The Speed of Light

About ...
News ...


Last updated Tue Dec 3 2002


In 1676 Roemer, while studying the satellites of Jupiter for an unrelated matter, was the first to find an indication that the speed of light was actually finite. As astonishingly great as that speed turned out to be (about 300,000 km/s), the real surprise came 2 centuries later.

By 1879, Michelson had determined the speed of light to an accuracy of +/- 50 km/s and for the first time the error was smaller than the back and forth speed of the Earth around the Sun (2x30 km/s), the next fastest accessible speed.

In one of physics' most celebrated experiments Michelson and Morley concluded in 1887 that the speed of light appeared unchanged by the movement of its source or that of the observer. This was unlike any speed from any other source and could not be explained by the laws of physics as they were then known.

In 1905 Einstein proposed the theory of Special Relativity to deal with this puzzle, and our understanding of the laws of Nature has become much more sophisticated. There is a special kind of relation between time and space, Einstein told us, and hence a special speed, and it appears that light has that speed. At, or near, that speed, things don't behave quite like what we are used to.

We propose to relive the key experiments of this fascinating history over the course of 2002 by recreating, hands on, some of these experiments. We will measure the speed of light and focus on using today's technology that is easily accessible to amateurs while keeping costs as low as possible.

And to raise the stakes ...    
  it's our 2002 contest!  
    ... and here are the rules.


With the notable exception of Empedocles, most classical thinkers either assumed or reasoned that the speed of light was infinite. The first recorded attempt to measure c was that of Galileo's in 1626 but he could not rule out that the speed was indeed infinite.

When Author Measured Method Result (km/s) Error
1626 Galileo Light Uncovering Lanterns infinity  
1676 Olaus Roemer Light Galilean Satellites of Jupiter 214,000  
1726 James Bradley Light Stellar Aberration 301,000  
1849 Armand Fizeau Light Toothed Wheel 315,000  
1857 Weber, Kohlraush ESU/EMU Ratio of Electrostatic to
Electromagnetic Units
1862 Leon Foucault Light Rotating Mirror 298,000 +/-500
1879 Albert Michelson Light Rotating Mirror 299,910 +/-50
1891 Blondlot Radio Parallel Wires 297,600 +/-15000
1907 Rosa, Dorsey EMU/ESU Electromagnetic Units 299,788 +/-30
1926 Albert Michelson Light Rotating Mirror 299,796 +/-4
1947 Essen, Gorden-Smith Radio Cavity Resonator 299,792 +/-3
1958 K. D. Froome Radio Radio Interferometer 299,792.5 +/-0.1
1973 Evanson et al Light Lasers 299,792.4574 +/-0.001
1983 CGPM Light Adopted Value 299,792.458 +/-0

source: K. D. Froome and L. Essen, "The Velocity of Light and Radio Waves", Academic Press, 1969

The table shows the merging of 3 different research lines (light, electric and magnetic units, radio waves). Indeed, over this period radio waves first emerged from the laws of electricity and magnetism and then the radio wave nature of light was established.


If you are, or will be, conducting experiments for this project, consider documenting your efforts for inclusion here. Send your pages, enquiries etc..., to webmaster@njsas.org.

Backyard Roemer:

Vincent Goffin is documenting an attempt to redo Roemer with a 3.5" telescope.

Modern Roemer:

Geoff Hitchcox, from New Zealand, has modified Roemer's methodology to use modern orbital information.

More Experiments

were carried out by NJSAS members and were presented at the Phila. convention. We hope to have them documented here soon.

Your Experiment:

by You! The contest year may be drawing to a close, but there will always be room on this site for more amateur contributions.


Lectures, history:

Michael Fowler's lectures from virginia.edu (local copy)
Katie McCullough's Speed of Light Formal Report (local copy)
Measuring c with a PMT, from hawaii.edu (local copy)
Usenet Relativity FAQ: measuring c (local copy)
Michelson at Mount Wilson: the 1926 experiment (local copy)
Ron Ebert's Web Pages


faster than c? NEC's Princeton experiment (local copy)
slowing light to a crawl: Cambrige's Hau shows how (local copy)