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scopic aid, may have been seen, and that no special notice wastaken of it, the planet being supposed to be a spot.
So far, then, the Copernican theory explains the occasionaloccurrence of transits of Mercury and Venus , as well as theiroscillation from side to side of the Sun. It also indicates that,when either of these planets is seen in transit, or passesslightly above, or below, the direction of a straight line drawnfrom the Earth to the Sun, it must turn its unilluminatedhemisphere towards the Earth . And a little consideration willshow that it further requires, not only that they should eachdisplay a fully illuminated hemisphere, when at their furthestfrom the Earth (in direct contradiction to the Ptolemaic theory);
Superior conjunction.
but that they should, at other times, pass through a regularsuccession of phases, such as are shown in Fig . XXXVIII.
It is very noticeable, however, in the above figure, that inone respect such phases decidedly differ from those exhibitedby the Moon , inasmuch as the apparent diameter of the discdoes not (as in the case of the Moon ) remain nearly constant;but becomes much wider as the phase becomes less and less ;because the planet at the same time approaches nearer andnearer to the Earth . Such phases are easily seen in a smalltelescope, and the change in the diameter of the disc is foundto correspond precisely with the change in the planet’s dis-tance from the earth caused by its passage round the Sun.
We have dwelt at some length upon this point, because itmay be considered to be a crucial test of the truth of theCopernican theory, in the case of Venus and Mercury . The