Jeremiah Horrocks

• 1. Introduction

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  Jeremiah Horrocks died on the 3rd of January 1641, at the age of twenty-two. He left behind no published books, no university degree, and no grave that anyone can prove. Yet Newton would later cite his work in the Principia, and he has been called the bridge connecting Newton to Copernicus, Galileo, Brahe, and Kepler.

  In the span of a few short years, working from a farmhouse in Lancashire with instruments he built himself, Horrocks did several things no one had done before. He showed that the Moon travels an elliptical path around the Earth. He predicted, and then watched, a transit of Venus that every other astronomer on the planet missed. And he suggested, years before Newton put it into a formal theory, that gravity was not confined to the Earth and its immediate neighbourhood.

  How did a young man from a modest Puritan family in Liverpool achieve all this? And why did it take decades for anyone beyond his small circle of correspondents to find out?

• 2. A Puritan Boyhood

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  Lower Lodge Farm in Toxteth Park, a former royal deer park near Liverpool, Lancashire, is where Horrocks was born in 1618. The neighbourhood was home to well-educated Puritan families, and his own was no exception. His father James had come to Toxteth Park to apprentice under a watchmaker named Thomas Aspinwall, and he eventually married Aspinwall's daughter Mary.

  Precision ran in the family on both sides. The Aspinwalls had become a successful family of watchmakers by 1600, and because Puritan beliefs shut them out of public office, they had channelled their energies into craft and trade instead. For young Jeremiah, this meant something unusual: his boyhood chores included measuring local noon, the moment used to set the town's clocks. He was learning to read time from the sky before most children his age could read a book.

  His upbringing also instilled a deep suspicion of astrology, witchcraft, and magic. For Horrocks, the heavens were orderly and rational, not a source of omens. That conviction shaped everything he would later argue against established astronomical tables. In 1632, at fourteen years old, he enrolled at Emmanuel College, Cambridge, as a sizar, a student who earned his keep through domestic work for the college.

• 3. Cambridge and Its Limits

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  At Emmanuel College, Horrocks encountered the mathematician John Wallis and the Platonist John Worthington. He was one of very few students there who accepted Copernicus's heliocentric theory. While the official curriculum remained cautious, Horrocks was quietly working through the writings of Johannes Kepler and Tycho Brahe on his own.

  By his own account, he read most of the astronomical treatises available to him and began marking their weaknesses. By seventeen, he was proposing new lines of research. Cambridge gave him access to serious minds, but its institutional pace was not built for a student whose curiosity had already outrun the syllabus.

  In 1635, Horrocks left Cambridge without graduating. The exact reason is unclear; historians have proposed everything from financial strain to a reluctance to take Anglican orders, which a degree from Cambridge would have required him to do. Whatever the cause, he returned to Lancashire, and his serious astronomical work began almost immediately.

• 4. Instruments and Ingenuity

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  Liverpool was a seafaring town, which meant navigational tools like the astrolabe and the cross staff were available in local markets. Specialised astronomical instruments were another matter entirely. No one was selling them, so Horrocks made them.

  His family connections turned out to be essential. His father and uncles were watchmakers, skilled in the construction of precise mechanical instruments. Horrocks apparently helped with the family business during the day, and in exchange his relatives helped him design and build the tools he needed to observe the night sky. By 1638, he had acquired the best telescope he could find.

  He had earlier owned a three-foot radius astronomicus, a type of cross staff used to measure angles between stars. By January 1637 he had pushed that instrument to its limits, so he built a larger, higher-precision version himself. A helioscope he later constructed for the Venus transit was characteristically simple: he focused the Sun's image through a telescope onto a flat surface, creating a projection safe to observe directly. The elegance of that solution reflects a mind that understood both the science and the practical constraints of working alone, far from any observatory.

• 5. The Moon's Elliptical Path

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  Horrocks was the first person to demonstrate that the Moon travels in an elliptical orbit around the Earth. To support this, he drew an analogy that was both ingenious and physically intuitive: he noted that a plumb bob, drawn back and released, traces an elliptical path, and that its major axis rotates in the same direction as the apsides of the Moon's orbit.

  He went further. He suggested that the Sun, not just the Earth, exerted an influence on the Moon's motion. That was an anticipation of ideas Newton would later formalise in the Principia, and Newton explicitly acknowledged Horrocks's contribution there. Horrocks also suggested that comets followed elliptical orbits, extending the logic beyond the Moon to other bodies in motion.

  In the final months of his life, he turned his attention to tides, trying to explain how the Moon produced the regular rise and fall of water. His death cut that work short, but the line of reasoning was continuous: gravity, for Horrocks, was something that operated at distance, between bodies, in ways that could be measured and predicted. That was not yet the consensus view.

• 6. The Transit of 1639

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  Johannes Kepler had published his Rudolphine Tables in 1627 and, two years later, added a pamphlet warning astronomers about upcoming transits. Kepler's tables predicted a near-miss for Venus in 1639, meaning Venus would pass close to the Sun's disc but not across it.

  Horrocks disagreed. He had spent years observing Venus himself and had also concluded that Kepler's tables, while better than those of Philip Van Lansberg, still needed correction. After making his own calculations, Horrocks determined that a transit would actually occur. He shared his prediction with his friend and correspondent William Crabtree.

  From his location in Much Hoole, Lancashire, Horrocks calculated the transit would begin at around 3:00 pm on the 24th of November 1639 by the Julian calendar, or the 4th of December in the Gregorian. The morning of that day was cloudy. At around 3:15 pm, the clouds parted, and Horrocks saw the tiny black disc of Venus moving across the face of the Sun. He watched for half an hour until the Sun set. Crabtree observed the same event from his home in Broughton near Manchester.

  They were the only two people in the world who saw it. Every other astronomer either lacked the prediction, lacked the equipment, or lacked the clear sky. From what he observed, Horrocks estimated the distance between the Earth and the Sun at 95 million kilometres. The accepted figure today is about 150 million kilometres, but his estimate was more accurate than any that had come before.

• 7. Venus in Sole Visa

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  Horrocks wrote up his findings in a treatise he called Venus in sole visa, meaning Venus seen on the Sun. He died before it was published. The manuscript survived the chaos of the English Civil War, though barely, and was eventually published by the Polish astronomer Johannes Hevelius at his own expense.

  When the treatise was revealed to members of the Royal Society in 1662, about twenty years after it was written, it caused immediate excitement. What members encountered was not a dry technical report. Horrocks had included humorous asides, passages of original poetry, and a conviction about the grandeur of the universe that ran through every paragraph. Writing about the long gap between Venusian transits, he composed lines addressed to the planet itself: "Thy return / Posterity shall witness; years must roll / Away, but then at length the splendid sight / Again shall greet our distant children's eyes."

  The treatise also contained his response to the theological attacks on astronomy that were common in his time. Horrocks argued directly that the irregularities blamed on the heavens were the fault of imprecise human observation, not of the celestial bodies themselves. He wrote that he could not allow that God had created the heavenly bodies more imperfectly than man had observed them. That confidence, in both the orderliness of nature and the possibility of understanding it, runs through every piece of work he left behind.

• 8. Legacy

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  Horrocks died suddenly on the 3rd of January 1641, having returned to Toxteth Park sometime in mid-1640. His friend Crabtree recorded the loss in words that have since been quoted many times: "What an incalculable loss!"

  The memorials came slowly and then in accumulation. A marble tablet and stained-glass windows were installed at the Parish Church of St Michael in Much Hoole in 1859. Toxteth Unitarian Chapel holds a memorial plaque. A crater on the Moon bears his name. The Jeremiah Horrocks Observatory was built at Moor Park in Preston in 1927.

  In 1993, the Jeremiah Horrocks Institute for Astrophysics and Supercomputing was established at the University of Central Lancashire, and in 2012 it was renamed to include Mathematics, Physics, and Astronomy. That same year, the transit of Venus was marked with a celebration at the church in Much Hoole, streamed live on the NASA website. Also in 2012, sculptor Andy Plant installed a work called Heaven and Earth at the Pier Head in Liverpool. It shows a telescope aimed at a working orrery in which Venus has been replaced by a figure of Horrocks himself, depicted as an angel. A play by David Sear, titled Horrox, was staged at Cambridge's ADC Theatre in 2023, from the 28th of March to the 1st of April, as part of the Cambridge festival.