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Thus far, then, we have shown that it is principally to Coper nicus that we owe the credit of the promulgation of the all-important statement that the planets revolve in orbits roundthe Sun ; while we are indebted to Kepler and to Newton forour knowledge of the more accurate nature of those orbitsinvolved in the laws we have just described.*
But we have uot yet seen how the Copernican theory accountsfor those special phenomena of the apparent planetary move-ments, which called forth the complicated cycles and epicyclesof Hipparchus and Ptolemy , nor have we, as yet, contrastedthe explanation afforded by it, with that which so long heldits ground amongst the philosophers of old. In other words,we have to show how so simple an arrangement as that theoryinvolves, of the movements of the planets with regard to theSun , involves all those intricate orbits seen from the Earth ,with the description and illustration of which we commencedthis lecture.
To begin with, let us take the case of Mercury or Venus ,whose apparent movements, as we have already seen, differin certain important respects from those of the other planets.It is, we think, very easy to understand, that, if they move,according to the assertion of Copernicus , in orbits round theSun as a centre, within the Earth ’s orbit, they must appear tooscillate from side to side of the Sun within a certain distanceof it, which distance will be much less for the former than forthe latter planet. It also follows that the greatest apparentdistance of either of them from the Sun , as seen from theEarth , will occur, when the relative positions occupied by themare such as are indicated in Fig . XXXVI.; i.e., when the
* It should also be mentioned, that calculations depending upon thelaw of gravitation, if accurately made, show (as careful observations alsoprove to be the case) that each planet’s orbit lies upon a plane, or level,of its own ; so that it moves just as a ball -would, if, while floating in avessel of water, it were made to travel round another larger one floating inthe same vessel and representing the Sun . In the case of the Earth thisplane is called the Ecliptic . The planes of the motion of the otherplanets are tilted at certain angles to it and to one another ; but thistilt is in every case slight, except in that of some of the small planetsbetween Mars and Jupiter , whose orbits are altogether so peculiar thatthey require separate consideration.