WEEK 11: MARCH 8-14

TOPIC: COMET 1P/HALLEY

     

To our ancestors of just a few centuries ago, comets were, at best, mysterious objects, very possibly of divine or supernatural origin. When one considers that bright comets could appear anywhere in the nighttime sky, seemingly out of nowhere, and after being visible for a few days or weeks would then disappear, it is little wonder why they thought so. Only gradually did human understanding of comets grow to the point where we know that they are regular members of our solar system – albeit still very interesting and worthy of study for a variety of reasons.

What could be considered the watershed moment in our understanding of comets is due to a British astronomer, Edmond Halley, who was born in Haggerston in 1656. Halley had a productive and colorful career, including two decades as Britain’s “Astronomer Royal.” He made many contributions to astronomy and other sciences, perhaps one of the most important of these being the “proper motion” of stars and thus the fact that stars, including our sun, travel through space. He was a contemporary and friend of the eminent British physicist Isaac Newton, and funded the publishing of the latter’s treatise Philosophiae Naturalis Principia Mathematica (Latin for Mathematical Principles of Natural Philosophy) – usually called the Principia – in 1687, wherein he described his laws of motion and gravitation.

In the early 18th Century Halley decided to apply Newton’s technique to calculating the orbits of 24 historical comets. Upon doing so he noticed that three of them, which had appeared in 1531, in 1607, and in 1682 (which he himself had observed as a young man), had orbits strikingly similar to each other, and moreover appeared at approximate intervals of 76 years. Halley then reasoned that these might not be three separate comets, but instead were returns of the same comet with that orbital period. He published his results in early 1705, and made the bold prediction that that same comet would return again in 1758.

Halley passed away in 1742 at the age of 85 and thus did not live to see whether or not his prediction would turn out to be correct. As 1758 approached several astronomers of that era, including the French astronomer Charles Messier who would become a champion discoverer of comets in subsequent decades, began making attempts for it, but these were initially unsuccessful. Finally, on December 25, 1758, a German amateur astronomer, Johann Palitzsch, discovered a comet in the constellation Cetus which a month later was recognized as being the long-awaited comet predicted by Halley. The actual perihelion date was March 13, 1759, and it became a conspicuous naked-eye object; meanwhile, in honor of his prediction and his recognition that comets are regular members of the solar system, the comet has been referred to as “Halley’s Comet” ever since.

             LEFT: Comet Halley in 1682 over Augsburg, Bavaria. RIGHT: Comet Halley over London in 1759, as painted by British artist Samuel Scott. Westminster Abbey is on the far left.

Once its periodic nature had been established, it has been possible to identify previous returns of Comet Halley, and indeed every return back to that of 240 B.C. has now been positively identified in the records of the respective times. It has almost always been a conspicuous naked-eye object, and has come quite close to Earth on occasion, the closest approach being to 0.033 AU in A.D. 837; according to Chinese records the tail around that time was approximately 90 degrees long. Other close approaches include ones of 0.088 AU in A.D. 374, of 0.090 AU in A.D. 607, and of 0.104 AU in 1066.

PERIHELION	q (AU)		PERIHELION	q (AU)
240 B.C. May 25.12	0.585		989 September 5.69	0.582
164 B.C. November 12.57	0.584		1066 March 20.93	0.574
87 B.C. August 6.42	0.586		1145 April 18.56	0.575
12 B.C. October 10.85	0.587		1222 September 28.82	0.574
66 January 25.96	0.585		1301 October 25.58	0.573
141 March 22.43	0.583		1378 November 10.69	0.576
218 May 17.72	0.581		1456 June 9.63	0.580
295 April 20.40	0.576		1531 August 26.24	0.581
374 February 16.34	0.577		1607 October 27.54	0.584
451 June 28.25	0.574		1682 September 15.28	0.583
530 September 27.13	0.576		1759 March 13.06	0.584
607 March 15.48	0.581		1835 November 16.44	0.587
684 October 2.77	0.580		1910 April 20.18	0.587
760 May 20.67	0.582		1986 February 9.46	0.587
837 February 28.27	0.582		2061 July 28.72	0.593
912 July 18.67	0.580		2134 March 27.90	0.593

The comet has managed to insert itself into our history from time to time. Its return in A.D. 66 was later believed by the Jewish historian Josephus to foretell the destruction of Jerusalem by Roman forces four years later. Its return in 1301 is alleged to have inspired the Italian painter Giotto di Bondone to make a comet represent the Star of Bethlehem in his fresco painting “Adoration of the Magi” in the Scrovegni Chapel in Padua. (The comet, in fact, had returned in 12 B.C.) The European Space Agency’s Giotto mission to the comet in 1986 was named in his honor.

        LEFT: Representation of Comet Halley over Jerusalem in A.D. 66, later believed by the Jewish historian Josephus to foretell the city's forthcoming destruction. Engraving from "Theatrum Cometicum" by Polish scientist Stanislaw Lubieniecki in 1668. RIGHT: The fresco "Adoration of the Magi" by Italian painter Giotto di Bondone in the Scrovegni Chapel in Padua, completed in 1305. Giotto was supposedly inspired by the appearance of Comet Halley in 1301 to include it as the Star of Bethlehem.

The return in 1066 not only involved a close approach to Earth, the comet was also apparently unusually bright. It is depicted in the Bayeux Tapestry, which shows King Harold II of England and his aides looking at it fearfully, as they took it as an omen of ill fortune. Across the English Channel, William the Conqueror took it as a good omen and launched his invasion of England; the comet turned out to be a self-fulfilling prophecy in both cases, with William defeating Harold’s forces at the Battle of Hastings. The comet’s return that year is also apparently depicted in Native American petroglyphs at Chaco Canyon in New Mexico.

        Representations of Comet Halley during its return in 1066. Left: From the Bayeux Tapestry, commemorating the Battle of Hastings. The Latin inscription reads "They marvel at the star." Right: Petroglyphs at Chaco Canyon in New Mexico. The bright star and crescent moon likely depict the appearance of the supernova of 1054 (which produced the Crab Nebula) and the series of concentric circles with streamers to the right is likely a depiction of the comet. Photograph courtesy Michael Lloyd of Wild Light Imaging Studio.

The return of 1531, in addition to being the first of the ones that Halley used in his calculations, is scientifically important for another reason. The German philosopher Petrus Apianus (Peter Apian), then a professor at the University of Ingolstadt in Bavaria, called attention to the fact that the comet’s tail always pointed away from the sun. It would be centuries before the explanation of this phenomenon was worked out – the solar wind in the case of ion tails, and solar radiation pressure in the case of the more visually prominent dust tails.

Since its return in 1759, Comet Halley has returned three more times. In 1835 it was recovered on August 6 by Father Etienne Domouchel and Francesco de Vico in Rome, en route to its perihelion passage in mid-November. It became as bright as 1st magnitude and exhibited a tail up to 20 degrees long, and was extensively studied by the astronomers of the era, including John Herschel – son of Uranus discoverer William Herschel – from the Cape of Good Hope in South Africa, and Friedrich Bessel in Germany. Bessel, in particular, made detailed studies of the “jets” in the inner coma, and surmised – correctly, as it turned out – that these acted as what are today known as rocket engines and, via Isaac Newton’s Third Law of Motion, caused the accelerations that had been observed in other comets, notably 2P/Encke. These motions, now called “non-gravitational forces,” are discussed as part of last week’s “Special Topics” presentation.

             LEFT: Representation of Comet Halley traveling through Leo during its return in 1531, as rendered by German philosopher Petrus Apianus (Peter Apian) at Ingolstadt, Bavaria. The sketches show the comet's tail pointing away from the sun. RIGHT: Sketches of the inner coma of Comet Halley in 1835 made by Friedrich Bessel, showing jetting activity.

Comet Halley's 1910 return, with perihelion passage taking place on April 20, was eagerly awaited by the astronomers of the time, in part because the viewing geometry would be excellent, but also because astro-photography and other relatively modern observing techniques had been developed. It was recovered on September 11, 1909, by Max Wolf at Heidelberg Observatory in Germany, being about 16th magnitude at the time. It brightened steadily after that, and was close to 8th magnitude when it disappeared into evening twilight in early March 1910.

Following conjunction with the sun, the comet emerged into the morning sky in early April, already a conspicuous naked-eye object of 2nd magnitude with a short dust tail. It continued brightening as it approached Earth – minimum distance being 0.151 AU on May 20 – and by the middle of May had become as bright as magnitude 0 with a tail in excess of 30 degrees. Shortly before its closest approach it passed directly between Earth and the sun, but while several attempts were made to observe it in transit across the solar disk, these were all unsuccessful, indicating that any solid component must be very small – which we now know to be true. Meanwhile, on the morning of May 19 the comet’s tail swept from well in front of Earth to well behind it, with measurements indicating a length of between 120 and 140 degrees – the longest cometary tail that has ever been observed.