Three years later Rödkier, at Copenhagen, March 3 and 4, 1764, saw the satellite of Venus with a refracting telescope 38 feet long, which should have been effective if longitude has any virtue. He could not see the satellite with another telescope which he tried. But several of his friends saw it with the long telescope. Amongst others, Horrebow, Professor of Astronomy, saw the satellite on March 10 and 11, after taking several precautions to prevent optical illusion. A few days later Montbaron, at Auxerre, who had heard nothing of these observations, saw a satellite, and again on March 28 and 29 it appeared, always in a different position.
It should be added that Scheuten asserted that during the transit of 1761 Venus was accompanied by a small satellite in her motion across the sun's face.
So confidently did many believe in this satellite of Venus that Frederick the Great, who for some reason imagined that he was entitled to dispose as he pleased of the newly discovered body, proposed to assign it away to the mathematician D'Alembert, who excused himself from accepting the questionable honour in the following terms:—
'Your Majesty does me too much honour in wishing to baptize this new planet with my name. I am neither great enough to become the satellite of Venus in the heavens, nor well enough (assez bien portant) to be so on the earth, and I am too well content with the small place I occupy in this lower world to be ambitious of a place in the firmament.'
It is not at all easy to explain how this phantom satellite came to be seen. Father Hell, of Vienna—the same astronomer whom Sir G. Airy suspects of falling asleep during the progress of the transit of Venus in 1769—made some experiments showing how a false image of the planet might be seen beside the true one, the false image being smaller and fainter, like the moons seen by Schort (as Hell called Short), Cassini, and the rest. And more recently Sir David Brewster stated that Wargentin 'had in his possession a good achromatic telescope, which always showed Venus with such a satellite.' But Hell admitted that the falsehood of the unreal Venus was easily detected, and Brewster adds to his account of Wargentin's phantom moon, that 'the deception was discovered by turning the telescope about its axis.' As Admiral Smyth well remarks, to endeavour to explain away in this manner the observations made by Cassini and Short 'must be a mere pleasantry, for it is impossible such accurate observers could have been deceived by so gross a neglect.' Smyth, by the way, was a believer in the moon of Venus. 'The contested satellite is perhaps extremely minute,' he says, 'while some parts of its body may be less capable of reflecting light than others; and when the splendour of its primary and our inconvenient station for watching it are considered, it must be conceded that, however slight the hope may be, search ought not to be relinquished.'
Setting aside Scheuten's asserted recognition of a dark body near Venus during the transit of 1761, Venus has always appeared without any attendant when in transit. As no one else claimed to have seen what Scheuten saw in 1761, though the transit was observed by hundreds, of whom many used far finer telescopes than he, we must consider that he allowed his imagination to deceive him. During the transit of 1769, and again on December 8–9, 1874, Venus certainly had no companion during her transit.
What, then, was it that Cassini, Short, Montaigne, and the rest supposed they saw? The idea has been thrown out by Mr. Webb that mirage caused the illusion. But he appears to have overlooked the fact that though an image of Venus formed by mirage would be fainter than the planet, it would not be smaller. It might, according to the circumstances, be above Venus or below, or even somewhat towards either side, and it might be either a direct or an inverted image, but it could not possibly be a diminished image.
Single observations like Cassini's or Short's might be explained as subjective phenomena, but this explanation will not avail in the case of the Copenhagen observations.
I reject, as every student of astronomy will reject, the idea of wilful deception. Occasionally an observer may pretend to see what he has not seen, though I believe this very seldom happens. But even if Cassini and the rest had been notoriously untrustworthy persons instead of being some of them distinguished for the care and accuracy with which their observations were made and recorded, these occasional views of a phantom satellite are by no means such observations as they would have invented. No distinction was to be gained by observations which could not be confirmed by astronomers possessing more powerful telescopes. Cassini, for example, knew well that nothing but his well-earned reputation could have saved him from suspicion or ridicule when he announced that he had seen Venus attended by a satellite.
It seems to me probable that the false satellite was an optical illusion brought about in a different way from those referred to by Hell and Brewster, though among the various circumstances which in an imperfect instrument might cause such a result I do not undertake to make a selection. It is certain that Venus's satellite has vanished with the improvement of telescopes, while it is equally certain that even with the best modern instruments illusions occasionally appear which deceive even the scientific elect. Three years have passed since I heard the eminent observer Otto Struve, of Pulkowa, give an elaborate account of a companion to the star Procyon, describing the apparent brightness, distance, and motions of this companion body, for the edification of the Astronomer-Royal and many other observers. I had visited but a few months before the Observatory at Washington, where, with a much more powerful telescope, that companion to Procyon had been systematically but fruitlessly sought for, and I entertained a very strong opinion, notwithstanding the circumstantial nature of Struve's account and his confidence (shared in unquestioningly by the observers present), that he had been in some way deceived. But I could not then see, nor has any one yet explained, how this could be. The fact, however, that he had been deceived is now undoubted. Subsequent research has shown that the Pulkowa telescope, though a very fine instrument, possesses the undesirable quality of making a companion orb for all first-class stars in the position where O. Struve and his assistant Lindenau saw the supposed companion of Procyon.