COMET

(Gr. xo,u.r,ryis, from xo/xfi, hair.) The name given to a numerous cla of celestial bodies belonging to the solar system. The luminous point which shines with greater or le brilliancy at the centre of a comet is called its nucleus. The nucleus is generally surrounded by a nebulosity, or luminous aurora. The train of light, of greater or le extent, with which most comets are accompanied, is called the tail. Formerly this name was only applied to the luminous train when it fell behind the comet in its diurnal motion; if it preceded the comet, it was called the beard; but this distinction has disappeared in modern works on Astronomy. The ancients gave the name of comet to every nebulous star or meteor which was observed to pa succe ively through different constellations. Modern astronomers apply the name, notwithstanding the etymology, to stars which have neither nebulosity nor tail. According to them, the distinctive characters of a comet are, 1st, that it po e es a proper motion; 2d, that it traverses space in a curve so elongated that in the distant parts of its orbit it ceases to be visible. The proper motion distinguishes comets from those new stars which occasionally appear, and become extinguished without changing their place in the sky. The elongated form of the orbit establishes a distinction equally marked between the comets and the planets. Orbits of Comets. — Some of the ancient philosophers regarded comets as simple meteors, engendered in the atmosphere. In order, however, to be convinced that they occupy a far more remote situation, it is only nece ary to compare simultaneous observations at very distant places on the earth. Tycho Brahe was the first who showed that their true place is in the planetary regions: since the time of Tycho, it has been discovered that they revolve about the sun according to regular laws, similar fee those which govern the planetary motions; and that their orbits are very elongated ellipses, having the sun in one of their foci. Comets are only visible during the short time they are near the perihelia of their orbits. But an elongated ellipse, and a parabola having the same summit and focus, only begin to diverge sensibly at a considerable distance from the common summit. In order, therefore, to represent the different positions of a comet during the short time it is visible, we may in general without any inconvenience substitute a parabola for an ellipse. If it happens that the orbit cannot be represented by a parabola, we conclude that the ellipse is not very elongated. Now, by means of three positions of a comet seen from the earth, all the elements of its parabolic orbit may be determined. These elements are, 1st, the line ot the nodes; 2d, the inclination of the orbit to the plane of the ecliptic; 3d, the perihelion distance, or least distance of the comet from the sun, expre ed in parts of the earth's semidiameter; 4th, the instant of the pa age through the perihelion; 5th, the longitude of the perihelion. When these elements are known, the path of the comet is completely determined; and it is only nece ary, in addition, to indicate the direction of the motion, that is to say, whether it is in the order Of the signs, or the contrary. The proper motion of all the planets is performed from west to east in the order of the signs; comets, on the other hand, appear to traverse the heavens in all directions indifferently. Of 129 comets whose orbits have been determined, there are 68 whose motion is direct, and 61 whose motion is retrograde, and their orbits intersect the ecliptic at all po ible angles. Out of the whole number there are only three whose returns to the sun in succe ive revolutions have been verified by observation. These are, 1st, Halley's comet, of which the period is about seventy-five and a half years; 2d, Enckc's, whose period is about three and one third years; and 3d, Biela's, which performs its revolution in six years and about eight months. Halley's Comet. — Newton was the first who submitted the motion of -a comet to calculation, and pointed out a method of determining its orbit from three of its observed positions. Halley applied Newton's method to a great number of comets, of which the positions had been observed; and on comparing the resulting elements, per  [s. 265]