COMETS. 109 



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

 peared 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 the cometary paths are immensely elongated 

 ellipses, their breadth bearing no proportion to their length. Only one end of the ellipse 

 lies within the visible limits of the solar system, in the case of the great majority of these 

 bodies. 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. The circumstances of their 

 motions plainly distinguish them from the planets, fixed stars, and nebula. Planetary 

 configuration 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 their 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. Herelius next ascertained the concavity of the orbits of 

 comets, which Tycho had supposed to be straight lines. Newton demonstrated their real 

 path round the sun to be either in a parabola, an ellipse whose transverse diameter is 

 extended to infinity, so that they appear, vanish, and are gone for ever, or in an ellipse 

 so elongated as to be insensibly different from a parabola within visible limits. 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 



