COMETS. 109 



when seen following the nucleus as the sun is approached. This distinction has clisap- 

 peai'ed from all modern astronomical works, and the latter name is given to the append 

 age, whatever its apparent position. Neither this luminous attendant, the tail, nor the 

 nucleus, are now considered essential cometary elements, but all bodies are classed as 

 comets which have a motion of their own, and describe orbits of an extremely elongated 

 form. There are several plain points of difference between comets and planets. The 

 planets move in the same direction from west to east, which is astronomically called 

 direct motion ; but the movements of comets are often from east to west, or retrograde. 

 The orbits of all the planets are confined to a zone of no great breadth on either side of 

 the ecliptic ; but the paths of comets cut the ecliptic in every direction, some being even 

 perpendicular to it, traversing the heavens in all parts. The contrast is striking likewise 

 between the forms of their respective orbits. A hoop will with no great inaccuracy 

 represent the courses of the planets, but cometary paths are of every possible eccentricity, 

 both elongated ellipses and open curves, as parabolas or hyperbolas. Only one end of the 

 ellipse lies within the limits of the system, in the case of the great majority of comets with 

 shut orbits. They only visit our gaze therefore during one part of their course, and that 

 a very small part, travelling during the rest of their journey far beyond the range of the 

 most distant planet, into spaces inaccessible to our sight. Those which describe parabolas 

 and hyperbolas are casual vistors only ; and depart to return no more. Planetary con 

 figuration is also uniformly globular, but the external appearances of comets exhibit great 

 diversities of form, from that of an irregular wisp of cloud to a simple spherical luminosity, 

 or a strongly- defined scimitar-shaped aspect. 



Most of the ancients, following Aristotle, regarded comets simply as meteors born and 

 perishing in the atmosphere of the earth. Seneca, however, clearly classed them with the 

 enduring realities of nature, having a definite path, and not wandering uncertainly through 

 a transient existence : " I cannot believe," he observes, " that a comet is a fire suddenly 

 kindled, but that it ought to be ranked among the eternal works of nature ; it has its 

 proper place, and is not easily moved from thence ; it goes its course, and is not extinguished, 

 but runs off from us ; " and in a passage already quoted, he anticipates the arrival of a 

 Newton or Halley to determine then: orbits, and the laws of their motions. Tycho Brahe 

 took the initial step in the path of true discovery by assigning them a place out of the 

 terrestrial atmosphere. By careful observation of the comet of 1577 he proved its extra- 

 lunar position in space. It yielded no sensible diurnal parallax, and was therefore beyond 

 the region of the moon. Hevelius next ascertained the concavity of the orbits of comets, 

 which Keppler had supposed to be straight lines. Newton succeeded in demonstrating that 

 comets are guided in their movements by the same principle as that which controls the 

 planets in their orbits, as the law of gravitation admits of revolving bodies describing any 

 one of the conic sections, or the four curves, the circle, ellipse, parabola, and hyperbola. Halley 

 finally, after a laborious comparison of elements, arrived at a measurable ellipse as the 

 orbit of one of these bodies ; and predicted the periodic return of the object, which has twice 

 appeared at the time appointed to verify his conclusion. The diagram represents a part 

 of the path of one of the long period comets, that of 1680, obviously but a very small 

 part, as it was described in little better than two months, and the periodic time is supposed 

 to be not less than five hundred years. The direction of the luminous train or tail is 

 shown, the frequent attendant of cometary bodies. This is nearly always away from the 

 sun, frequently assuming a curved form. It increases in length with its proximity to the 

 solar body, but does not acquire its greatest extent until after the perihelion or the point 

 nearest to the sun is passed. If we regard the train as vaporisation produced by the 

 intense heat to which the body of the comet is exposed upon approaching the sun, this 

 accounts for its increasing length and greatest extent after the perihelion, just as it is after 



