Christopher M. Cevasco, Author


Halley’s Comet – Part 4 (16th – 18th Centuries)

This is the fourth in a series of posts about the storied history of Halley’s Comet (officially designated 1P/Halley). If you missed the earlier posts, you can still read Part One here, Part Two here, and Part Three here.

Edmond Halley1P/Halley made its first post-medieval appearance in 1531, which also marked the start of a more rigorous scientific investigation of the comet. That particular apparition was observed by Peter Apian, the German humanist who later Latinized his own name to Petrus Apianus, and also by Italian physician and scholar Girolamo Fracastoro.  Both men are credited with being the first to note that the comet’s tail always pointed away from the sun, although Fracastoro is perhaps better remembered for giving us syphilis–well, the name at least, which is derived from his 1530 three-part epic poem, Syphilis sive morbus gallicus (“Syphilis or The French Disease”) about a shepherd named Syphilus who is punished with a terrible disease after insulting the gods.

During the comet’s next apparition in 1607, it was observed by German astronomer and mathematician Johannes Kepler at Prague and by Danish astronomer Christian Longomontanus at Malmo and Copenhagen, although each man’s views on the phenomenon proved somewhat lacking. Kepler seemed to believe comets moved along straight lines, notwithstanding his celebrated work on the elliptical orbits of planets. As for Longomontanus, despite his otherwise scientific mind, he adhered to the ancient belief that comets were heralds of great evil. Nevertheless, the records of both men, as well as those of Apianus, were later to be used by the figure most closely associated with the comet’s history–Edmond Halley himself (pictured above).

Halley (most commonly pronounced to rhyme with either “valley” or “daily” but sometimes pronounced as though the first syllable were spelled “haul”) was born in 1656 in Haggerston, Shoreditch, England (now part of Greater London). He went on to become a renowned astronomer, geophycisist, mathematician, meteorologist, and physicist, and was the second person to be named Astronomer Royal–a senior post in Britain’s Royal Household first held by John Flamsteed from 1675 to 1719 and persisting to this day.

Among his many scientific contributions, Halley convinced Sir Isaac Newton to publish Prinicipia Mathematica Philosophiae Naturalis in 1687, when he discovered Newton had solved the problem of proving Kepler’s laws of planetary motion but had failed to publish his solution; Halley even funded the publication. One of Halley’s more colorful theories, put forth to explain anomalous compass readings, was that the Earth was hollow, consisting of a 500-mile-thick shell around two concentric inner shells and an innermost core; he speculated that atmospheres existed between the shells, that each shell had its own magnetic pole and rotated at a different speed, and even that the inner regions were luminous and had atmospheres such that they might be inhabited; he thought gases escaping from these inner shells could have been the cause of the Aurora Borealis (Northern Lights).

Fortunately, Halley’s application of historical astronomy to the study of comets yielded more plausible results. When 1P/Halley returned in 1682, Halley was able to make his own first-hand observations. Until the previous century, the consensus on comets followed the view of Aristotle that they were disturbances in our planet’s own atmosphere. The Danish astronomer, Tycho Brahe, disproved this in 1577 by using parallax measurements (accounting for displacement or difference in an object’s apparent position viewed along two different lines of sight) to show comets must lie somewhere beyond our moon. Notwithstanding Brahe’s calculations, many remained skeptical that comets actually orbited the Sun, assuming instead like Kepler that they followed straight paths through the Solar System. Halley’s friend Newton had suspected two comets that appeared in close succession in 1680 and 1681 were in fact the same comet before and after passing behind the Sun, and although this was later shown to be correct, he was unable to fully reconcile comets into his laws of gravity and motion.

In the end, Halley himself used Newton’s laws to calculate the gravitational effects of Jupiter and Saturn on cometary orbits, and he published his findings in his 1705 Synopsis of the Astronomy of Comets. His calculations permitted him to compare his own observations of the 1682 apparition to the earlier apparitions in 1531 and 1607 and determine that they shared nearly identical orbital elements. He concluded all three comets were the same object returning every 76 years (the period has since been amended to every 75-76 years). He predicted the comet would return in 1758.

Edmond Halley did not live to see his prediction proven correct; he died in 1742.  But on Christmas Day (or possibly the following day), 1758, German farmer and amateur astronomer Johann Georg Palitzsch was the first to officially spot the returning comet in the heavens. Notwithstanding the possibility, discussed in an earlier post in this series, that 1st-century Jewish astronomers had already recognized the comet as periodic, this 18th-century confirmation of its return is generally viewed not only as the first time a comet was recognized as periodic but as the first time anything other than planets was shown to orbit the Sun. The following year, French astronomer Nicolas Louis de Lacaille named the comet in Halley’s honor.

Up next in this series: Part 5: 19th – 21st Centuries

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  1. Halley’s Comet – Part 5 (19th – 21st Centuries) | Christopher M. Cevasco

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