Note 41 — CONTENTS. (continued 4)

It may be necessary to throw in here a few words of explanation, lest the non-astronomical reader should run away with the idea that the so-called exact science is a very inexact science indeed, so far as comets are concerned. The comet of 1680 was one of those which travel on a very eccentric orbit. Coming, indeed, from out depths many times more remote than the path even of the remotest planet, Neptune, this comet approached nearer to the sun than any which astronomers have ever seen, except only the comet of 1843. When at its nearest its nucleus was only a sixth part of the sun's diameter from his surface. Thus the part of the comet's orbit along which astronomers traced its motion was only a small part at one end of an enormously long oval, and very slight errors of observation were sufficient to produce very large errors in the determination of the nature of the comet's orbit. Encke admitted that the period might, so far as the comparatively imperfect observations made in 1680 were concerned, be any whatever, from 805 years to many millions of years, or even to infinity—that is, the comet might have a path not re-entering into itself, but carrying the comet for ever away from the sun after its one visit to our system.