He placed us in the sky

kw: book reviews, nonfiction, biographies, mathematicians, astronomers

Sunday coming, in just a few days, will mark the 549^th birthday of Nicolaus Copernicus, or very close to it. Born Niklas Koppernigk in German-speaking Varmia, then part of Prussia, now part of Poland, he Latinized his name as an adult. He is the classic example of one who, while seldom straying a day's journey from home, roamed the cosmos in his mind, saw better than others how it worked (at least the local billion miles or so), and wrote the book that dragged the rest of educated humanity, kicking and screaming, into the new view. His book On the Revolutions of the Heavenly Spheres was only removed from the Roman Catholic Index of Prohibited Books 177 years ago, after a residence of more than two centuries.

There are partial biographies of Copernicus aplenty, but none other as accessible yet so thorough as A More Prefect Heaven: How Copernicus Revolutionized the Cosmos by Dava Sobel. One great value of the book is placing Copernicus in his milieu as a canon, an unordained church official, nonetheless required to live celibate. His official duties occupied much of his time and energy. Yet he was able to compile a great mass of observations, particularly of the Moon. He had as a primary ambition the desire to revise the Moon's orbit; the accepted orbit inherited from Ptolemy required the Moon's apparent size to vary by a factor of four, which it clearly does not do. He succeeded in this, and these and other calculations convinced him that the Sun, not Earth, was at the center of all orbits except the Moon's.

Secondly, from close reading of many of his letters and other writings, Ms Sobel is able to open a window into the thought processes that enabled Copernicus to unveil a sun-centered Solar system. Now that he has led the way, we expect every child to come to the same realization, to understand that the apparently solid ground under their feet is in fact rotating about an axis that spins both it and them at hundreds of miles (or km) per hour. Not only so, the yearly motion about the Sun requires a planetary velocity of about 66,000 mph, or nearly 110,000 kph. Copernicus did not know this latter figure, having no accurate parallax for the Sun, but he guessed its magnitude within a factor of about ten.

Parallax is the measurement of distance using trigonometry, by measuring the angular difference between measurements taken at different locations. Accurate measurements of stellar occultations and near misses by the Moon, taken by observers a few degrees apart in longitude (say, one in London and another in Paris, if both have a clear sky!) suffice to measure the Moon's parallax of about a degree (normalized to Earth's radius). That is, if one observer plots the Moon's position just at moonrise, at the same time a colleague plots its position while the moon is near the zenith—because the two observers are some 10,000 km apart—, their measurements will differ by about a degree. But the Sun is 400 times as far away as the moon, and naked-eye observations cannot discern a parallax of 1/400 degree, or nine arc-seconds.

As a church official, Copernicus knew there would be strong opposition to his model of the Universe. He knew as well as anyone the verses in the Bible that implied the Earth is "fixed forever" and that the Sun moved, except when Joshua commanded that it stand still long enough for a key battle to be finished. He published a short document that outlined his mathematical calculations using a new system with only 33 epicycles (Ptolemy had needed more than 100), without clearly stating that he believed the Earth was moving about the Sun. He delayed publishing anything further, until he was prodded by a Lutheran mathematician named Rheticus. Even then he dragged his feet and the book was only finished as he lay on his deathbed. He died at age seventy, having, perhaps, seen the last few pages of the book's galley proofs, but leaving it to others to proofread and complete. In April of 1543 the book was finished, and he died in May.

So carefully had the writing been done, and so hedged about with arguments intended to deflect scripture-based criticism, that it took the Holy Office of the Inquisition (established in 1542) 73 years to determine it belonged on the Index, and only as a book needing "correction", not wholly prohibited. The prohibition was dropped in 1835, after 219 years.

Concerning one fascinating bit of detective work, the author relates the work of Owen Gingerich, who studied about 600 copies of early editions of On the Revolutions (or De Revolutionibus, as it is frequently called). The book had three major editions and several minor ones, and was always printed with wide margins. Nearly every copy has copious notes, which indicates that anyone who thought enough of it to purchase a copy, read it carefully and checked calculations and thought about its implications. Interestingly, Johannes Kepler's copy has fewer annotations than most, but those that exist are very thorough and insightful. Nearly the only major scholar of the years prior to 1700 for whom a copy of the book is not known is Newton! Yet his work indicates he was familiar with it.

A word about production values. I love a book as well thought out as this, with such care taken to make the reading pleasant. The carefully-chosen quotes that begin each chapter, the archaic type face for chapter titles, and the decorated letter used in preference to a bare drop-capital, all indicate a fine historical sense.

One of my professors once remarked about such embellishments, nearly universal in well-produced 19^th Century books, that they showed the author had enjoyed the writing, and presaged an enjoyable reading experience. What a contrast to the dryness of most modern historical publications!