Written by Salisbury Cathedral volunteer Ken Smith
Elliptical orbits, Jovian moons and tides –
Kepler and Galileo
In my continuing forays into the treasures of the Cathedral library, I have discovered that the evolution of modern Astronomy can be traced into the seventeenth century in the discoveries made by Johannes Kepler. Kepler had worked with Tycho Brahe and had used some of the latter’s extensive observations of stars and planets. After Brahe’s premature death in 1601, Kepler replaced him as Imperial Mathematician to the Holy Roman Emperor Rudolph II.
In his book “Astronomiae Pars Optica” from 1604 – held by the library – Kepler pioneered optical theory, discovering amongst other things, that the human eye produces an inverted image. He postulated that the brain corrected the image, which we now know to be true.
Brahe’s amazing collection of observations allowed Kepler to discover that the orbit of Mars, hence all the other planets, was elliptical rather than circular as Copernicus and Brahe had believed. This is described in another of his books in the library – “Astronomia Nova” from 1609. It is fascinating to pore over these venerable leather-bound tomes in the library with their hundreds of pages of Latin text, calculations, tables of observations and woodcut diagrams of planetary orbits.
It is humbling too, when it is recalled that Kepler produced this astounding work whilst being often short of money* and weighed down with family tragedies including the deaths of several of his children and his first wife. If this were not bad enough, he and his family were banished from Syria for his Lutheran beliefs. For the rest of his life he lived in the eye of the appalling religious storm we now call the Thirty Years War.
Another contemporary polymath genius of Kepler was Galileo. Famed for developing the telescope, Galileo is less well known today as an engineer, mathematician and a pioneer in physics especially in kinematic motion. He and Kepler corresponded but never met. Galileo’s 1610 discovery of the moons of Jupiter was welcomed by Kepler who later observed them himself using a borrowed telescope. Subsequently, Kepler developed an improved telescope with paired convex lenses thus giving greater magnification. Curiously, although there was much mutual regard between the two, Galileo never agreed with Kepler’s elliptical planetary orbits or indeed with the role of the gravitational pull of the moon on tides – an idea endorsed by Kepler.
These differences are evident in another book held in the library, a large folio edition of Galileo’s “Mathematical Collections and Translations” from 1661. All of the books mentioned show their author’s struggles, not only with established authority but also when their discoveries went against their own religious and philosophical beliefs. This underlines, even more distinctly, their moral courage, tenacity and intellectual honesty.
*Although a salaried member of the Imperial Court, Kepler’s wages were often in arrears. To make ends meet he produced and sold horoscopes for the Emperor and Nobles of the court. Thanks to Kepler’s astute understanding of the political and military situation of the time, these horoscopes were often very accurate.