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.