A 17th Century astronomer on my twitter stream.

My brief post on Hevelius seems to have hit a nerve and has generated many comments and much traffic throughout the blogahedron. One of the most interesting comments was made by Alun Salt both on G+ and on his blog. Alun is an expert for ancient astronomy and he drew readers attention to the fact the amongst his other achievements Hevelius was the originator of some of the forty extra stellar constellation that have been added to the forty-eight listed by Ptolemaeus in his Syntaxis Mathematiké. Now over the centuries many astronomers suggested new constellations or new names for existing ones but unlike Hevelius not all were successful in getting their suggestions accepted by the astronomical community.

One of my various activities is to give guided tours of the astronomical instrument collection of the Germanisches Nationalmuseum in Nürnberg. During these tours I always draw the visitors attention to a rather curious half celestial globe displaying the constellations of the northern hemisphere (the southern half has not survived). This celestial globe was made by a seventeenth century German astronomer and globe maker who due to his deep Christian belief wanted to replace the heathen Babylonian, Greek and Islamic images and names of the constellations with European Christian ones. The half globe therefore has in place of the usual pictures of Pegasus or Orion the coats of arms of the royal and the aristocratic families of Europe. The maker of this globe was the professor of mathematics at the University of Jena, Erhard Weigel (1625 – 1699), who has been tweeting regularly on my twitter stream for several months and was also one of those with whom my Hevelius post found favour.

Alun categorised Hevelius as an important largely unknown astronomer a description that applies even more so to Erhard Weigel. As Weigel played a not insignificant role in the history of seventeenth century astronomy and as I said is now tweeting merrily on my twitter stream I thought it would be nice to introduce him to my readers.

Erhard Weigel

Erhard Weigel was born on 16^th December 1625 in Weiden in Bavaria but his Protestant parents were forced to flee to Wunsiedel when he was three years old by Bavarian Counter Reformation Catholic troops, which reminds us that Weigel was born both temporally and geographically into the middle of the Thirty Year War. He studied at the University of Leipzig and in 1653 was appointed professor of mathematics at the University of Jena where he would remain for the rest of his life. There are no laws, discoveries or other scientific achievements named after Weigel who made his mark on the history of science as an educator, a teacher. The big name major event version of the history of science ignores the educators but a good teacher contributes at least as much if not considerably more to the development of a scientific discipline as a great discovery or a famous scientist and Weigel was an excellent teacher. In the decades that he taught Weigel educated many, many mathematicians and astronomers who made small but significant contributions to the progress of those discipline mainly in Southern Germany and it would beyond the scope of this post to consider them all so I will indulge in a bit of cherry picking.

Weigel’s most notorious pupil was of course Gottfried Wilhelm Leibniz who although he studied for a comparatively short period in Jena received his introduction to the mathematical sciences from Weigel. He would of course go on to become one of the greatest seventeenth century mathematicians.

Leibniz

I am now going to sketch the achievements of three of Weigel’s lesser-known pupils, Gottfried Kirch, Johann Christoph Sturm and George Christoph Eimmart.

After studying under Weigel in Jena Kirch (1639 – 1710) went on to study under Hevelius in Danzig.

He established himself as a successful observational astronomer and calendar writer (a standard method for astronomers to earn a living in the Early Modern Period) and in 1700 he was appointed Prussian State Astronomer in Berlin leading what was to become the observatory of the Berlin Academy, the German answer to the Paris and Greenwich observatories. Kirch was respected and acknowledge as a leading astronomer throughout Europe.

Georg Christoph Eimmart (1638 – 1705) who was born in Regensburg studied under Weigel in Jena from 1655 to 1658.

Eimmart by himself

In 1660 he settled in Nürnberg where he established himself as an engraver and became very successful also as the owner of a small private art school.

Eimmart’s Planaphaerium Coeleste

With the money that he earned from his artistic activities he set up the first European public observatory on the city walls of Nürnberg.

Eimmart’s Observatory

Like Kirch Eimmart enjoyed a European wide reputation and several of the assistants he trained at the observatory went on to become important Middle European astronomers and cartographers. As I posted last year both Kirch’s wife Maria and Eimmart’s daughter Maria were with the help of husband and father early female astronomers in their own right.

Johann Christoph Sturm (1635 – 1703) studied under Weigel from 1656 to 1662 and in 1669 he was appointed professor for physics at the Nürnberg University in Altdorf.

Sturm played a very significant role in the history of physics in Germany as a transitory figure between the Aristotelian physics of the scholastic university and the new experimental physics of Newton and especially Robert Boyle.

Illustration from Sturm’s Experimental Physics Book

Weigel was not just a teacher but also an important, innovative and successful celestial globe maker. Apart from his failed attempt to introduce new constellation images mentioned above some of his globes had an ingenious and fascinating novelty. Viewed from the earth the stars and the constellations appear to be on the inside surface of a gigantic sphere, the so-called sphere of the fixed stars. It was this imaginary sphere that formed the basis of celestial globes. However nearly all globes were constructed as if the viewer were standing outside the sphere of the fixed stars, the so-called God’s eye view. That means on a celestial sphere one did not see the stars as one saw them from the earth. In some of his larger celestial spheres Weigel drilled hole at the positions of the principal stars and constructed a trapdoor in the southern hemisphere. It was therefore possible by lighting the sphere from outside and viewing the interior through the trapdoor to see the stars in the night sky as one would see them in nature.

Another Weigel innovation in celestial cartography was his eclipse map from 1654. An eclipse map is a map that shows the path on the surface of the earth from which a solar eclipse will be visible. Weigel’s was the first such printed map ever produced. This honour is usually falsely accredited to Edmund Halley for his 1715 eclipse map.

Weigel’s Eclipse Map

In 1676 Johann Christoph Sturm followed the example of his teacher and produced an almanac of eclipse including two eclipse maps.

Sturm’s Eclipse Map

Another area where Weigel served the whole of society was in the question of calendar reform. When Pope Gregory introduced his new reformed calendar in 1582 it did not meet with universal approval. The protestant rulers in Germany always suspicious of anything to do with the Pope consulted the leading German protestant astronomer Michael Maestlin, professor of mathematics in Tübingen and most well known as Kepler’s teacher, as to whether they should also introduce the new calendar. Maestlin’s answer was a resounding no, largely based on religious prejudice rather than scientific grounds and so the protestant states continued to use the Julian calendar. Now given the patchwork nature of the German states in the seventeenth century this was not exactly convenient. It meant that people who lived within a couple of hundred metres of each other geographically existed ten days apart temporally. The only people happy with this situation were the calendar makers who made good money producing double calendars in both old and new style. In the second half of the seventeenth century Weigel campaigned tirelessly to persuade the protestant states to finally adopt the new calendar. His efforts were rewarded shortly after his death as the German and Scandinavian protestant states finally decided to introduce the reformed calendar at the beginning of 1700. Unfortunately they introduced Weigel’s suggestion for the calculation of Easter i.e. based on the actual astronomical dates of the vernal equinox and the new moon as opposed to the Catholic calendar that was based on a nominal equinox and the mean moon. Eimmart’s one time assistant, the Nürnberger astronomer Johann Leonard Rost, noticed that in 1724 and 1744 the Protestant and Catholic Easters would fall on differing Sundays. In 1755 the protestant states adopted the full Gregorian calendar.

Since 2003 there is a Weigel Society at the University of Jena and those who read German can learn about him here.

Some of Erhard Weigel’s heraldic celestial globes:

Greenwich

Thüringer Landesmuseum Heidecksburg

Franklin Institute

Staatliche Kunstsammlungen Dresden

The Dresden Globe on an East German stamp 

The eclipse maps are downloaded from Michael Zeiler’s excellent Eclipse Maps website. The picture of Eimmart’s Nürnberg observatory is from the Astronomie in Nürnberg website. The rest of the illustration are as always borrowed from Wikipedia.