In astronomy, 99.9% of the information we receive from the sky comes to us in the form of electromagnetic radiation that covers the entire spectrum from gamma rays, through visible light, to radio waves. This radiation travels at the universe’s top speed of 300,000,000 metres per second. However we take this as read because we haven’t the tools to check this ourselves and we have faith in the generations of scientists that have checked it! Wouldn’t it be nice to check it ourselves?

During an astrocamp at Haddon Grove last year, I was chatting with some other campers about the speed of light, and the subject of its measurement using a spinning toothed wheel cropped up.

Afterwards, I got to think that the simplicity of the experiment (developed and carried out in 1849 by Hippolyte Fizzeau) should make it easy enough for me to replicate. However the speed of the wheel, the number of teeth required and the distance over which the experiment would need to be carried out (several miles), put me off.


Fizeau’s friend, Léon Foucault made improvements that resulted in a method involving a spinning mirror, which in theory was much more sensitive, and therefore could be reproduced with a slower spin rate and a shorter distance. The basic apparatus is as follows:-

A narrow beam light source and a telescope are collimated so that the spot of light from the beam falling on a distant object can be seen in the centre of the field of view of the scope. An eyepiece with a measuring scale is used.

The telescope and light source are pointed at a mirror that can spin at a known rate. Turn the mirror slightly to a different position and the spot of light still appears in the same position in the eyepiece even though the background view has changed.

A large mirror or reflector is placed at a known distance so that the reflected light from the spinning mirror can sweep across it and be reflected back.

With the mirror spinning fast, and because of the time delay for the light to traverse the experiment, the spot of light appears in a slightly different position in the eyepiece. This can be measured on the scale and can give an angle of deviation.

The speed of light can be calculated from the angle of deviation, the rate of spin and the distance between the two mirrors.

Luckily I had plenty of odds and ends, and with help from John Bardwell, and advice from John Rose, Mario Stephenson and Bill Phenix, was able to assemble the equipment we needed.


In the evening, a small group of us (Bill and Stella, Johns Bardwell and Rose, Mario and myself, went to Highfields Park in Chesterfield. There weren’t many people around but we had to be careful where the beam pointed. We mounted the reflector (made from strips off high-viz jackets) and measured out 150m, then set up the rest of the equipment. With the mirror stationary, we measured the position of the spot of light against the scale at 5.0. We set the mirror spinning (thought to be 15-20 revs/sec), and the spot was now at 5.1, about the expected deviation. Bill stayed at the eyepiece as the mirror was switched off and watched as the spot of light returned to its original position. Although an accurate reading for the speed of light could not be obtained, we had detected the time delay between the light leaving the spinning mirror and returning (about a millionth of a second).


Johns Bardwell, Brown, Rose and myself went to the park again with a better idea of the spin rate, and carried out the experiment over 100m, 200m and 300m. Unfortunately the mirror’s spin rate was found to be difficult to determine precisely. Nevertheless, from our most reliable results we obtained a speed of 265,000,000 m/s, about 14% out, but we are definitely in the right area.


I’ve tried the experiment over a distance of 20m, well within the realms of repeating it in the carpark at the observatory; this would be something to show people on cloudy nights, but we need to improve the spinning mirror. If anyone want to help out in any way, please let me know!

Rob McGregor






















Many thanks for this Rob and to the helpers.