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stars involved in the description, near to the times when theymake their closest approaches to the Earth , of the smallerloops in the apparent orbits shown in Figs . XXIV.—XXVIII.The explanation, although not very obvious, may, we hope, beunderstood without much difficulty.
In order that we may clearly realize it, let us briefly restate,from the earlier part of this lecture, what it is that we nowwish to show that the Copernican theory explains. It is this.If any planet be watched, and its place amongst the stars benoticed night after night,—although, upon the whole, it movesamongst them in a direction opposite to that in which theyappear to go round the Earth day by day ; or, in other words,although its path is for the most part from west to east; itwill be found that, at a certain epoch, before or after the planetpasses through one of its nearest approaches to the Earth , itseems to stand nearly or quite still for a while; and thatbetween these two stationary positions it reverses the directionin which it travels amongst the stars, and appears to go fromeast to west.
In order to indicate how the Copernican theory explains allthis, let us, first of all, take the case of a planet nearer to theSun than the Earth, such as Venus ; and let us suppose v 1; v 2 ,v 3 , etc., to represent successive positions of Venus in its orbit;while e 1? e 2 , e 3 , etc., are corresponding positions of the Earth ,which is moving at a slower rate round the Sun. An observerupon the Earth will see Venus amongst the stars, in the pro-longation of the line joining him to it at any time, and willrefer its place to an imaginary celestial sphere situated at agreat distance. We will represent such places, correspondingto the positions Ej and Vj, e 2 and v 2 , e 3 and v 3 , etc., respectivelyof the Earth and Venus , by the small letters v v v 2 ,v 3 , etc. Wemay also notice in passing that, when the Earth is at e 5 andVenus at v s , the latter is said to be in the position of inferior con-junction with the Sun, so that it might then, for a few hours, beseen in transit across its disc; but that in general the tilt of itsorbit will carry the line of view clear of the Sun, so that it willbe referred to v 5 , and may be altogether invisible for a day or twoowing to the glare of the surrounding Solar light. It should
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