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Ptolemaic system is here utterly at fault, and would have beenknown to be so from the first, if the phases of these planetshad been visible without telescopic aid.
It may further be well to mention that certain apparentirregularities in the greatest distances to which Venus andMercury are seen to depart from the Sun, or in their move-ments in general, are also found to agree exactly with thoserefinements in the Oopernican theory, which, as we havepreviously stated, involve the knowledge of the tilt of theirorbits and of their elliptical form about the Sun as a focus.
But to keep to more simple matters, we may remind ourreaders, that, in the table given in connection with Fig.XXXIII., it is said that Mercury goes round the Sun in about88 days ; while observation shows (as is also indicated inCassini’s diagrams, see Figs. XXIV. and XXVI.) that theaverage time between that planet being seen at its greatestdistance on either side of the Sun, and reaching a similarposition upon that same side of it again, is in general about116 days. We may ask, do these facts agree with one anotheraccording to the Copernican theory? Does it offer any simpleexplanation of them ? Yes!—It tells us that the onwardmovement of the Earth only allows Mercury to overtake it,upon an average, with about fths its own speed of angularmotion round the Sun; and that, therefore, in order to regaina similar position to that which it occupied with regard to theSun as seen Irom the Earth at any given date, it takes about^rd longer than it occupies in circling once round its ownorbit.
In like manner, the Copernican theory explains, equally aswell as the old Ptolemaic system, why it is that about Ifyears, or 584 days, elapse, between Venus being seen twice insuccession in any given position relatively to the Sun, althoughits own orbital period is only 225 days, or -g-ths of a year.The apparent effect is, in fact, the same as would take place ifthe Earth were supposed to stand still in its orbit, and Mercurywere to revolve round the Sun in 116 days, Venus in 584, in.such paths as are shown in Fig . XXXIX.
It is also evident from the above figure, that a planet, thus