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INFOMAN

 

HALLEY’S COMET

Astronomy

Halley’s Comet is the only periodical comet that can become a bright naked-eye object. It has a mean planetary period of 79.66 years, an eccentricity of orbit of 0.96727 and is inclined to the ecliptic at 162.239r. Its distance at perihelion, inside the orbit of Venus, when it is travelling at 120,000 mph is 0.58720 AU and at aphelion, beyond Neptune, when travelling at less than 2,000 mph, is 35.33 AU. At perihelion it arcs 180 in just over three months, whereas the remaining 180 take 75 years to cover. At each return it loses approximately 300 million tonnes of material.

The calculated years of return of Halley’s Comet between 722 BC and AD 9 are as follows: 690, 616, 540, 466, 391, 240, 164 (Perihelion: 12 November), 87 (Perihelion: 12 November) and 12 BC (Perihelion: 10 October). It may have been the comet recorded in 467 BC; certainly in 240 BC by Chinese astrologers as it came to perihelion on 25 May; and in 87 BC the Han-shu records, "In autumn during the seventh month (August 10 to September 8) there was a bushy star in the east", although in fact the month for Halley must have been the sixth.

In 12 BC it was followed by Chinese astrologers for 56 days (26 August - 20 October) through the constellations of Gemini to Scorpio. An imperial edict following its appearance read, "Lately, reproaches in the form of solar eclipses and meteors have been in the sky. These great strange signs were repeated and yet those in official positions remained silent; rarely has there been loyal advice. Now a bushy star has been seen in Tung-chin. We are very dismayed. The ministers, grandees, doctors and advisors are each to think solemnly as to the meaning of these changes and compare them clearly with the Classical texts: nothing is to be concealed..." The earliest known mention of the comet in Europe occurs in Dion Cassius’s account of a number of portents preceding the death of Agrippa, in his Roman History: "The star called comet hung for several days over the city and was finally dissolved in flashes resembling torches."

Since then it has been recorded at every perihelion. These occur every 74-80 years, its irregularity due to pertubations by Jupiter and Saturn. It returned in AD 66 (Perihelion: 26 January) and was first sighted by the Chinese on 31 January, later appearing in Capricorn and disappearing around 10 April; 141 (Perihelion: 22 March), when it was described as pale blue by the Chinese, being last seen in Leo; 218 (Perihelion: 17 May), when it was seen between Auriga and Virgo; 295 (Perihelion: 20 April), appearing in Andromeda and being last seen in Leo; 374 (Perihelion: 16 February), first seen in Aquarius on March 6; and 451 (Perihelion: 28 June), appearing in Taurus on 10 June and disappearing from Virgo on 15 August.

In 530 (Perihelion: 27 September) it was first seen in China on 29 August in Ursa Major, and described as pure white. Byzantine writers first saw the "huge and terrible star" in September, and, because it resembled a burning torch, it was called Lampadias; and at its next return in 607 (Perihelion: 15 March) it was first seen 30 March in Pegasus, conjuncting the Sun and finally vanishing in Gemini. Halley’s Comet was first depicted in the Nuremburg Chronicle of AD 684, when it was first sighted by the Chinese (and the Japanese, for the first time) on 6 September 684 in the western sky, but had disappeared by 9 October, having reached perihelion on 2 October.

It reappeared in AD 760 (Perihelion: 20 May) on 16 May in Aries and was last seen over 50 days later in Virgo. Byzantine chroniclers recorded Halley’s Comet as "like a glittering beam". A seperate comet was sighted in the opposite side of the sky on 18 May, and this was regarded as having serious astrological consequences, especially since an eclipse of the Sun, very large in Eastern Europe, followed on 15 August.

The return of 837 (Perihelion: 28 February) marked by far the closest approach to Earth, little more than ten times the Moon’s distance, at about three million miles. As a result the motion of the comet was strongly disturbed by Earth’s gravitational field. It appeared on 22 March in Aquarius. When nearest to Earth, the comet was moving across the sky at roughly 2 per hour and its tail was observed to be more than 60 in length and was distinctly seen to branch into two parts. The Chiu-t’ang-shu records, "the Emperor summoned the Astronomer Royal and asked him the reason for these star changes." Without contemporary observations, accurate to within .01 of a day, it would be impossible to calculate perihelions prior to this due to the perturbations in its orbit caused by Earth on this return. Had Halley’s comet returned just four days later it would have almost come within the Moon’s orbit, with a consequent huge alteration in its motion.

The 912 return (Perihelion: 18 July) is not recorded by the Chinese, but the Japanese saw it in the north-west from July 19 to 28. The 989 return was seen by the Chinese between 12 August and 9 September. Their observations suggest that the date of perihelion may have to be corrected from 5 September to 9 September. An Arab chronicle records "a planet with a tail" visible for 22 days from Cairo.

Halley made one historic return in 1066 (Perihelion: 20 March), while William of Normandy was preparing his invasion of Britain. William is supposed to have remarked to his courtiers, "A comet like this is only seen when a kingdom wants a king". The Bayeux Tapestry shows an ailing Harold on the throne pointing to his eye as his courtiers gesture towards the comet in terror. The caption reads isti mirant stella, ‘They wonder at the star’. According to the Anglo-Saxon Chronicle, "there was seen all over England a sign such as no one had ever seen. Some said that the star was a comet, as some called the long-haired star, and it appeared first on the eve of the feast of the Greater Litany (April 24)." The cathedral archives at Viterbo in Italy record it as having a tail streaming like smoke up to nearly half the sky. At Baghdad its reappearance after conjunction is described by Ibn al-Jawzi, "It reappeared on Tuesday evening at sunset with its light folded around it like the moon. People were terrified and distressed." The Koreans describe it as being as large as the Moon, and it is recorded in local chronicles all over Europe and Russia. It was first seen in China on 3 April and was observed both before and after conjunction for a total of 67 days, suggesting an unusually bright return. After perihelion they describe its broom-like vapour and compare it to a one-tenth peck measure.

Because it is not at the same position relative to Earth it is not equally brilliant at each return. It was bright in 989 and in 1301 (Perihelion: 25 October), when it was depicted by Giotto di Bondone (b. 1266-1276, Vespignano; d. 8.1.1337, Florence) in his 1303 painting Adoration of the Magi as the Star of Bethlehem, but less so in its intervening returns of 1145 (Perihelion: 18 April) and 1222 (Perihelion: 28 September), and in 1378 (Perihelion: 10 November) when it passed within 10 degrees of the north celestial pole. The 1145 return may appear in the Eadwine (Canterbury) Psalter which depicts a comet at the bottom of the page where Psalm 5 begins, and it is thought the monk Eadwine drew it because the comet appeared as he was completing the page. An Anglo-Saxon comment beside the drawing translates as, "Concerning the star called comet. A suchlike ray has the star known as Comet, and in English it is called ‘the hairy star’. It appears seldom, after [periods of] many winters, and then for an omen".

In 1456 (Perihelion: 9 June), however, it was sufficiently brilliant for Pope Calixtus III to issue a declaration against "the Devil, the Turk and the comet". The doom of the entire Christian world was expected at the hand of the Turkish army of Mehmet II beseiging Belgrade, due to Halley’s Comet. Detailed observations were made by Paolo del Pozzo Toscanelli in Florence from 8 June - 8 July, but these were not available to Halley. They were rediscovered in 1864 and allowed Giovanni Celoria to establish the comet’s orbit on this 1456 return with great precision.

It next came to perihelion on 25 August 1531 at 1 39’ Aquarius, and on 27 October 1607 at 2 16’ Aquarius. Johannes Kepler recorded his observations of the comet at Prague from 26 September to 26 October, and Christian Longmontanus at Malmo and Copenhagen from 1- 26 October. These records were used by Halley to calculate the comet’s orbit, though Kepler apparently believed comets moved along straight lines, despite his work on the elliptical orbits of planets.

Dr Edmond Halley (b. 8.11.1656, London, d. 14.1.1742, Greenwich), Britain’s second Astronomer Royal and a friend of Sir Isaac Newton (b. 25.12.1642, Woolsthorpe, nr Grantham, d. 20.3.1727, London), calculated the path of a bright comet from observations made by John Flamsteed (b. 19.8.1646, Denby, nr Derby; d. 31.12.1719, London), Astronomer Royal at Greenwich, and his assistant, though he observed the comet himself from 26 August to 10 September 1682 (Perihelion: 15 September 1682 at 2 53’ Aquarius), and noticed how similar its path was to the recorded movements of comets seen in 1531 and 1607. He concluded in 1695 that the three comets were one and the same and that its path was orbital, but it was not until 1704 that his conclusions were published. He anticipated the return of the comet "about the end of the year 1758 or the beginning of 1759". It duly reappeared, being first seen on Christmas night 1758, coming to perihelion 13 March 1759 and proving his theory. Had any doubt remained, its 1835 return, at perihelion on 16 November, dispelled them. Mark Twain (b. 30.11.1835, 0445 LMT, Monroe City MO, in Florida) said, "I was born with Halley’s Comet and expect to die on its return". He died on 21 April 1910, in Redding CT, two days after its next perihelion.

This return was first detected on 12 September 1909 by Max Wolf in Germany. It was then still 25 million miles from the Sun. It came to perihelion on 19 April at 5 Aquarius. On 18-19 May 1910 it passed directly between the Earth and the Sun, travelling at 3 million miles per day, but could not be seen, confirming the flimsy nature of cometary substance. It is thought that Earth actually passed through its tail on the 19th. On 5 June 1910 it lost its plasma tail, leaving the original tail far behind the comet’s track.

Its next return was first sighted on 16 October 1982, using the Mount Palomar telescope, three years before its return. Five spacecraft were sent to examine Halley’s Comet at its return in 1985: two Japanese (Suisei and Sakigake), two Russian (Vegas 1 and 2) and one European (Giotto). Giotto was launched 2 July 1985 and actually passed through the comet’s head, within 335 miles of the velvet black nucleus, obtaining 2,112 close-range images of it, until the probe was jolted by hitting a rice-grain sized dust particle. It revealed that the nucleus was an irregular lumpy potato-shaped object, 9.3 miles in length, and had a rotation period of 53 hours, with a 7.3-day rotational period around this axis. Its temperature on the side nearest the Sun was 47C.

Halley’s Comet was found to be in effect a "dirty snowball"; eighty per cent of all the molecules being thrown out from the comet were water, but close in the dust was rich in hydrogen, carbon, nitrogen and oxygen, indicating that the nucleus is covered in a layer of organic material. The property of deuterium (heavy hydrogen) in the water of Halley’s Comet is closer to the proportion in Earth’s oceans than to the abundance in the Sun and the gassy planets. It is possible, therefore, that Earth’s oceans could be made of melted comet. Comets could also have contributed material to Earth’s atmosphere while discharging water.

The most likely cause of Halley’s dark surface is polyoxymethylene, a chain of formaldehyde molecules, following Giotto’s detection of the first polymers in space a year after Halley’s approach. The appearance of Halley’s surface enhanced the idea that at least some asteroids might be exhausted comets. Giotto confirmed that comets are ancient relics of pristine material which have remained virtually unchanged since the formation of the Solar System between 4,500 and 5,000 million years ago.

Halley was last at perihelion on 9 February 1986 at 0640 hr GMT, at 15 35’ Aquarius, 75.81 years after the last, and was at close approach to Earth on 27 November 1985 and 11 April 1986. Since 1531 Halley’s Comet has invariable come to perihelion in the sign of Aquarius.

It is possible that this was Halley’s Comet’s final return. In 1987 a Carmelite Nun named Sister Maria Gabrel claimed that Halley’s Comet would suffer an explosion in 1991, and on 12 February 1991 two Belgian astronomers, Olivier Hainant and Alain Smette of the Southern European Observatory in Chile, discovered an outburst of Halley’s Comet, then 1,243 million miles away. It was surrounded by a cloud of 19,000 miles, where normally it would occupy only 9 miles, and was 6 magnitudes or 300 times brighter than normal. They concluded that Halley’s Comet had collided with another body and could now have fragmented entirely, unless it has become two or more smaller comets. A scientific forecast society claimed that Halley’s Comet’s orbit had been knocked out of orbit and was returning to the Sun. Its next return should occur in 2061 but this is now open to some doubt.

Astrology

It is said by astrologers that the task of Halley’s Comet is to drive mankind deeper into materiality, and this could have been justifiable at certain stages of our historical evolution. However, it is by overcoming the opposing forces given by this comet that we may develop the strength to leave the abyss. Halley’s Comet is thought to persuade people to remove the physical excuses and hypochondria they use to avoid undesirable things, and teaches an organic holism, creating renewed vitality.

Since 1682 Halley’s Comet has witnessed man’s descent into what has been seen as a shallow materialism; for example in 1759 when its return coincided with the Enlightenment which preceded the French Revolution; or in 1910 when its return preceded the First World War.