Jeremiah Horrocks

English astronomer

• Born: c. 1619
• Birthplace: Toxteth Park, England
• Died: January 3, 1641
• Place of death: Toxteth Park, England

Horrocks’s observation of the transit of Venus, in 1639, is the earliest on record. He applied Kepler’s laws of planetary motion to the Moon, comets, and planets.

Early Life

Jeremiah Horrocks (jur-uh-MI-uh HAWR-uhks) was born in Toxteth Park, near Liverpool, England, around 1619, but the records from Toxteth Park for that year have been lost. There has been some speculation that his father may have been William Herrocks, a farmer, but historical research suggests that his father was actually James Horrocks, a watchmaker. James Horrocks was married to Mary Aspinwall, who was from a family of influence in Toxteth Park. Horrocks entered Emmanuel College, Cambridge University, as a sizar, or poor scholar, in 1632. At that time, the curriculum included mostly the arts, divinity, and classical languages, although it is likely Horrocks also learned geometry and some classical astronomy. He studied at Cambridge until 1635, when he returned to Toxteth Park, where he is believed to have become a tutor.

In Horrocks’s time, ideas about planetary motion were in transition. Since the ancient Greeks, it had generally been accepted that the earth was the center of the universe. In 1543, however, Nicolaus Copernicus published De revolutionibus orbium coelestium (1543; On the Revolutions of the Heavenly Spheres, 1952; better known as De revolutionibus), which argued that it was the Sun that was at the center, with the heavenly bodies, including the earth, moving around the Sun. In 1608, Johannes Kepler’s analysis of careful position measurements of the planets and the stars, taken by the Danish astronomerTycho Brahe, demonstrated that the planet Mars moved in an elliptical path around the Sun, and Kepler developed a series of laws that described planetary motion.

The precision of Kepler’s laws of planetary motion may have appealed to Horrocks’s uncle, who is believed to have been a clockmaker. Tradition says it was this uncle who first stirred Horrocks’s interest in astronomy. Shortly after returning to Toxteth Park, Horrocks constructed his own telescope, and he developed projection techniques that enabled him to view the Sun. He used his telescope to measure the positions of the planets and used these measurements to correct errors in existing astronomical tables.

Life’s Work

As Horrocks’s interest in astronomy deepened, he constructed simple instruments to measure the size of the Moon, Venus, and the Sun. To measure the size of Venus, he made a pinhole in a piece of paper and mounted the paper on the end of a stick. He determined the size of the pinhole by measuring the size of the pin, using an ingenious technique. Horrocks wrapped thread around the pin many times, then unwound the thread and measured its length. By dividing this length by the number of times he had wrapped the thread around the pin, he was able to accurately determine the circumference of the pin. Then he looked at Venus through the pinhole, adjusting the distance of the pinhole from his eye until Venus just filled the hole. By measuring the distance of the card from his eye, he calculated the angular size of Venus.

The passage of a planet across the disk of the Sun is called a transit, and the prediction of the exact date and time when a transit will occur requires a very precise calculation of the motion of the planet. Only transits of the two inner planets, Mercury and Venus, can be observed from the earth. These transits occur much less frequently than eclipses of the Sun by the Moon. There are generally about thirteen transits of Mercury each century. However, because Venus’s orbit is considerably larger than Mercury’s orbit, transits of Venus are far rarer, occurring in pairs separated by eight years. More than a century elapses between each transit pair. Seven transits of Venus have occurred since the invention of the telescope (1631, 1639, 1761, 1769, 1874, 1882, and 2004).

The French astronomer Pierre Gassendi was the first person to record that he had observed a planetary transit, that of the planet Mercury in 1631, but the time of this transit differed by six hours from that predicted by Kepler. A transit of Venus occurred just one month later. Gassendi attempted to observe it, but he failed because this transit was visible from America but not visible from Europe. According to Kepler, this was to be the only transit of Venus of the seventeenth century. While the planet would come close to making a transit in 1639, by Kepler’s calculations, it would just miss moving across the face of the Sun as seen from Earth: The next transit of Venus would not occur until 1761.

Kepler’s calculations in this regard were the basis of Horrocks’s most important discovery. Horrocks made his own precise observations of the position of Venus over a four-year period, and he observed slight deviations from the positions predicted by Kepler. In 1639, Horrocks, who had just moved to Much Hoole, near Preston, used his measurements to calculate that a transit of Venus would occur at about 3:00 p.m. on November 24, 1639, and that this transit would be visible from Much Hoole.

On the day of the predicted transit, Horrocks projected the image of the Sun onto a sheet of paper in a darkened room. Late November is not generally a time of clear skies in that part of England, but Horrocks was able to view the Sun. However, November 24, 1639, was a Sunday, which meant that, as a good Puritan, he was expected to be in church at the time of the transit. Further, since Horrocks’s calculation of the exact time of the transit might be in error, as he suspected Kepler’s calculation was, Horrocks intended to observe the Sun for as much of the day as possible, beginning at sunrise. He was called away several times in the morning but maintained his vigil until mid-afternoon.

The transit began at about 3:15 p.m., and Horrocks watched Venus move across the Sun for about thirty minutes before the sun set. By observing the projection of the Sun with Venus moving across its face, Horrocks was able directly to compare the angular diameter of Venus with that of the Sun. His observations showed the apparent diameter of Venus to be only 1′ 12,″ compared with the Sun’s diameter of 30′. This was much smaller than the 11′ angular diameter reported for Venus by Kepler.

In 1640, Horrocks returned to Toxteth Park, where he wrote his Venus in sole visa (1662; The Transit of Venus over the Sun , 1859) and started work on his next treatise on solar dimensions. He had begun essays on comets, tides, and the Moon as well. On January 3, 1641 Horrocks died suddenly. Much of his work was lost, but the surviving material was published much later as Opera posthuma (1678).

Significance

Horrocks is generally regarded as the father of British astronomy. He excelled at both making precise observations and using those observations to predict planetary motion. Although Kepler recognized that transits of Mercury and Venus could be used to determine the distance from Earth to the Sun, he was unable to measure this distance because he died before the 1631 transits of Mercury and Venus took place. While Horrocks obtained only a rough estimate of the distance from Earth to the Sun, because he had observed the transit of Venus from only a single location, his value of 59 million miles (95 million kilometers) was significantly more accurate than previous estimates. It was not until a century later, following the transits of Venus in 1761 and 1769, that a more accurate value, about 93 million miles (150 million kilometers), was obtained. His results were used extensively by later astronomers, and even Sir Isaac Newton cited Horrocks’s measurement of the diameter of the earth in the first edition of his Philosophiae Naturalis Principia Mathematica (1687; The Mathematical Principles of Natural Philosophy, 1729; best known as the Principia ).

Bibliography

Aughton, Peter. The Transit of Venus: The Brief, Brilliant Life of Jeremiah Horrocks, Father of British Astronomy. London: George Weidenfeld & Nicholson, 2004. A two-hundred-page account of Horrocks’s contributions to astronomy, focusing on his observation of the transit of Venus across the Sun.

Chapman, Allan. “Jeremiah Horrocks, the Transit of Venus, and the ’New Astronomy’ in Early Seventeenth-Century England.” Quarterly Journal of the Royal Astronomical Society 31 (1990): 333-357. A detailed account of Horrocks’s research methods and his scientific accomplishments, which attempts to correct myths about Horrocks’s life story.

Van Helden, Albert. Measuring the Universe: Cosmic Dimensions from Aristarchus to Halley. Chicago: University of Chicago Press, 1985. Includes an account of Horrocks’s observations, detailing his effort to determine the distance from Earth to the Sun.

‗‗‗‗‗‗‗. Transits of Venus: New Views of the Solar System and Galaxy. Proceedings of IAU Colloquium 196. Cambridge, U.K.: Cambridge University Press, 2004. Includes articles by Allan Chapman and John Walton detailing Horrocks’s life and his astronomical observations.