Tycho Brahe and Johann Kepler, together with Copernicus and Galileo, founded modern astronomy. An unusual partnership--Danish nobleman and poor scholar, yet both obsessed with the heavens. That was the time when casting horoscopes was a still a major duty of astronomers and when alchemy (a sideline of Tycho, an obsession of his kinsman Erik Lange) drew just as much attention. By the late 1500s a great change had begun: Copernicus had proposed his heliocentric theory, and before church dogma entered the fray, it was viewed seriously, even if not generally accepted. But the telescope, which after 1609 drew the public to the wonders of the sky, had not yet appeared.

      Kitty Ferguson has written a delightful and eminently readable book around these two very different personalities. Meant for the general reader, it describes in an easy and concise style the science and history of an era very different from ours. That was when nobility was all-powerful, when the king of Denmark could take an island of freeholders and make it Tycho's personal domain, leaving its residents no voice in the matter. Religious intolerance was widespread--indeed, events were moving towards the 30 years' war, Europe's most destructive religious fight, mirrored by the civil war in Britain. Kepler was forced out of Graz, among all other employees of Protestant colleges in town, after the ruling archduke decreed they must leave the city by nightfall, that same day. It was also an era when Kepler's mother was arrested for witchcraft, when most of his numerous children died in childhood, and when Tycho's marriage was regarded as a second-rate "slegfred" union because his chosen wife was not from the nobility.

      And yet, there was also science. The Ptolemaic system, by which sun, moon, planets and stars all revolved around the Earth, was still the accepted foundation of astronomy. It was a cumbersome model, where effects now known to arise from the motion of Earth were represented by epicycles, secondary orbits traced by planets on top of their primary ones. Further corrections were still needed, so centers of planetary orbits were displaced, and astronomers assumed ad-hoc that their orbital progress was constant, not as seen from that center but as viewed from some different "equant" point. Even with the inaccurate observations of the time, this still left discrepancies. All models assumed circular orbits, including the sun-centered model of Copernicus, which also suffered from discrepancies.

      From his teenage years Tycho was fascinated by astronomy. While his peers pursued rank and power in the service of the king, he himself resolved to devote his life and energy to more accurate observations of the stars, perhaps good enough to provide a real test. In that early era it was often hard to predict what was feasible and what was not. In 1572 a new, very bright star appeared, not too far from the northern pole star (it is still known as "Tycho's supernova" and its remnants were recently studied by the orbiting Chandra X-ray telescope). Was it closer than the Moon--or was it more distant, in a region where Aristotle proclaimed no changes ever occurred? The rotation of the Earth shifts an observer several thousands of kilometers in a single observing night, yet Tycho's accurate observations failed to detect any corresponding shift of the new star relative to its neighbors. He concluded it was more distant than the moon, and also found the same held for a comet. Trivial conclusions to us, but novel ones at his time.

      Tycho later tried to use the same method to estimate the distance to Mars, which he knew was the closest outer planet. Unaware that because of the great distance, any shift was too small for the unaided eye to discern, he kept seeking its value, at long last convincing himself he had actually found it. He was also the first to evaluate the shift of the positions of stars near the horizon, caused by light refraction in the atmosphere. However, Tycho's preoccupation with astronomy also made him neglect his political position. It eroded so much that he felt it wiser to exile himself to Prague--a comfortable exile, to be sure, an honored scholar among the nobility.

      Unlike Tycho, Kepler grew up in near poverty, with a father who was mostly absent and who ended abandoning his family altogether. Younger than Tycho by 25 years, he displayed early in life a sharp mind, and was helped to a good education by Germany's Lutheran institutions. He ended up teaching in one of them in Graz, Austria.

      Where Tycho was a supreme experimenter and socially adept, Kepler was deeply religious, inclined to mysticism and speculation, but also skilled in the mathematics of his day. That skill was helped by his enormous tenacity, grabbing hold of problems like a bulldog and not letting go. In our age of space exploration, Tycho may well have become a leader of glamorous space missions with NASA, politically powerful and generously funded. Kepler might have been the talented theorist laboring in relative obscurity, sifting the data returned by those missions and extracting their nuggets of discovery. Though the two might not have admitted it, the work of each would have been essential to the success of the other.

      Something like that certainly happened in their actual encounter, in what Ferguson dubbed (title of chapter 17) "a dysfunctional collaboration." Each had what the other lacked. Tycho knew that his accurate observations held the key to understanding how the planets moved--especially Mars--but he also realized that Kepler was the only one around him sharp enough to solve the puzzle. Yet he had misgivings: Kepler was a relative latecomer, less trusted than Tycho's longtime assistants Longomontanus and Tengnagel. Furthermore, Kepler believed in the Copernican model, while Tycho had devised his own "Tychonic" system, in which the planets orbited the Sun, but the Sun orbited Earth. Tycho felt quite possessive over that idea and bitterly defended his claim to it. Yet when he painfully died in 1601, he was well aware that Kepler would probably use his data to promote the Copernican view. "Let me not have died in vain" he pleaded on his deathbed.

      History is full of ironies. Kepler indeed used Tycho's data to support Copernicus, but by doing so, he ensured Tycho's immortality. Had the two failed to meet and collaborate, the precision of Tycho's observations--supreme in his days--would have soon been surpassed by telescopic instruments. Someone else would then have used telescopes to decode the motions of the planets, and Tycho's work would have become a forgotten footnote.

      This is a great story, told with skill and verve. Kathy Ferguson is not the first to do so, and those seeking finer details (e.g. of Kepler's math) may want to look elsewhere, but her book is set at just the right level--not too technical for lay readers, not too shallow for science-savvy ones, not too long nor too brief. Like Goldilock's porridge, it is just right, and the interplay of religion and politics, academic battles and a gallery of unusual characters, all these have their places too. As do occasional tidbits: who would have suspected that the names of Rosencranz and Guildenstern, the doomed courtiers in Shakespeare's "Hamlet" (and in Tom Stoppard's play) were patterned after those of two of Tycho's kinsmen?