Astronomy in a nutshell, the chief facts and principles explained in popular language for the general reader and for schools . s, may return to perihelion in aperiod so long that a second return has notbeen observed. A better division is intocomets of short period, and comets of long,or unknown, periods. Still, many astronomers are disposed tothink that the majority of comets do nottravel in elliptical but in parabolic, and afew in hyperbolic, orbits. This calls for a fewwords of explanation. Ellipses, parabolas,and hyperbolas are all conic-section curves,but the ellipse alone returns into its
Astronomy in a nutshell, the chief facts and principles explained in popular language for the general reader and for schools . s, may return to perihelion in aperiod so long that a second return has notbeen observed. A better division is intocomets of short period, and comets of long,or unknown, periods. Still, many astronomers are disposed tothink that the majority of comets do nottravel in elliptical but in parabolic, and afew in hyperbolic, orbits. This calls for a fewwords of explanation. Ellipses, parabolas,and hyperbolas are all conic-section curves,but the ellipse alone returns into itself, orforms a closed circuit. In each case the sunis situated at the focus where the perihelion,or nearest approach, of the comet occurs,but only comets travelling in ellipticalorbits return again after having once beenseen. A comet moving in a parabola wouldgo back into the depths of space nearly inthe direction from which it had come, andwould never be seen again; and if it moved ina hyperbola it would go off toward another Comets 203 quarter of the celestial sphere, and likewisewould never return. Now it is true that. Fig. 17. Ellipse, Parabola, and Hyperbola. The figure shows graphically why it is so difficult to tellexactly the form of a comets orbit. The three kinds ofcurves are nearly of the same form near the focus (theSun), and it is only in that part of its orbit that thecomet can be seen. Moreover a comet is, at best, amisty and indefinite object, which renders it so muchthe more difficult to obtain good observations of itsprecise position and movement. the forms calculated for the orbits of themajority of comets that have been observedappear to be parabolic (a very few seemto be hyperbolic), and if this is the fact such 204 THe Solar System comets cannot be permanent members ofthe solar system, but must enter it from far-off regions of space, and having visited thesun must return to such regions without anytendency to come back again. In that casethey may pay simil
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