March, 1907.] 



KNOWLEDGE & SCIENTIFIC NEWS. 



57 



Halley's Comet. 



By I-'. VV. Henkel, B.A., F.R.A.S. 

 (Late Directot 1 < Markrei V ... 



'I he .shortly expei ted return of this well-known object, 

 which was the first of these bodies known to move in 

 closed paths round the Sun, and the remarkable 

 phenomena attending its last appearance (in 1S35 and 

 1836) render Halley's Comet a peculiar object of in- 

 terest at the present time. 



Newton, in the third section of the Principia, first 

 showed that a body moving under the influence of a 

 force varying inversely as the square of the distance 

 from the centre of force, will describe one or other 

 of the curves known as the " conic sections," i.e., eith 1 

 an ellipse (or circle, as a special case), a parabola, or a 

 hyperbola. These three curves may all be obtained by 

 cutting a cone in different ways by a plan.-, but perhaps 

 they may be more intelligibly defined to the non-mathe- 

 matical reader as obtained by throwing the shadow 

 of a circular disc upon a plane, such as the surface of a 

 table. If, however, the disc is held parallel to 

 the table, we shall gel a circle ; if it is held 

 edgeways to the light, the shadow will be a straight 

 line. If now we raise our disc so that its highest point 

 is on a level with the source of light, we shall get a 

 curve known as a parabola, which will be oval at one 

 end, but the two sides will open out. If now we hold 

 our disc still higher, we shall get another curve still, 

 whose two sides will separate even further from one 

 another. This curve is known as the hvperbola. 



Whilst the planets move in ellipses, so little differing 

 from circles that if represented on paper the deviation 

 is not perceptible, on the other hand, most comets are 

 found to move in orbits so nearly parabolic that only 

 in a few cases are they known to be otherwise. A 

 great cornet which appeared in 1680, and approached 

 very close to the Sun, was the first whose path was 

 calculated as a parabola, though there is some reason 

 to believe that it was not truly so, but an enormously 

 elongated ellipse. 



In 1682 a comet was observed by Newton. Halley. 

 and others, and on examining the circumstances of its 

 motion, Edmund Halley computed its orbit on the sup- 

 position that this was a parabola, Comparing his 

 results with observations of previous cometSi for which 

 purpose it was necessary lor him to compute their orbits 

 from the necessarily imperfeel observations of earliei 

 times, he found that in 1531 and 1607 comets had ap- 

 peared which followed si 1 nearly the same path as this 

 one that he ventured to assert its identity with them. 

 and to predict its return in a period of about 75 years. 

 It was afterwards ascertained that comets had been 

 seen in 1066, 1378. and 1456 whose paths were the 

 same as thai of the comet oi 1682, and it is now known 



that all these were apparitions oi one and the same 

 body. In 1066 its appearance was figured on the 

 BayCUX tapestry, and it was regarded (after the event) 

 as an omen of the Norman Conquest. In [456 the 

 comet is said to have been of extraordinary splendour, 

 its tail (10 degrees long, and it is slated that a papal 



bull was fulminated against tin- 'Turks and the comet 

 and it was ordained that the bells nl all churches should 

 be rung at mid-day. Athough Halle) had predicted its 

 reappearance, he did not live to observe this himself, 

 dying in 1742, at the age oi 83, aftei having been Astro 

 nomer Royal for 23 years. He pointed out that the 



comet must have passed very near the planet Jupiter in 

 the interval between 1607 and 1682, and its velocity 

 increased, thereby resulting in a shortening of its period 

 volution. Thus he concluded that, whilst the 

 interval between 1607 and 1682 was only 75 years, the 

 following revolution would probably take a longer time; 

 but the then state of Mathematics did not enable him to 

 make the necessary calculations to determine this with 

 accuracy. Were the Sun and comet alone existing in 

 space, the hitter's path would be an exact ellipse, and 

 the period of its revolution always the same. This is, 

 however, not the case. Besides the Sun I 

 tlie planets, and these, by the law ol gravitation, attract, 

 and are attracted by, one another, and other bodies. 

 Their masses, however, being very small, in comparison 

 with that of the Sun, the general nature of the paths 

 pursued by planets and comets is not changed by this 

 action ; but deviations nevertheless arise, which are the 

 more perceptible as their masses are greater and their 

 approaches more close. 



Thus Jupiter, the giant planet of our system, whose 

 mass is about lT ,\ TTJ that of the Sun, has at tin 

 greater effect on comets when near to him than the Sun 

 itself. Lexell's comet of 1770 must have been at one 

 time fifty-eight times less distant from Jupiter than 

 from the Sun, and so the planet's attraction (inSr. th at 

 of the Sun) must have been three times greater. 



"The celebrated Ciairaut. who so greatly advanced the 

 science of astronomv by his work on the Moon, as well 

 as by his researi he- in pure Mathematics, undertook 

 the great labour of calculating the effect ol the action 

 of the planets upon Halley's Comet for a period of about 

 130 years, and in a memoir presented to the Academic 

 des Sciences, at Paris, he predicted the date of perihelion 

 as the 1 St h April, [759, subject to an uncertainty of 

 about a month. As the result of his calculations he 

 estimated that the period "I revolution of the comet 

 was increased by 100 days on account of the action of 

 Saturn, and 518 days by Jupiter. It was first seen by 

 Palitsch, a Saxon peasant, about the end ol 1758, and 

 came to perihelion on March 12, 17511, just a month 

 earlier than the time assigned by Ciairaut. Before its 

 next return the orbit was calculated by no less than 

 lour mathematicians, Damoiseau, Pontecoulant, Rosen- 

 berger, and l.ehmann. and they all agreed in giving 

 a day in the month of November, 1835, as the tin 

 its perihelion passage It was first seen at Rome early 

 in August of that year, aim\ was visible up to the 16th 

 November in the Northern Hemisphere. 



After this, passing its perihelion on that day. it was 

 seen at the Cape and at Melbourne up to the early 

 part of May, 1836, when it finally disappeared from 

 view. Very careful observations and elaborate draw- 

 ings oi its appearance were madeb) Sir John Herschel, 

 who was then in South Africa. At first it presented the 

 appearance of an almost round nebula, having a bright 

 nucleus not quite at its centre. By the beginning ol 



October. [835, a small tail appeared, and this 



a length of about 20" by the middle ol" the month. 



this the tail diminished, so that before the time ol" 



lion 1N1 \ niiui in it had again disappeared. 

 On the 2nd ot October, the dav when the tail was first 

 seen, an emission of light was seen coming from the 



IS, on the side presented towards the Sun. 'This 

 .•mission ceased for a time and then recommenced on 

 the 8th oi that month. At this time one 



1 i ived w hat he called a " second tail," in a direct 101 



posit, ■ to the original tail- thus presented towards the 



Sun. The sl\a|H' and brightness of the emanations con- 



