NATURE|Vol 462|24/31 December 2009 OPINION Johannes Kepler on Christmas Kepler’s interpretation of the supernova of 1604, De Stella Nova, interwove the science of astronomy with astrology and theology in an attempt to determine the correct birthdate of Jesus, explains Martin Kemp. When was Jesus born? On 25 December Kepler’s treatise can be acclaimed in the year that starts the Christian as a work of science in the modern calendar, is what wide belief would sense. For instance, he uses the lack have it. Greek and Russian Orthodox of diurnal parallax from the new star churches retain the date of 7 Janu- — that is, the inability to measure its ary in accordance with the old Julian distance using the angle at two dif- ARCHIVES/SPL CALTECH calendar, rather than the Gregorian ferent points on Earth’s surface — to revision that was introduced in 1582. infer that it resided in the sphere of Internal evidence from the Bible the fixed stars. This provided fur- rules out the designated year because ther evidence that the sphere was Herod the Great, who was responsi- not as immutable as the ancients ble for the massacre of the innocents, had assumed. But De Stella Nova, as from which the infant Jesus escaped, is its extended title declares, explicitly known to have died in 4 bc. This is one involved metaphysics and astrology, reason why the now-obscure Polish and it climaxed in an appendix on scholar, Laurence Suslyga, argued in biblical chronology. his 1605 thesis on the birth and death Within this, we cannot separate of Christ that Jesus was actually born what we regard as the real science in 5 bc. from the astrology and theology. For Suslyga’s tract unexpectedly gained Kepler, mathematics and meaning a significance beyond the specialist comprised a unified whole. The great realm of biblical chronology. The cir- astronomer, best known for identify- cumstances are best explained by the ing the elliptical paths of the planets, great German astronomer, Johannes became increasingly concerned in Kepler: “I found for sale at Graz a the years following publication of De small book by Laurence Suslyga of Stella Nova with the construction of Poland,” with which Kepler agreed a ‘purified’ version of ancient astro- that at least “four years must be added logical wisdom, purged of the myths to the Epoch of Christianity now in Artwork from De Stella Nova shows the constellation Ophiuchus with that had accumulated around the use”. Suslyga provided Kepler with the 1604 supernova (N; in the foot, lower left) observed by Kepler. zodiacal signs. He later referred to the necessary licence to correlate the the mystic symbolism of traditional dramatic appearance of a ‘new star’ in 1604 which it faded from view as the flash from its astrology as “filthy mud” from which “one with the act of stellar navigation performed by explosion declined in intensity. can glean even an occasional escargot, oysters the biblical Magi when they journeyed to see In 1606, Kepler published a pamphlet on the or an eel for one’s nutrition”. the Christ child in Bethlehem. new star in German, while planning a more Kepler gained his “nutrition” from the In 1604, astronomers keenly awaited the substantial account in Latin: On the New Star mathematical ratios of the ‘aspects’ between shift of the conjunction of Jupiter and Saturn in the Foot of the Ophiuchus and on the Fiery the planets. He defined an aspect as “a geo- into the ‘Fiery Trigon’ of the zodiac (Sagit- Trigon that Began Anew at its Rising. A Booklet metrical construction [of an angle] between tarius, Aries and Leo), which initiated a new Full of Astronomical, Physical, Metaphysical, light beams of two planets here on Earth”. The cycle of conjunctions in the Trigon — an event Meteorological and Astrological Disputations ratios were integral to the celestial geometry that was calculated to recur about every 800 (beginning in Latin, De Stella Nova in Pede that manifests the mathematical “music” of the years. Mars also moved close to Saturn and Serpentarii …). The second of two substan- heavens. He explained that earthly nature can- then Jupiter, thus outlining a triangular array tial appendices in the book dealt with the year not help but respond to the dictates of heavenly that was of great interest to astrologers. of Christ’s birth “in the light of the new pro- harmonies, and said that nature is affected by During his time in Prague as mathemati- nouncements of Laurentius Suslyga”. an aspect “just as a farmer is moved by music cian to the Holy Roman Emperor Rudolf II, In a series of arguments that even he admit- to dance”. Kepler observed this remarkable cluster of ted were hard to follow, Kepler claimed that the De Stella Nova serves to remind us that it was planets and the new star on 17 October 1604, star followed by the Magi was the equivalent of not possible in the era of Kepler and Galileo when the clouds over the city finally lifted. the stella nova of 1604–5, and that it had arisen to pursue astronomy in such a way that the The star burned brightly in the evening and during a series of related planetary conjunc- mathematical study of the heavenly bodies was even visible as a morning star, located in tions in the years 7–5 bc — which he took to was divorced from the theology of a heaven the foot of the constellation of the Serpent cover the period of Christ’s conception and the inhabited by God. ■ Bearer, or Ophiuchus (pictured). He made his Magi’s journey to Bethlehem, as recounted in Martin Kemp is emeritus professor in history of last observation of the star a year later, after Matthew 2:9–10. art at the University of Oxford, Oxford, UK. 987 © 2009 Macmillan Publishers Limited. All rights reserved.