Flamsteed Astronomy Society

Relative Values  by Dr Eddie Yeadon

— October 2005

Dr Eddie Yeadon, founder member of the Friends’ astronomy group the Flamsteed Astronomy Society, reveals two other anniversaries this year of significance to the NMM besides Trafalgar

Transcribed from f@nmm magazine Issue 2 Winter 2005

100 years ago, Einstein published his first paper on the Theory of Relativity, and fifty years ago the first caesium atomic clock was built. These two events have been responsible for remarkable developments in astronomy, cosmology, and the science of timekeeping – all of great interest to the Royal Observatory.


Why, wondered Einstein, did the same ‘mass’ appear in both of these Laws? This had been known as the Equivalence Principle, but there was no theoretical justification why gravitational mass and inertial mass should be equal.

Einstein’s reasoning led him to develop the General Theory of Relativity, published in 1915, which considers gravity to be not a force in the conventional sense, but the consequence of curvature of space-time produced by the presence of mass or energy.

The first triumph of Einstein’s theory was in being able to explain the previously mysterious precession of the major axis of Mercury’s orbit.

 It was further verified by Sir Arthur Eddington, who measured the deflection of rays of light from a star passing close to the Sun during a total eclipse in Africa in 1919. The apparent deflection was, in fact, due to the rays of light travelling along a ‘straight’ line through ‘curved’ space. The theory has since been tested with exquisite precision by astronomical observations, laboratory experiments and various spacecraft. General Relativity has allowed us to increase our understanding of stars, galaxies, and ‘black holes’.

In addition to contributions to astronomy and cosmology, the General Theory of Relativity has many more everyday applications, including corrections to the orbits and signals from the GPS navigational satellites. This navigational system relies on precise measurements of time differences from atomic clocks in each satellite, with the whole operation being regulated by the caesium clocks, first developed by Louis Essen at the National Physical Laboratory in May 1955, which define the modern international timescale.

 So the 100-year old Theory of Relativity, together with the 50-year old atomic clocks, enables navigation at sea today with a speed and accuracy which Nelson - using sextant, chronometer and Nautical Almanac - could never even have imagined. I think he would have approved.

Albert Einstein 1905

It was in 1905 that Albert Einstein, a young clerk in the Swiss Patent Office, submitted five scientific papers, which rocked the foundations of science. One of these papers considered the implications if the speed of light in a vacuum appeared to be the same to all observers in the Universe, regardless of the speed they were travelling relative to each other. There was already some experimental evidence for this from the observations of Michaelson and Morley in America. His conclusions became the Special Theory of Relativity, according to which the dimensions of a body – including time – shrank as its speed approached that of light. And it led, in another paper published later the same year, to the famous equation relating energy and mass

E = mc2


 Einstein went on to develop his theory, especially regarding the property of ‘mass’. In 1687 Isaac Newton had published a theory, which related the orbit of the Moon about the Earth to the fall of an apple. ‘Mass’ came into Newton’s Laws of Motion in two apparently independent ways: the attractive gravitational force between two bodies depended on their masses, and the motion of the body resulting from an applied force also depended on the mass.

Einstein and Sir Arthur Eddington


Louis Essen (R) with the first atomic clock


Modern atomic clock  © NMM