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COMENIUS. knowledge a pleasure rather than a task; and (6) making instruction universal. While the formulation of many of these ideas is open to criticism from more recent points of view, and while the naturalistic conception of education is one based on crude analogies, the importance of the Comenian influence in education has now been recognized for half a century. The educational writings of Comenius comprise more than forty titles. In 1892 the three-hundredth anniversary of Comenius was very generally celebrated by educators, and at that time the Comenian Society for the study and publication of his works was formed. Consult: Laurie, John Amos Comenius, Bishop of ihe Moravians: His Life and Educational Works (London, 1884); Quick, Essays on Educational Reformers (London, 1868); Raumer, Geschichte der Pädagogik, vols. i.-iv. (Gütersloh, 1874-80); Müller, Comenius, ein Systematiker in der Pädagogik (Dresden, 1887); Löscher, Comenius, der Pädagog und Bischof (Leipzig, 1889).

COMET (AS., Lat. comet a, comet, from Gk. Kopf^ris, kometes, having long hair, from KOfxav, komun, to wear long hair, from Kbij.ri, komC, hair). The word "comet' had its origin in the hairy appearance often exhibited by the haze or luminous vapors, the presence of which is at first sight the most striking characteristic of the celestial bodies called by this name. The gen- eral features of a comet are: a definite point or nucleus, a nebulous light surrounding the nu- cleus, and a luminous train preceding or following it. Anciently, when a train preceded the nucleus — as is the case when a comet has passed its perihelion and recedes from the sun — it was called 'the beard.' being termed 'the tail' when seen following the nucleus as the sun is ap- proached. This distinction has disappeared from all modern astronomical works, and the latter name is given to the appendage, whatever its apparent position. Xeither this luminous at- tendant, the tail, nor the nucleus, is now con- sidered an essential cometary element, but all bodies in the solar system are classed as comets if they 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 almost perpendicular to it. The orbits of all the planets are nearly circular; or, more properly speaking, are ellipses of very small eccentricity. The orbits of comets, on the other hand, present great variety of eccentricity, many of them being ellipses or elongated closed orbits of various degrees of elongation : others, though very rarely, may be In-jierbolas ; while the majority have a fonn of orbit not differing sensibly from the parabola, which is the limiting form of cun-e to which both the ellipse and hyperbola usually approximate. Any attraction, linwever, of an extraneous body, like a planet, interfering with the attraction of the sun. might change the orbit from the ellipse to the hyper- bola, and vice versa, or from the parabola to either. As, however, there is only one parabola corresponding to infinite sets of ellipses and hy- perbolas, an interfering cause is not likely to change the orbit from an ellipse or hyperbola to the parabolic form. Of about 3o0" comets whose orbits have been obtained with more or less accuracy, 00 appear to have described el- lipses, 275 orbits cannot be distinguished from parabolas, and in two cases the hyperbolic form of orbit is extremely probable. The discovery that comets are celestial bodies extraneous to our atmosphere is due to Tycho Brahe, who ascertained the fact by obsenations of the comet of 1577. Newton succeeded in demonstrating that their movements are subject to the same law which controls the planets in their orbits. Hallcy was the first, by determining the para- bolic elements of a number of comets from recorded observations, to identify the comets of lt!82 with one which had been observed in 1007 and with one obsened by Apian at Ingolstadt in 15.11, and thus confidently to predict the return, at the end of 1758 or the beginning of 1759, of a comet which would have the same parabolic elements. This prediction of the first "periodic' comet moving in a closed oval orbit simply meant that the portion of the closed orbit lying nearest the sun, and therefore the only observ- able portion of the orbit, would very closely resemble the parabolas or open curves in which this comet had been supposed to be moving at its earlier appearances.

Parabolic cometary elements are the following: (1) The inclination: (2) the longitude of the node; (3) the longitude of the perihelion or point of nearest approach of the sun : ( 4 ) the perihelion distance, or nearness of approach to the sun: (5) the direction of motion, whether direct or retrograde. The first two of these elements determine the plane of the orbit. To determine these parabolic elements, three observations of the comet are sufficient; and by a table of such elements, calculated from recorded observations, it is possible at once to ascertain, as Halley did, whether any newly observed comet is identical with any that has been previously observed. However, to predict with accuracy the time of the return of a comet, a much more accurate calculation must be made of the orbit, taking into account the perturbations of the planets to i'hose inflvience it is subject. This ditficult problem was solved, in the case of Halley's comet, by the joint work of Lalande, Mme. Lepante, and Clairaut, who announced in November, 1758, just as astronomers began to look out for the return of the comet, that it would take CIS days more to return to the perihelion than on the preceding revolution. The perihelion passage was fixed about the middle of April. 1750; but Clairaut distinctly stated that, beins pressed for time, he had neglected small values which collectively might amount to about a month in the seventy-six years. The comet passed the perihelion on ilareh 12, 1759, exactly a month before the time annoimced. but within the assigned limits of divergence from that date. The elements of its orbit proclaimed it to be the comet of the former appearances by their similaritv. For the next perihelion passage, the diflferent calculations executed by Damoiseau and De Pontecnulant fixed the 4th. the 7th. and the 13th of Xovember. 1835. Subsequently, observations indicated the 10th — that is to sav. a deviation of onlv three liar's from