14th December was the anniversary of Tycho Brahe’s birthday and as is now usual on such occasions various people, including me, posted their Tycho scribblings on Twitter, Facebook et. al.
As is also, unfortunately, usual several of them referred to Johannes Kepler either as Tycho’s pupil or student.
Now, both on Hven and later in Prague, Tycho ran what has been called a large scale research centre employing, over the almost thirty years he collected astronomical data in a systematic programme, a fairly large number of observational assistants. Some of these came to work with the noble Dane as experienced astronomical observers; others came to learn from him. Some of those who came to learn only stayed for a short period and having learnt returned home, some having learnt stayed and worked for a time as assistants. These can justifiably called pupils or students. For example, Simon Marius, whom I have blogged about on several occasions came to Prague as a student for a few months in 1601, shortly before Tycho’s death, and returned home relatively quickly.
The great Dutch cartographer Willem Janszoon Blaeu visited Tycho on Hven for six months in 1595-6 both learning and contributing. He took Tycho’s star catalogue, in the form from a simple celestial globe with him when he left to start his own celestial globe production.
Kepler’s presence in Prague was of a completely different nature. A graduate of the University of Tübingen, he had already worked as a maths teacher and district mathematicus in Graz for six years before he moved to Prague to work with Tycho. He had in 1596 already published his first volume of astronomical speculations, Mysterium Cosmographicum, and it was this that attracted Tycho’s attention in the job seeking German scholar. Kepler’s bizarre cosmological speculations were of less interest to Tycho than Kepler’s very obvious mathematical abilities.
Tycho didn’t take on Kepler as a pupil or student to learn the trade of astronomical observer, for which he would have been fairly useless having suffered from a visual defect since a childhood illness, but as a mathematician to reduce Tycho’s observational data to ephemerides, the practical tables of planetary positions used by cartographers, navigators and astrologers. The production of ephemerides was the principle function of astronomy from antiquity down to the seventeenth century. Tycho didn’t even employ Kepler himself but secured him a position at the court of Rudolph II, employed specifically to produce those ephemerides. Kepler was not Tycho’s pupil or student but his astronomical colleague at court, which is the principle reason why he succeeded him as Imperial Mathematicus. By the way, it took him tweet-six to produce those ephemerides, The Rudolphine Tables, complaining often over the years how tedious the task was. They were, because of their level of accuracies, however the principle reason why people began to accept the heliocentric system over the geocentric and helio-geocentric ones, so it was a well spent twenty-six years.