Astronomy and the Speed of Light - Prof Bob Lambourne – 21st November 2018


To measure distance these days you need, not a ruler, but a clock! For the metre is the length of the path travelled by light in vacuum during a time interval of 1/299792458th of a second.

The metric system was originally conceived as a system of measurement derivable from unchanging phenomena; but technical limitations necessitated the use of artefacts (the prototype metre and prototype kilogram for example) when the metric system was first introduced in France in 1799.

But in the same way that has been done for the metre, from 20th May 2019 the kilogram, ampere, kelvin, and mole will be defined by setting exact numerical values for the Planck constant (h), the elementary electric charge (e), the Boltzmann constant (k), and the Avogadro constant (NA), respectively.

The November lecture from our Vice President Prof Bob Lambourne, focussed on the history of the determination of the speed of light, such that now it is deemed to be a fixed value – a constant in the same way as the Planck or Boltmann constants.

Galileo in 1638 started it, coming up with a value of about 200,000 km/s, but admitting it was actually too fast to measure. Then through the work of Bradley, Huygens and Newton in the 18th Century; Fresnel, Fizeau, Foucault, Michelson and Morley in the 19th Century to arrive at estimates closer to 300,000 km/s; culminating in the theories of Lorentz and Einstein in the 20th Century. Along the way, the notion of the “ether” was invented and discarded, and Einstein’s Theory of Relativity showed how to relate measurements made by observers in a specified state of relative motion. Whilst “c” does represent the speed of light, its real significance is that it is the fundamental constant in all space-time physics.

Not strictly a lecture about astronomy, this was a welcome revision of this corner of the history of science. The Society wishes Bob a happy retirement from his post at the Open University and we are extremely pleased that he will continue to be our Vice President and wants to come back and give us more lectures!

Sandy Giles