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straight line joining- the Earth to the planet is a tangent tothe path of the latter.
Moreover, we think it is also evident, in the above diagram,that, if it were not for the tilt of the orbits to which we haverecently referred, each of these two planets of which we arespeaking, would, at frequent intervals, pass exactly betweenthe Earth and the Sun , when in such a position as is markedi.c. But that tilt necessitates that there can only he twopoints in each of their orbits, in, or very near to which, theycan he in a position to do so ; viz., the two points in whichthose orbits cut the plane of the ecliptic in which the Earth Fig
. XXXVI.—The Earth being at e, an inferior planet when seen at p| or po 13 at itsgreatest apparent distance from the Sun.*
moves. In other words, it is only possible that either of themshould be exactly in a straight line between the Earth and theSun , at two particular times in each year, when the Earth , as itwould he seen from the Sun , lies in the direction of one of thetwo points in question ; which points are, of course, differentfor the orbit of Mercury from those which belong to that ofVenus . This may be best understood by the considerationof such a diagram as is drawn in Fig . XXXVII., in which theEarth is supposed to move in the plane of the paper.
The above figure shows that, if the Earth is in such a posi-tion as e or e', when the planet also happens to be at, or very
* If the Earth be at e, and a planet be in the position i.c., it is said tobe seen in inferior conjunction with the Sun ; if it be at s.c., it is in superiorconjunction.