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(and not as a planet) in two different positions, which, as soonas time permitted, would by their comparison have demon-strated its onward movement, and consequently have provedits real character.*
4Ve think we are indeed justified in calling this a triumph.Perhaps it may be some day repeated, and irregularities inNeptune ’s orbit be used to deduce the place of a yet moredistant planet. The movements of Neptune have indeed beenalready watched with this object in view, but any attempt tosolve the problem is at present premature. The investigationwould, of course, be much more difficult than it was in thecase of Uranus . Nor are we, at present, able to calculate theorbit of Neptune itself with such accuracy that we can beperfectly certain of the extent to which irregularities occur inits motion; although we are aware that the attractions of someof the other planets do undoubtedly, and very considerably,perturb its path.
One striking illustration of such perturbations of its move-ment, first brought under our notice by the kindness ofthe Astronomer Royal (Mr. Christie), may be both interestingand instructive. It is, that, at the end of 1881, Neptune was about 500,000 miles further from the Sun than it wasin 1876, or than it probably will be in 1887, or near to that date.
Now the year 1881 is that in which, if Neptune ’s motion wereundisturbed, it would pass through its Perihelion, or make itsnearest approach to the Sun ; but the result of the above per-turbation (which is chiefly due to Jupiter ) is, that its distance isreally 500,000 miles less, about five (or six) years, both before,and after, that date. That is to say, it practically passesthrough two Perihelia, one of which occurred in 1876, whilethe other will be in, or about, 1887. This may, at any rate,
* It should also be mentioned that, just as it was found after th<discovery of Uranus that it had been previously observed, and its placerecorded as that of a star ; so in like manner two observations of Neptune were found to have been made by Lalande, on May 8th and 10th, 1795,fifty-one years before its discovery in 1846. Indeed Lalande noticed thatthe places observed on these two days did not agree, but, instead ofimagining that the object might be a planet whose proper motion causedthe discordance, he simply rejected that of May 8th as erroneous.