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Hendrikus Jelckama was a Frisian mathematician, astronomer and engineer most famous for his work on tidal motion and discovery of the Jelckaman moons of Jupiter.
Jelckama was born in Seisbierrum, Fryslân in 1561, the son of the village's mayor, Tjerk Jelckama and his wife Nannetje Siurds, and brought up in the Calvinist faith. This put him at odds with the prevailing religious mood in Fryslân which either cleaved to Catholicism, Lutheranism (increasingly supported by Fryslân's Anglian rulers) or more radical Anabaptist belief. Recognised from a young age to be adept at mathematics, and apparently spurred by the sight of the Great Comet of 1577, Jelckama was sent to the University of Leiden, where he supported himself by casting horoscopes. Repudiating Calvinism in 1582 he gained a government post with the Anglian administration and quickly became the province's chief mathematician.
In 1586 he was made head engineer of the Lauwersmar polders which were slowly reclaiming arable land from the flood of 1280. He quickly made improvements to sluice gates and pumps, implementing a new meshing gear system which improved efficiency threefold, while he also drew up ambitious and fanciful plans to build a massive dyke system along the length of the Frisian Isles to reclaim the Wadden Sea. It was during this time he became interested in the motion and causes of tides. At that time many had already commented on the co-incidence of higher tides during full moons and equinoxes however Jelckama was the first to discuss the tides in pure scientific terms, dismissing many misconceptions. He argued clearly for the moon as the cause of tides but was at a loss to explain how, merely leaving the subject open to further research.
In 1591 at the request of the Queen, Catherine of Hesse, he assumed the duty of tutor to her two children, Henry of Ipswich and Anna of Ljouwert, instructing the pair in languages, mathematics and philosophy. Here his hydrological engineering took a backseat and he engrossed himself more in the heavens. With a grant from the queen for money and land he would build 'Uraniawert' (after the Greek muse of astronomy, Urania), a small artificial hill on which he placed his home with an observatory in the roof. The construction apparently absorbed 1% of Anglia's entire budget for 1599 (when the Frisian polders took 3%) and was the subject of much discussion in the Witenage, and ended in severe censorship for the Queen.
This public dressing down, embarrassed the Frisian court and Jelckama was urged to adopt a lower profile, spending more time on his pupils. Henry sadly died in 1600 though Anna flourished under his tutelage. He would also design fortifications, especially for Ljouwert, Drylts and Hertford, which he visited in 1602.In 1604 and under the patronage of his old student Anna, now Queen regnant of Anglia, Jelckama renewed his interest in the heavens, systematically observing and recording a supernova. His detailed and eloquent description partially helped to undermine the Aristolian ideal that the heavens were immutable, an idea which he himself had personally believed in and had written on its defence on more than one occasion. Indeed his tidal theory rejected the notion that the earth moved at all. He was an early adopter of telescopes, improving and refining their design and would personally demonstrate their use to Anna II and various Anglian lords in Lincoln in early 1610. By 1612 he was issuing pamphlets ridiculing his old beliefs and fully supporting the idea of Heliocentrism, popularised by the Auvergnese mathematician Claude Loiseau.
The old ideas took a further battering when in 1618 Jelckama described three 'stars' in orbit around Jupiter. This discovery came almost a year after the Vinlander Jóhannes Ólafursson discoveries of the same. Ólafursson's telescopes were of better quality and indeed he had described four moons rather than three, however as Ólafursson wrote in Vinlandic rather than Latin his ideas did not have the same traction in Europe and Jelckama was given credit. After correspondence with other astronomers who confirmed his claims, Jelckama named the three Mnemosyne (Memory, a lover of Jupiter) and two of their daughters; Urania and Euterpe, the muses of Astronomy and Song respectively. This naming system would be extended to Ólafursson's fourth moon (Erato, muse of poetry) and others as further moons were discovered.
After the abdication of Anna II in October 1610 Jelckama fell out of favour with Anglia's new rulers and, though she would still fund some of his astronomical work using her estates, he was forced to take mathematics teaching posts at seminary schools in Grönnen and then Münster.
In 1581 he married Sjoukje Dircx and they had six children. He would die in 1623 in Münster.