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Updated: June 29, 2025
If any doubt remained as to the misleading character of Encke's deduction, so long implicitly trusted in, it was removed by Powalky's and Stone's rediscussions, in 1864 and 1868 respectively, of the transit observations of 1769.
Some physicists believe that Encke's Comet, with its retarded motions, will some day fall into the sun; while others fancy that such a consummation can never take place, because successive portions of its substance will be thrown off by the tail-forming process with each perihelion return; so that long before the cometic mass could reach the sun, it will have been altogether dissipated into space, and nothing will be left to accomplish the final state of the fall.
Tobias Mayer of Göttingen, however, whose short career was fruitful of suggestions, struck out the right way to the same end; and Laplace, in the seventh book of the Mécanique Céleste, gave a solar parallax derived from the lunar "parallactic inequality" substantially identical with that issuing from Encke's subsequent discussion of the eighteenth-century transits.
The hypothesis, then, of a resisting medium receives at present no countenance from the movements of comets, whether of short or of long periods. Although Encke's comet has made thirty-five complete rounds of its orbit since its first detection in 1786, it shows no certain signs of decay. Variations in its brightness are, it is true, conspicuous, but they do not proceed continuously.
If the thin vapour experienced any resistance while moving, its free passage would be checked, although that resistance was many thousand times less than the one the hand feels when waved in the air. It is found, however, that Encke's Comet does indicate the presence of some such resistance.
But it was generally pretty one-sided, because I sailed by them the same as if they were standing still. An ordinary comet don't make more than about 200,000 miles a minute. Of course when I came across one of that sort like Encke's and Halley's comets, for instance it warn't anything but just a flash and a vanish, you see. You couldn't rightly call it a race.
Although on that occasion, owing to the position of the earth, invisible in the northern hemisphere, Sir Thomas Brisbane's observatory at Paramatta was fortunately ready equipped for its recapture, which Rümker effected quite close to the spot indicated by Encke's ephemeris. No great comet is of the "planetary" kind.
Examples, on the other hand, are not wanting of the diminution of stellar light under similar circumstances; and we meet two alleged instances of the vanishing of a star behind a comet. Wartmann of Geneva observed the first, November 28, 1828; but his instrument was defective, and the eclipsing body, Encke's comet, has shown itself otherwise perfectly translucent.
Still more important were the great star-maps of the Berlin Academy, undertaken at Bessel's suggestion, with the same object of distinguishing errant from fixed stars, and executed, under Encke's supervision, during the years 1830-59. They have played a noteworthy part in the history of planetary discovery, nor of the minor kind alone. We have now to recount an event unique in scientific history.
Encke's comet, above all, has served as an index to much curious information, and it may be hoped that its function in that respect is by no means at an end. The great extent of the solar system traversed by its eccentric path makes it peculiarly useful for the determination of the planetary masses.
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