This e-book chapter is excerpted from Ken Shulman's Venus in Sole Visa, or Venus as Seen against the Sun (Smashwords, 2012). Used with permission of the author.
On November 6, 1639, in a stone farmhouse in the Lancashire village of Much Hoole, a university dropout and amateur astronomer named Jeremiah Horrocks sat down to pen a letter to his friend William Crabtree. Steeped in dampness and the odor of burning lamp oil, Horrocks outlined the final steps he and Crabtree needed to take to observe the transit of Venus that Horrocks predicted would occur on November 24. In the letter, Horrocks assured Crabtree that the event would be memorable. If, he added, it actually transpired.
Jeremiah Horrocks was the only person in all of England—and probably in the entire world—convinced that the 1639 transit would take place. Son of craftspeople and perhaps farmers—there were also a few watchmakers among his forbears—Horrocks had been a local wunderkind who entered Emmanuel College in Cambridge at age 14 as a sizar—a poor student whose duties, along with studies, included the preparation of meals, waiting on tables, and custodial work. In 1635, three years after his arrival, Horrocks left the university—in all probability due to lack of funds--and returned to Lancashire, where he continued to observe the heavens with a small telescope he either purchased or received as a gift from one of the landed families whose children he tutored.
Horrocks knew that Venus had last crossed between the Earth and the sun eight years earlier, on December 6, 1631. That eclipse—and so many other astronomical events—had been accurately predicted in Johannes Kepler's Rudolphine Tables. According to Kepler, Venus would next cross the sun in 1761. There was no mention of a 1639 transit.
Published in 1627 on commission from Holy Roman Emperor Rudolf II, the Rudolphine Tables were by far the most accurate interplanetary timetable ever written. They were the product of a comprehensive set of data and a revolutionary discovery. The comprehensive data belonged to Tycho Brahe, who'd harvested them over many years at the magnificent observatories he'd built on the island of Hven in Denmark's Oresund.
The revolutionary discovery was Kepler's, and would be his most memorable contribution to science. One century before Kepler, Nicolas Copernicus stated that the sun—not the Earth—was the center of our planetary system. Heliocentrism helped astronomers reconcile cosmic theory with the real-life cosmos they saw before them. Yet there were still many phenomena that Copernicus' bold shift did not explain: retrograde motion—the apparent backtracking of planets—was one; others included eclipses and planetary conjunctions that should not have occurred if the Copernican model of the solar system was accurate.
Kepler intuited that these discrepancies were due to the true shape of planetary orbits. While Copernicus had been bold enough to set the sun at the center of the solar system, he did not think (or perhaps dare) to revise the traditional model of planetary orbits, which had planets traveling in perfect circles, at perfectly constant speeds. Copernicus was no more daring when it came to the distance between Earth and sun. His astronomical unit of 1142 Earth radii is little changed from the estimate put forth by Hipparchus of Rhodes in second century BC. Copernicus also states the ratio of the sun-Earth to moon-Earth distances as 19, a figure that falls smack on the median of the range prescribed even earlier by Aristarchus of Samos.
After a prolonged and trying period analyzing Brahe's data (Kepler gained access to Brahe's figures while working as Tycho's assistant in Prague in the 1590's,) Kepler concluded that Mars traveled around the sun in an ellipse and not in a circle. So, he soon realized, did all the other planets. The discovery—or divine intuition, as Kepler himself might have considered it—of elliptical orbits then led Kepler to another insight. Not only did the planets not follow a circular path around the sun. They also did not travel at uniform speeds. Planets accelerated as they approached the sun, and decelerated as they traveled away from it. These observations then led Kepler to a formula by which he could calculate the relative distances of the known planets from the sun. The unit of measure was the space between sun and Earth. Expressed in Kepler's Third Law, this formula would establish the solar distance as the yardstick of the universe. Whoever could put a real number on that measure could know the size of the solar system.
The Venus transit of 1761, predicted by Kepler and attended by hundreds of researchers across the globe, was a celestial cotillion with dozens of invitations and directions sent out decades in advance. The Venus transit of 1639, predicted by the solitary Horrocks, was a humble provincial tea, with no one other than himself, his friend William Crabtree, and perhaps Horrocks' brother Jonas on the guest list. Horrocks cottage observatory in Much Hoole was a far cry from Tycho Brahe's palatial skylabs in Denmark and Bohemia.
But Horrocks was not delusional. In 1610, Galileo had used a telescope of just 10x magnifying power to confirm that Venus was a planet and not a star; the finding cast the nature of the entire universe into doubt. In 1631 Pierrre Gassendi had successfully observed a transit of Mercury with a makeshift camera obscura he'd mounted in a spartan Paris garret. (The French amateur was particularly surprised at the small size of Mercury when he saw it in profile against the sun. "I thought rather that is was a spot which I had not noticed on the sun on a previous day," Gassendi wrote.)
Horrocks' telescope was at least as powerful as Galileo's, and certainly powerful enough to observe a transit of Venus. His strategy was to train his looking glass directly at the sun; the image would project from the eyepiece onto a circle Horrocks had drawn on a sheet of paper tacked to the opposite wall. Venus, when (and if) it traversed the sun, would appear as a black spot or shadow. Horrocks also graded his circle; the graduations would help him quantify the planet's progress, and estimate its size relative to that of the sun.
The technology, while rough, was solid. All that was necessary now was for Horrocks' computations to be right. But for Horrocks computations to be right, Kepler's needed to be wrong. Either Venus would eclipse the sun, or it wouldn't. Horrocks knew that the Rudolphine Tables were the finest astronomical timetable in existence. Yet he'd found some discrepancies in the ways that events Kepler had predicted had played out in the skies. Kepler had been spot on predicting the 1631 Venus transit. But he'd miscalculated where the event might be seen. Night had already fallen on Gassendi's Paris before Venus began its trek across the sun. And contrary to Kepler's predictions, the transit was blacked out for much of Europe.
It wasn't that the German's math was flawed, Horrocks saw. It was that Kepler had misunderstood the nature of the force that causes the planets to travel around the sun in ellipses. Kepler believed the sun first pulled the planets toward it, and then, when they were close, repelled them. This alternating push and pull, according to Kepler, was the force that generated the elliptical orbits. Horrocks believed this was wrong, and that the error had skewed Kepler's calculations.
The Englishman was a very unlikely challenger for such a heavyweight. Kepler had studied with the finest professionals of his day, had enjoyed royal patronage, and had access not only to Tycho's magnificent data set but to his equally magnificent facilities. In contrast, Horrocks was a poor university dropout working in a remote provincial town that most likely did not even have a library, let alone an observatory. His mind, of course, was keen. But it was also a mind that worked in almost total isolation, and in a country that had never attributed great importance to the study of the stars.
Still, Horrocks continued to trust his own eyes and his intuition. He constructed a pendulum and studied its Earthward and upward swings; from this simple experiment he concluded that a planet, left to its own devices, would always travel in a straight line. And that the sun, conversely, would attempt to cause the planet to revolve around it in a circle. (Horrocks' description of the dynamic between sun and planet is very close to the force that his compatriot Isaac Newton would identify as gravity some four decades later.) It was the ongoing dialogue between these two forces, Horrocks concluded, that dictated the elliptical orbits, not the push me–pull you sun of Kepler's cosmos. More importantly, it was this difference in dynamic that accounted for the inaccuracies he'd found in the Rudolphine Tables.
The distinction between the theories—and the distortions that distinction might produce—was minor, Horrocks knew. But scaled against the solar system, at distances that could be cadenced in thousands of Earth radii, it was just large enough to turn a non-event into a full-blown transit of Venus. Kepler's Rudolphine Tables had Venus passing just above the sun on November 24, 1639. Horrocks predicted that Venus would pass in front of the sun, just as it had eight years earlier.
And this was not all. Horrocks believed that this, the second Venus transit of the decade, was no fluke. Yes, he agreed, Kepler was right about the frequency of Venus transits. They recurred at intervals 105.5 and 121.5 years. But Horrocks now claimed that they recurred in pairs. After 105.5 years, two Venus transits would occur, eight years apart. Then, after 121.5 years, two more Venus transits would follow, again eight years apart. Horrocks believed that Venus transits would no longer be isolated events—nor had they ever been.