65 



COMETS. 



COMETS. 



66 



comet, which continued visible 70 days. The tail is said to hav 

 covered a fourth part of the heavens, and occupied four hours in 

 rising. 



43 B.C. In this year, during the celebration of the games held in 

 honour of Venus, a great comet appeared at Rome, which was visibl 

 before sunset. This celestial prodigy was supposed to have some con 

 nection with the departed soul of Julius Caesar, whose assassination ha< 

 shortly before occurred. 



389 A.D. There appeared a comet, which is said to have almos 

 rivalled Venus in brightness. The tail, which was of immense length 

 was curved like a scimitar. The apparition of so extraordinary a phe 

 nomenon excited universal terror. 



891 A.D. Contemporary European writers allude to a great come 

 which appeared in this year. In the Chinese annals, wherein allusion 

 to it is also to be found, it is stated that the tail was 100 in length. 



1106. A great comet appeared, which was visible over all Europe 

 The tail is said to have resembled a fiery beam. According to Matthew 

 Paris, it was visible in the day-time. 



1264. This year was distinguished by the apparition of a niagni 

 ficent comet, which is alluded to by several European writers, and is 

 also mentioned in the Chinese annals. The tail is said to have attainec 

 a length of 100. 



1 402. Two comets of extraordinary splendour appeared in the course 

 of this year. The first became visible in the beginning of spring, am 

 towards the close of March was so bright as to be visible in the 

 day-time. The second comet of the year was not less conspicuous than 

 the first, if we are to believe the statements of contemporary writers 

 The tail is said to have extended from the horizon to the zenith. It ii 

 stated, also, as in the case of the first comet, to have been visible in f ul 

 daylight. 



1456. A magnificent comet was visible throughout all Europe. The 

 tail is said to have been 60 in length. The apparition of the pheno- 

 menon excited universal terror, in consequence of its being simul- 

 taneous with the capture of Constantinople by the Turks. With the 

 view of averting the evil influence of its presence, Pope Calixtus II. 

 ordered prayers to be offered up in all the Western churches ; he also, 

 in a famous bull, anathematised at once the Turks and the comet. It 

 has been satisfactorily established in modern times that this was one oi 

 the early apparitions of Halley's comet. 



1472. The comet of this year was undoubtedly the most splendid of 

 the century. Towards the end of January it was visible in full daylight. 

 In Europe Regiomantanus observed it. In China its successive posi 

 tions with respect to the stars were also carefully recorded. 



1531. An early apparition of Halley's comet. Observed in Europe 

 by Peter Apian, at Ingoldstadt. An account of this apparition is also 

 to be found in the Chinese annals. 



1532. A comet appeared this year, which is stated by Cardan to have 

 been visible in full sunshine. 



1556. Apparition of a great comet, which has been supposed by some 

 astronomers to be identical with the comet of 1264. 



1577. The comit of this year is memorable in history from having 

 furnished the data which enabled Tycho Brahe' to demonstrate that the 

 regions traversed by cometary bodies in general lie beyond the moon's 

 orbit. 



1607. An apparition of Hslley's comet. The phenomenon was ob- 

 served on the Continent by Kepler and Longomontanus, and in 

 England by the celebrated mathematician, Hariot. The head is said 

 to have equalled in size the planet Jupiter, but to have shone by a pale 

 and watery light. The tail, which was of a very conspicuous bright- 

 ness, was about 7 long. 



1618. The third comet of this year was one of the most splendid of 

 which history makes mention. Longomontanus states that the tail was 

 100 long. 



1652. Apparition of a conspicuous comet, which is minutely described 

 by Hevelius. 



1664-5-8. Each of these years was distinguished by the apparition of 

 a comet of considerable brightness. 



1680. The comet of this year is, for several reasons one of the most 

 remarkable of ancient or modern times. It was first seen by Godfrey 

 Kirch, at Coburg in Saxony, on the 14th of November. After its 

 passage of the perihelion on the 20th of December, it shone with great 

 splendour, the tail appearing in some places to extend over an arc of 

 90'. This comet approached nearer the sun than any other comet 

 recorded in history, with the exception of the great comet of 1843. It 

 has been already stated that the observations of this comet furnished 

 the data by means of which Newton was enabled to demonstrate that 

 the orbits of comets are conic sections, having the sun situate in their 

 common focus. 



1744. This was the most brilliant comet of the 18th century. It was 

 discovered at Haarlem by Klinkenberg, on the 9th of December, 1743. 

 On the 7th of February the tail was 20 in apparent length. On the 

 1st of March, when the comet passed the perihelion, it was seen in full 

 daylight. Remarkable physical changes were observed to occur in the 

 head of this comet, on the occasion of its approach to the perihelion. 



1759. An apparition of Halley's comet. 



1769. This comet is memorable for the immense tail by which it was 

 accompanied. Its passage of the perihelion took place on the 8th of 

 October. On the 10th of September its tail appeared at Paris to be 



ARTS ASD 8CI. DIV. VOL. Ul. 



60. According to Pingre', the apparent length of the tail in tropical 

 countries measured 97**. 



1807. The comet of this year was very conspicuous to the naked eye. 

 It was first discovered at Castro Giovanni in Italy, on the 9th of Sep- 

 tember, by Parisi, an Augustine monk. The passage of the perihelion 

 occurred on the 19th of the same month. This comet was carefully 

 observed by Sir William Herschel, who, in a paper published in the 

 ' Philosophical Transactions of the Royal Society' for 1808, has recorded 

 many interesting facts respecting it. 



1811. The first comet of this year is in many respects one of the 

 most remarkable of modern times. It was discovered by M. Flaguerges, 

 at Viviers, on the 26th of March. The passage of the perihelion took 

 place on the 12th of September. From that time till the end of the 

 year it formed a very conspicuous "object in the heavens, the effect 

 being enhanced by the circumstance of its apparent path lying so near 

 the North Pole that it always remained above the horizon. It was 

 last seen in Siberia, by Wisniewski, a Russian astronomer, on the 17th 

 of August, 1812. 



1835. An apparition of Halley's comet. 



1843. One of the most splendid comets recorded in history. It was 

 seen with the naked eye, close to the sun, in Italy, the Cape of Good 

 Hope, and America, on the 28th of February, the day of its passage of 

 the perihelion. In some places the tail was observed to extend over an 

 arc of 65. It generally disappeared from observation about the begin- 

 ning of April. This comet is remarkable for having approached nearer 

 the sun than any other comet of modern times. 



1853. A very fine comet appeared in the autumn of this year. 

 Throughout Europe it was distinctly visible to the naked eye shortly 

 after sunset. 



1858. The comet discovered by Donati on the 4th of June in this 

 year, is one of the most splendid of which history makes mention. It 

 first became generally visible to the naked eye on the 5th of September. 

 The passage of the perihelion took place on the 30th of that month. 

 The comet attained its greatest slendour about the 10th of October. 

 The tail then appeared to extend over an arc of about 40. The comet 

 ceased to be visible in Europe about the 20th of October, but it con- 

 tinued to be observed by Mr. Maclear at the Cape of Good Hope till 

 the beginning of March in the following year. 



Theory of the Movement! of Comett. 



The theory of the movements of comets resolves itself into two great 

 subjects of research. One of these relates to the determination of the 

 orbit of a comet from a definite number of observed positions, sup- 

 posing it to revolve in a conic section around the sun ; the other takes 

 cognisance of the effects of planetary perturbation upon its motion. 

 Newton's method for determining the elements of a comet's orbit was 

 founded on the hypothesis of its revolving in a parabola. His opinion 

 indeed was, that all comets revolve in very elongated ellipses ; but he 

 remarked, that in any of such cases the orbit near the perihelion does not 

 deviate sensibly from a parabola. By supposing the path of the comet 

 k> be parabolic, the investigation of its elements is considerably sim- 

 plified ; but even with this assumption the problem is one of the most 

 difficult in astronomy. It is plain, also, that the parabolic hypothesis 

 cannot assign the major axis of the orbit, nor consequently the time of 

 revolution. Newton, as already stated, remarked that the time of 

 revolution might be found by comparing the intervals which elapsed 

 Between the apparitions of comets having the same parabolic elements. 

 For this purpose, it is necessary to form a catalogue of the elements of 

 all those comets the orbits of which have been computed ; then, when 

 a new comet has been observed, and its parabolic elements calculated, 

 a reference to the catalogue will serve to indicate whether it has been 

 observed on any former occasion ; and if the newly calculated elements 

 should thus turn out to be identical with those of any comet in the 

 catalogue, the interval between the passages of the perihelion will give 

 ;he time of revolution, supposing the two apparitions to be consecutive. 

 'n this way Halley determined the time of revolution of the comet 

 which bears his name ; and the same mode of ascertaining the periodi- 

 city of a comet is, in consequence of its easy application, constantly 

 >ractised in the present day. The method most commonly used for 

 computing the parabolic elements of a comet is one invented by the 

 Herman astronomer Olbers, towards the close of the last century. 



It is plain that the determination of the elements of a comet's orbit 

 upon the parabolic hypothesis, is subject to the defect of not giving the 

 major axis by direct investigation. In order to ascertain this element, 

 t is necessary that the comet should have been observed at two con- 

 secutive passages of the perihelion. Nay, it may happen that, although 

 he comet has been observed on more than one occasion, the apparitions 

 may not be consecutive ; and yet there exists no criterion by which a 

 lefinitive conclusion on this point may be arrived at. Geometers have 

 ccordingly investigated methods for computing the elements of a 

 omet's orbit, independently of any hypothesis with respect to the 

 pecies of conic section in which it may be revolving. In this branch 

 f research, Laplace and Gauss have laboured with eminent success. 

 Lccording to the method devised by the latter geometer, the six 

 lliptic elements of a comet's orbit may be derived from three (in some 

 ases four) observed positions of the body. 



When a comet has once been discovered, three observed positions 

 enerally suffice, by the aid of Olbers's method, for ascertaining the 



