CHAPTER XI.
THE ORBITS OF COMETS.
All Cometary Orbits sections of a Cone.—The different kinds of Sections.—The Circle.—The Ellipse.—The Hyperbola.—The Parabola.—The last-named the most easy to calculate.—An ellipse very troublesome to calculate.—The elements of a Comet's Orbit.—For a Parabolic Orbit 5 in number.—Statement of various details connected with Orbits.—Direction of motion.—Eccentricity of an Elliptic Orbit.—The various elements represented by certain symbols.—Number of comets whose orbits have been calculated.—The significance of the different orbits pursued by comets.
It has already been stated, but without much explanation, that all comets move in orbits which are either elliptic, parabolic, or hyperbolic—3 of the 4 possible sections of a cone.
Fig. 91
THE VARIOUS SECTIONS OF A CONE.
Fig. 91 represents the various sections of a right cone, as it is called, and will convey a better idea of Cometary Orbits than a verbal description would do. When a right cone is cut at right angles to its axis the resulting section A B, will be a Circle: no comet, however, is known to revolve in a circular, or even in anything like a circular orbit, though, on the other hand, all the planetary orbits may be said to be nearly circular. When a cone is cut obliquely, so that the inclination of the cutting plane to the axis of the cone is greater than the constant angle formed by the generating line of the cone and the axis, as C D, the resulting section will be an Ellipse, the shape of which will vary from almost a circle on the one hand to almost
