118 ANSWERS TO PRACTICAL QUESTIONS 



icSf they must both revolve about their common center of 



gravity ? 



(See Astronomy, note, p. 200.) 



The earth is not stationary as regards the moon, for both 

 it and our satellite revolve together about their common center 

 of gravity. Again, it is not the earth alone which revolves 

 about the sun in the elliptical orbit, but this common center of 

 gravity. The sun, also, is not stationary, but it and the planets 

 revolve about the common center of gravity of the whole 

 system. 



46. During a transit of Venus, do we see the body of 

 the planet itself on the face of the sun ? 



(See Astronomy, p. 277.) 



During a transit, Venus appears as " a perfectly round Nock 

 spot on the disk of the sun." The planet turns its unillumined 

 side toward us, and is, strictly speaking, invisible. 



47. How many real motions has the sun ? How many 

 apparent ones ? 



It has two real motions : one around its axis, and one with 

 the solar system around the Pleiades. It has three apparent 

 motions : one along the ecliptic, its yearly motion ; one through 

 the heavens, its daily motion ; and one north and south. 



48. Hoiv many real motions has the earth ? 



Three. One on its axis ; one around the sun ; and a third, 

 its "wabbling motion," which causes Precession. 



49. Can an inferior planet have an elongation of9O ? 



No. Venus recedes only 48 from the sun. 



50. How do ive know the intensity of the sun's lit/lit on 

 the surface of any of the planets ? 



The intensity of the heat and light varies inversely as the 

 square of the distance. 



51. IVJiy is the Tropic of Cancer placed where it is ? 



Because it is the farthest place north where the sun is ever 

 seen directly overhead. 



