Halley and His Comet

Halley and His Comet

The occasional apparitions of the comets disturbingly challenged the notion of an unalterable and divinely ordered Cosmos. It seemed inconceivable that a spectacular streak of milk-white flame, rising and setting with the stars night after night, was not there for a reason, did not hold some portent for human affairs.

So the idea arose that comets were harbingers of disaster, auguries of divine wrath – that they foretold the death of princes, the fall of kingdoms. The Babylonians thought that comets were celestial beards. The Greeks thought of flowing hair, the Arabs of flaming swords. In Ptolemy’s time comets were elaborately classified as “beams”, “trumpets”, “jars” and so on, according to their shapes. Ptolemy thought that comets bring war, hot weather and “disturbed conditions”.

Comet Halley has played an interesting role in human history and may be the target of the first space vehicle probe of a comet, during its next return in 2062. Image: Princess on Purple Magic Horse. Fantasy Art. © Megan Jorgensen (Elena)

Some medieval depictions of comets resemble unidentified flying crucifixes. A Lutheran “Superintendant” or Bishop of Magdebourg named Andreas Celichius published in 1578 a “Theological Reminder of the New Comet”, which offered the inspired view that a comet is “the thick smoke of human sins, rising every day, every hour, every moment, full of stench and horror before the face of God, and becoming gradually so thick as to form a comet, with curled and plaited tresses, which at last is kindled by the hot and fiery anger of the Supreme Heavenly Judge”. But others countered that if comets were the smoke of sin, the skies would be continually ablaze with them.

The most ancient record of an apparition of Halley’s (or any other) Comet appears in the Chinese Book of Prince Huai Nan, attendant to the march of King Wu against Zhou of Yin. The year was 1057 B.C. The approach to Earth of Halley’s Comet in the year 66 is the probable explanation of the account by Josephus of a sword that hung over Jerusalem for a whole year. In 1066 the Normans witnessed another return of Halley’s Comet. Since it must, they thought, presage the fall of some kingdom, the comet encouraged, in some sense precipitated, the invasion of England by William the Conqueror. The comet was duly noted in a newspaper of the time, the Bayeux Tapestry. In 1301, Giotto, one of the founders of modern realistic painting, witnessed another apparition of Comet Halley and inserted it into a nativity scene. The Great Comet of 1466 – yet another return of Halley’s Comet – panicked Christian Europe; the Christian feared that God, who sends comets, might be on the side of the Turks, who had just captured Constantinople.

The leading astronomers of the sixteenth and seventeen centuries were fascinated by comets, and even Newton became a little giddy over them. Kepler described comets as darting through space “as the fishes in the sea”, but being dissipated by sunlight, as the cometry tail always points away from the Sun. David Home, in many cases an uncompromising rationalist, at least toyed with the notion that comets were reproductive cells – the eggs or sperm – of planetary system, that planets are produced by a kind of interstellar sex. As an undergraduate, before his invention of the reflecting telescope, Newton spent many consecutive sleepless nights searching the sky for comets with his naked eye, pursuing them with such fervor that he fell ill from exhaustion. Following Tycho Brahe and Johannes Kepler, Newton concluded that the comets seen from Earth do not move within our atmosphere as Aristotle and others had thought, but rather are more distant from the Moon, although closer than Saturn. Comets shine as the planets do, by reflected sunlight and “they are much mistaken who remove them almost as far as the fixed stars; for if it were so, the comets could receive no more light from our Sun than our planets do from the fixed stars”. He showed that comets, like planets, move in ellipses; “Comets are a sort of planets revolved in very eccentric orbits about the Sun”. This demystification, this prediction of regular cometary orbits, led his friend Edmund Halley in 1707 to calculate that the comets of 1531, 1607 and 1682 were apparitions at 76-year intervals of the same comet, and predict its return inn 1758. The comet duly arrived and was named for him posthumously.

Edmond Halley, the British astronomer Royal whose name is immortalized by a certain popular comet, was born on November 8, 1656.

Before Halley observed his famous namesake, the comet that bears his name visited the inner solar system regularly since at least 240 B.C. Yet not one understood that comets orbits the Sun. It was believed that these mountain-sized chunks of frozen gas and dust appeared only once and then vanished. Many even thought that comets were freakish storms high in Earth’s atmosphere.

Halley, a gentleman adventurer who explored the seas, studied the atmosphere, and charted the skies, developed an interest in comets when he saw the very bright comet of 1680 while in Paris. He got the records of its movements from the director of Paris Observatory and tried – without success – to calculate the comet’s orbit.

The South Polar Cap of Mars. Photo by NASA of public domain

Later, Halley befriended Isaac Newton, the physicist and mathematician who devised the three laws of motion and the law of universal gravitation. Halley later convinced Newton to record his work in a volume known as the Principia, and he even paid for the book`s publication.

Newton told Halley that he believed comets circle the Sun in elliptical orbits. Using Newton`s laws, Halley computed the orbits of twenty-four comets. He found that the comets of 1456, 1531, 1607, and 1682 had very similar orbits. Halley decided they represented just one object, and he predicted that the comet would return in 1758.

And it did. On Christmas night 1758, sixteen years after Halley`s death, amateur astronomer George Palitzsch saw the comet on its way toward a rendezvous with the Sun in early 1759.