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.