Kepler was a deeply religious man who had originally aspired to become a Lutheran minister. Like Phillip Melanchthon, his astrology was part and parcel of his Christianity. As the Imperial Mathematician, he not only interpreted horoscopes for the emperor and his court, he published regular almanacs and predictions and made himself available for questioning on astrological and meteorological matters by the people of Prague, about which he complained, “…those of the lower classes with straight-forward and active minds…I get such a working over that I might as well call them my teachers.”
Kepler’s biographer Caspar relates an incident which occurred as a series of sextile aspects were shaping up in the heavens:
“Kepler swore 15 days before, in front of doubters, that there would be wind and rain on that day. In due course, on the day in question, came a fierce gale, driving black clouds, so that at noon it was as dark as half an hour before sunset. Amazed, the people asked themselves what was happening. Then the cry grew loud, ‘Kepler comes’.”
In the course of his work for the Imperial Court, as ‘district mathematician and calendar maker’ in Graz, and later as astrologer for the famous General Wallenstein, Kepler made some interesting and accurate predictions that have been preserved in his publications and biographies. For instance, in 1595, he predicted a peasant uprising, an invasion by the Turks, and an especially cold winter, all of which came to pass and bolstered his reputation. His calendar for 1618 said that ‘if a true comet should appear in the heavens’ then the other calendar writers would have to ‘sharpen up their pens.’ Three comets in all appeared that year, including one with a spectacularly bright tail.
Kepler also published extensively on his passion for reforming astrology: something of a hot topic in his time. His astrological works have only just recently been translated into English – again, some would say, they were deliberately suppressed. It is surprising how, even today, many English-speaking astronomers and physicists will adamantly deny that Kepler had any genuine interest in astrology.
In 1601 Kepler published De Fundamentis Astrologiae Certioribus, or, On the More Certain Fundamentals of Astrology, in which he explains his opinions on how, and to what extent, astrology works. He published Tertium Intervens, or the Third Man in the Middle, in 1610. In this classic of astrological reform, he presents himself between the two extremes of those who practice superstitious star-gazing and those who want to throw astrology out altogether. In 1619 he published his masterpiece, Harmonices Mundi, which was also not translated into English. Kepler poured twenty years of his life’s work into this grand synthesis of geometry, arithmetic, music, astrology and astronomy, which also contained his third law of planetary motion.
Astrology was not something that Kepler did merely to make money. He cared deeply about it and he saw the world through his own unique Pythagorean, harmonic paradigm. He was, as he described himself to his mentor, Michael Maestlin, a ‘Lutheran astrologer’. He was not, as later biographers have styled him, a radical rationalist out to make the world safe for science by ridding it of medieval superstition: and neither were his contemporaries. It was because of his passion for astrology, and not in spite of it, that he made the discoveries that brought him lasting fame as one of the greatest astronomers of all time. The quote below from his correspondence reveals just how personal Kepler’s astrology was. In a letter to Johan Herwart from 1599, he discusses his own horoscope:
“In my case, Saturn and the sun work together in the sextile aspect (I prefer to speak of what I know best). Therefore my body is dry and knotty, and not tall. My soul is faint-hearted and hides itself in literary corners; it is distrustful and fearful; it seeks its way through harsh brambles and becomes entangled in them. Its habits are similar. To gnaw bones, to eat dry bread, to taste spiced and bitter things is a joy to me. To walk over rugged paths, uphill and through thickets, is a holiday treat for me. I know no other way of seasoning my life than science; I do not desire any other spice and I reject it if it is offered to me. My fate is precisely similar to this attitude.”