is finite and produced a measure of that speed.