# Jost Bürgi

Jost Bürgi (born 28 February, 1552 in Sankt Gallen, died 31 January 1632 in Kassel) was a Swiss watchmaker, astronomer and mathematician. He became famous in his time for the construction of various instruments for astronomical observations. In this context, he developed new, mathematical methods that correspond to modern logarithms.
Bürgi has got a crater on the Moon and an asteroid named after him.

## Biography

Apart from the fact that Bürgi was born in 1552 in the small town of Lichtensteig in the Swiss canton of Sankt Gallen, little is known about his upbringing. He received no higher education and must on his own have studied and gained experience with clocks and other instruments used by astronomers. For the same reason, he never learned Latin, which at that time was used in scientific works.

## Kassel

In 1579, Bürgi was permanently employed by Count Wilhelm IV of Hesse-Kassel to build instruments that the astronomically interested count himself used in exploring phenomena in the firmament. With this in mind, he had built an observatory in Kassel which at that time had about the same significance as Tycho Brahe's Star Castle on the island of Hven between Copenhagen and Helsingborg. The count was in regular contact with Brahe. In this context, Bürgi constructed the first sextant made of metal. It was smaller and lighter, but still gave greater accuracy and was easier to use. To make the observations, Bürgi constructed several celestial globes where one could plot the positions of the stars. Similarly, he built astronomical clocks that could display the positions of the Sun or Moon at any given time. Using Copernicus' heliocentric description of the solar system, he also made clocks that could show the course of the planets.
He succeeded in making clocks so accurate that they could even set seconds, something that had not previously been possible. Only with the introduction of pendulum clocks almost a hundred years later could this be made more accurate. With such a precise time indication, one could follow the daily rotation of objects in the sky. In this way, all the coordinates of the horizon system could be determined and soon became the prevailing observation technique.

## Prague

In 1592, the German-Roman emperor Rudolf II expressed a wish to receive a gift from the Count of Kassel in the form of a celestial globe made by Bürgi. It was to be handed over in person by the watchmaker himself at the court in Prague. This meeting must have been so successful that it was the beginning of an ever closer contact between Bürgi and the emperor. The Count died the same year, but Bürgi was offered by his son to continue his work in Kassel on the same terms as before. But in 1604, Bürgi was persuaded to move to Prague where he was employed as an imperial watchmaker. At that time, Kepler was also at court as an imperial mathematician and was in the middle of his work to establish his laws for the movements of the planets. This work required very accurate, numerical calculations and Bürgi could assist with this as an assistant to Kepler.
When Rudolf II died in 1612, Bürgi was still able to continue in the same position in Prague under the new emperors. But as he got older, he made more and more frequent visits to Kassel. This was in the middle of the Thirty Years' War. In 1631 he finally moved back permanently and died in Kassel the following year.

## Mathematical contributions

In addition to having a great talent for the construction of ingenious timepieces and astronomical instruments, Bürgi was also a gifted mathematician. Also in this field he contributed original work that testifies to extraordinary abilities. Part of the explanation for this may be that he could not read Latin and Greek so that he could not learn from the ancient, classical works. He therefore largely had to develop new methods based on his own ideas. But since he could not write in these languages either, few of his works were printed and therefore little