Page:The Scientific Monthly vol. 3.djvu/531

 WHAT WU KNOW ABOUT COMETS 525

every case a parabola seemed to represent their motion satisfactorily. This Laplacean view that comets are wanderers through the great stellar system and are only chance visitors to the solar system was the prevailing one throughout the nineteenth century. Evidences to the contrary began to appear as early as 1860, but so firmly rooted was the hypothesis that only in the twentieth century have astronomers in gen- eral been convinced that the comets are members of the solar system. Several lines of evidence, all in good agreement, have brought us to this conclusion.

1. Since the solar system is traveling through the stellar system in the direction of the constellations Lyra and Hercules, with a speed of twelve and a half miles per second, if comets come in from inter- stellar space we should meet more comets coming from the Lyra- Hercules direction than there are comets overtaking us from the oppo- site part of the sky, for precisely the same reason that if we are travel- ing very rapidly by automobile from San Diego to Los Angeles we should meet more autos than would overtake us and pass us. Now the comets do not show that preference. As early as 1860 Carrington studied the directions of approach of all the comets, 133 in number, which up to that time were considered to have parabolic or hyperbolic orbits. He found that only sixty-one* of these comets met the solar system, so to speak, whereas seventy-two* comets overtook us — extremely strong evidence that the comets are traveling along with us, just as all of our planets are traveling with the sun while revolving around it. Many later astronomers, especially Fabry, using the more plentiful and more accurate data now available, have confirmed this conclusion that there is no tendency for comets to meet us, as we rush through inter- stellar space, rather than to overtake us. It is a fact, however, that the observed comets have not had their directions of approach distrib- uted uniformly over the surface of the sphere. Their deviations from reasonable imiformity appear to be due in small measure to a prefer- ence of comets to travel in planes making small angles with the ecliptic, with motion around the sun from west to east as in the case of the planets; but the chief discrepancies arise from the heterogeneous cir- cumstances under which comets are discovered.

Nearly all discoveries of comets made by means of telescopes prior to forty years ago were made in the northern hemisphere, at observa- tories situated in latitudes north of +40°. The southern hemisphere is still very much in arrears in the matter of comet discoveries, though the discrepancy is not now so great as it once was.

There is more searching for comets in the northern hemisphere during the northern summer and in the southern hemisphere during the southern summer than in their respective winters. There is also

3 The disparity in the numbers is thought to be purely accidental.

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