Vol. 17
March Issue
Year 2016

Off the Beaten Track

in Vol. 17 - March Issue - Year 2016
The Speed Of Light

Apparent duration of Io's orbits seen from Earth

Aberration of light

On the outskirts of Paris in the middle of the seventeenth century, a man stared intently into the eyepiece of his telescope, patiently waiting to witness an event that he would then scrupulously record in his notebook. At the same moment, on an island off the coast of Denmark, another man awaited the same event, his own telescope also pointing towards the heavens. Both men were observing the mighty planet Jupiter. What was it that kept their attention riveted to the sky?


Since ancient times, there has been speculation about the nature of light. The questions revolved around whether light had substance and whether it had a finite speed or if instead it was instantaneous. The Greek philosopher Empedocles, who lived in the fifth century BC, elaborated the first recorded theory on this subject. He argued that light was emitted from the eye, thus enabling sight, and that light had a finite speed. His views were accepted by later Greek philosophers such as Euclid (fourth century BC) and Ptolemy (second century AD). To the contrary, Aristotle, who lived about a century after Empedocles, claimed that light was a substance and that it had no speed because it did not move.

The matter remained a moot point until the Middle Ages, when Arabic scholars attempted to address these unresolved issues. In his Book of Optics published in 1021, Ibn al-Haytham reversed the ancient theory by Empedocles and surmised that light traveled from the object to the eye and that it must have a finite speed. This speed, he argued, was variable and decreased in denser media. Another eleventh century Arabic scholar, Abu Rayhan al-Biruni, agreed and added that light moves much faster than sound.

The debate raged on for many years, with various philosophers and scholars, such as Bacon, Witelo, Kepler, Descartes and Fermat, giving their conflicting views. It was all quite pointless, however, as long as there was no way of either proving or disproving the various theories.

At the beginning of the 1600s, Isaac Beeckman suggested measuring the speed of light by observing the flash of a cannon reflecting off a mirror placed at a certain distance. In 1638. Galileo Galilei tried something similar by using covered lanterns. Both attempts were inconclusive.

As occasionally happens, the breakthrough came while scientists were looking for something else. For many years, experts had been searching for a practical way of determining longitude, essential for navigation and for creating accurate maps. In 1616. Galileo proposed to calculate longitude by measuring the time of day. This would be achieved by timing the eclipses of the moons of Jupiter, which Galileo had discovered in 1610 with his new invention, the telescope. Unfortunately, in that period, it was not possible to accurately time the moons' eclipses and Galileo's method could not be applied.

The accuracy of timepieces improved considerably over the following years and by the second half of the century, the Royal Observatory in Paris was applying the method suggested by Galileo. This was done by timing the eclipses of Io, one of Jupiter's moons, from two different observatories simultaneously, one located in Paris and the other on the island of Hven near Copenhagen. By noting how many minutes after midday either the start or the end of the same eclipse was observed in Paris and on Hven, it was possible to calculate the difference in longitude between these two locations.

However, it was soon noted that there were inexplicable differences in these timings, depending on the distance between Earth and Jupiter at the moment at which the timings were made. Ole Rømer, a young Danish astronomer working at the Royal Observatory in Paris, realized that the differences could only be explained by the time it took the light, i.e. the image of the eclipse, to travel between Jupiter and Earth. By comparing the apparent duration of Io's orbits as Earth moved towards Jupiter and as Earth moved away from Jupiter over a period of several years and by calculating the position of Earth and Jupiter in their orbits for each of the eclipses, Rømer estimated that it took light eleven minutes to cover a distance equivalent to the radius of Earth's orbit. James Bradley, who demonstrated the principle of the aberration of light in 1727, confirmed the validity of Rømer's method. Rømer never actually calculated the speed of light, but his correspondent Christiaan Huygens combined Rømer's estimate with the estimated radius of Earth's orbit and came up with 220,000 km/s, about 26% less than the actual value.

By Giovanni Gregorat, Contributing Editor MFN

Author: Giovanni Gregorat