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Comets - Visitors from the Frozen Edge of the Solar System Transcript Comets - Visitors from the frozen edge of the Solar System Transcript Date: Wednesday, 20 November 2013 - 1:00PM Location: Museum of London 20 November 2013 Comets - Visitors from the frozen edge of the Solar System Professor Carolin Crawford Comets arrive to grace our skies every year – some are new to the inner Solar System, and some are old friends on a repeat visit, but only comparatively rarely do they reach sufficient brightness to become visible to the unaided eye. The main inspiration to give an introduction to comets for today’s lecture is that we might be in line for a sight of a ‘great comet’ later this winter, Comet ISON – but also to explain why we also might not…There’s a great quote from the famous comet-hunter David Levy: Comets are like cats; they have tails, and they do precisely what they want. As you’ll see, nothing is ever certain about comets, and this unpredictability is part of their charm and interest. A History of Comets The name comet comes from the Greek aster kometes meaning ‘long haired star’, a surprisingly friendly term for an object whose appearance would bring feelings of dread and fear to people many different historical cultures around the world. Comets do not behave like any other object that we can observe in the night sky with the unaided eye. Stars remain fixed in the pattern of their constellations, and are regular in their motion through the sky from one night to the next, and from one month to the next. Even a planet (also from the Greek for ‘wandering’ stars) follows a fairly slow and predictable path. By comparison, a comet is a totally different kind of event: it will appear unexpectedly, and at any place in the sky; it will change position from one night to the next relative to the background of stars; and its path will be along a separate direction and path across the sky from the planets and stars. During the few weeks or months that it is observable it will first steadily increase in brightness from one night to the next, may change its shape – growing bigger, longer or extra tails – and then wane to invisibility, never be seen again. As a consequence, people in many cultures grew to regard comets as messages from the gods. One of the earliest depictions of a comet portrays one as the Star of Bethlehem in Giotto’s Adoration of the Magi from 1305, a move thought to have been inspired by an apparition of Halley’s comet in 1301. This particular image is unusual in portraying the comet as a positive portent. Sudden comet apparitions were more commonly viewed as ill omens promising disasters such as war, famine, flood or the death of rulers. A selection of 17th century woodcuts portray a comet as a fiery sword blazing across the night sky, signifying war and death, and leaving chaos and disaster in their wake. Indeed, plenty of past comets have been blamed by the astrologers of their day for bringing or marking misfortune. ‘Caesar’s comet’ appeared a few months after the assassination of Julius Caesar in 44BC, and was recorded as visible even during daylight, and may be even the brightest such known. Caesar’s adopted son took advantage of the apparition and declared the comet to be the soul of his father rising up to heaven, exploiting this ‘omen’ so he could eventually seize control of the Roman Empire as Augustus Caesar. In South America the Incas observed a comet shortly before the cruel invasion of Francisco Pizarro. And the great comet of 1811, which was visible to the unaided eye for 260 days, was linked (among other events) to Napoleon’s invasion of Russia in 1812. A character in Tolstoy’s novel War and Peace describes observing an enormous and brilliant comet [...] which was said to portend all kinds of woes and the end of the world. Halley’s comet (of which more later) has by itself been blamed for several calamities. It was observed between April and June in 1066, presaging Harold II’s death at the Battle of Hastings. Chinese records of this apparition suggest it was twenty times brighter than Venus, and it is depicted on the Bayeux Tapestry. Halley’s comet also appeared three years after the fall of Constantinople to the Turks in 1453, leading (allegedly) to its excommunication as an instrument of the devil by Pope Callixtus III. Fear of the comet persisted even to its relatively recent times. There was widespread panic prompted by the prediction that its 1910 approach would be so close that the Earth would pass through the outer regions of its tail, which was known to contain traces of cyanogen. This was sensationalized in the media of the day, who stoked the fear with suggestions that mass cyanide poisoning would result. Entrepreneurs did a brisk business selling gas masks and comet pills. This is all despite astronomers reassuring people that the wispy gas in the tail would be far too diffuse to bring harm; it couldn't penetrate Earth's dense atmosphere, and even if it could, there wasn't enough cyanogen to bring any real harm. There are have been many spectacular comets throughout history; on average we are visited by what is termed a ‘great comet’ about three times a century. This appellation is saved for those comets that reach exceptional brightness (sometimes so much that they are visible during the day); there are records of them showing features such as multiple tails, haloes, red or green colours. When you read the accounts and view the paintings that record them, it does sometimes begin to feel like we’ve been shortchanged over the last few decades! Partially because they provoked such terror, apparitions of comets have been noted through history. Some of the oldest and most extensive observations on record come from Chinese astrologers/astronomers, with comet atlases that date back to the Han Dynasty (206 BC – 220 AD) and which incorporate knowledge from much earlier centuries. The track taken across the sky was recorded, along with a comet’s physical appearance, as the different forms were thought to be associated with different kinds of disaster. Despite all these observations, the Chinese did not seem to note any scientific thoughts on how comets were manifested. The early views accounting for their appearance came from the Ancient Greeks, and in particular from Aristotle (4th Century BC), who offered an interpretation of comets as phenomena high in the earth’s atmosphere. The prevailing Greek opinion demanded that the realm of the Heavens was a place of unchanging perfection and order, and thus they could not host something as unpredictable and changeable as a comet; instead it had to be associated somehow with the human domain of the Earth. This explanation persisted, and was not investigated scientifically until the 16th Century. The great Danish astronomer Tycho Brahe was inspired to test Aristotle’s idea by the apparition of the Great Comet of 1577. He arranged careful observations of both the comet and the Moon simultaneously from two separate locations. The aim was to look for any differences in their positions relative to the fixed locations of stars – ie observing any ‘parallax’ that results when a nearby object is seen projected against a different part of a background when seen from two different viewing points. The lack of an observable parallax - both observers saw the comet in exactly the same place - demonstrated that the comet could not be closer to Earth than the Moon, and that it could not be located in the high atmosphere. Tycho concluded that the comet had to be at least six times further away than the Moon, and this simple observation provided the first recognition of comets as truly cosmic bodies separate from the Earth, and members of the Solar System. The most famous of all comets is Halley’s comet; not that it is the most spectacular, but study of its orbit by the English astronomer Edmond Halley (1656-1742) was fundamental to pinning down the real nature of comets. During the 17th century, Halley was using Newton’s new mathematics of calculus to try to characterise the orbits of twenty-four comets from observations recorded over the previous four centuries. He realised that the orbital path of the bright comet recently seen in 1682 was very similar to that followed by two other comets – one observed in 1531 and one (seen by Kepler) in 1607. All moved in a retrograde direction (ie opposite to the revolution of the planets round the Sun), following an elliptical orbit that had a similar orientation to the plane of the planets’ motion. The great comet of 1456 was also known to have travelled in a retrograde direction. Halley’s inspiration was to realise that these were four apparitions of the same comet following a set path around the Sun, but which only became apparent to observers on Earth when its orbit returned it into the inner Solar System, after an interval of about 76 years. Although he did not live to see the success of his prediction of the comet’s return in 1758, when the comet was recovered on schedule it was given his name. Subsequently at least 23 previous apparitions of Halley’s comet have identified from historical records, the first known in a Chinese text dating from 240BC. Halley’s comet last swung round the Sun in 1986, and it is has long since sped back out into the cold of space, not due to return till 2061.
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