The Celestial Sphere

Venice is known for masquerades. This sculpture decorating the Venetian Hotel in Las Vegas represents an armillary sphere. The armillary was invented by Erastothenes in 255 BC. The gold rings represent the equator, longitude and latitude. The wide band angled from the equator represents the ecliptic with constellations of the zodiac. These devices usually represented Earth at the centre, which the modern artist chose to omit. Erastothenes was also famous for calculating Earth's diameter with amazing accuracy.

At about the same time Aristarchus of Samos wrote two books about the Universe. His work "On the Sizes and Distances to the Sun and Moon" contained estimates that were strikingly accurate. Those distances were measured from the Earth, fitting an Earth-centred cosmology. The enormous distances to solar system objects may have started Aristarchus thinking about alternatives. He wrote another book that is now lost and known only through citations by contemporaries like Archimedes. This second book introduced a cosmology with the Sun in the centre and Earth circling around it. Aristarchus also believed that the stars were infinitely distant, to explain their lack of parallax. Another contemporary wrote that Aristarchus should have been put on trial for the impiety of suggesting that Earth was not the centre! The cosmology of Aristarchus was incredibly prescient for its time, and the reaction to Aristarchus predated that given to Copernicus and Galileo.

Ptolemy made Earth the centre of his cosmology. From Ptolemy until the time of Copernicus, most people would believe that Earth was the centre of the Universe. In some ways this was a reasonable assumption to make. A navigator on Earth need not take into account the distances to stars, for they are too great to affect a navigator's calculations. Many scientists, including this one, keep on the shelf a clear globe with stars drawn on its surface. Though we realise today that this is just a convenience, for most of history humans believed that the stars were truly fixed to an immense sphere.

Anyone observing the planets for any length of time will see that they often appear to reverse course and travel backwards in their paths. To explain this retrograde motion, Ptolemy's cosmology relied upon epicycles, spheres within spheres. The planets were each attached to an invisible sphere, in turn attached to bigger spheres circling the Earth. These dark spheres dominated Ptolemy's Universe, opposing the tendency of objects to fall Earthward. Even today, it is tempting to explain the Universe with dark energies opposing gravity.

Old interpretations of Special Relativity treat the velocity of light as constant. In some ways this is a reasonable assumption to make, for c change is only visible at high redshifts. Special Relativity does not take gravity into account, for that is under the command of General Relativity. Linking the two Relativities leads to some surprising predictions. The best-known prediction is that c is not constant but changes over time. Change in c is visible in observations of high-redshift supernovae and other phenomena.

It is also possible to explain our observations with dark energies opposing gravity. Future posts will give time to these alternatives.