156

THE PLACET VENUS.

Sun , the longer is its year, and the slower its velocity of move-ment ; while a little arithmetical calculation would showthat the numbers stated above agree very satisfactorily withKepler’s 3rd Law, which was explained in the same Lecture.

There is, however, one peculiarity connected with the orbit ofVenus , which distinguishes it from every other planetary orbit;viz., that it is more nearly circular than any other. We havestated, in Lecture VI., that the fraction 1-tli represents the oval-ness of Mercury’s orbit, because the Sun is situated at one-fifthof the distance between the centre and one end of the majoraxis of the ellipse which that planet describes. For the ellipsein which the Earth rotates around the Sun the correspondingfraction is ^th, which indicates an orbit much more closelyapproximating to a circular shape. But for that of Venus it isonly about T | T th; as a consequence of which it follows, that thedistance of Venus from the Sun never exceeds or falls short of itsmean value of about sixty-seven millions of miles by so much ashalf a million. The whole change in its distance is therefore lessthan one million of miles ; while Mercury ’s distance from theSun varies from twenty-nine to forty-three millions of miles;—or, in other words, changes by as much as fourteen millions ofmiles in the course of its year ;—and that of the Earth variesfrom ninety-one and a half to ninety-four and a half millionsof miles; or changes by three millions of miles.

If we compare the semi-minor axis and the semi-major axisof the orbit of Venus by the rule which we gave in connectionwith our explanation of the form of an ellipse, in Lecture V.,page 119 ; we find that they only differ by about T -.r^- afl th partof either; that is, by less than 1,600 miles.

It is also worthy of notice that the plane of the path of Venus is tilted to that of the Earth , or, in other words, to the planeof the ecliptic, less than one-half as much as that of Mercury .With the exception, however, of some of the minor planets, thetilt is greater than in the case of any other planetary orbit.

Venus may, of course, be seen, from time to time, at a muchgreater apparent distance from the Sun , and at a much lessactual distance from the Earth, than Mercury , as is indicatedby our previous diagram (Fig . XXXIX.), in which the com-