66 4.NSWERS TO PRACTICAL QUESTIONS 



7. Can any inferior planet have an elongation of '90 f 

 No. Venus recedes only 48 from the sun. 



8. How do we know the heat of the surfs rays at any planet? 

 The intensity of the heat and light vary inversely as the 



square of the distance. 



9. Can yon give any other proof than that named in thf 

 book, of the rotundity of the earth ? 



Aeronauts, when at a proper height, can distinctly see the 

 curving form of the earth's surface. 



10. In what way is the force which acts on a spinning-top 

 opposite to that which produces precession (p. 125) ? 



Gravity^acting on the top, tends to draw C P (Fig. 34) ./>*" 

 the perpendicular. The attraction of the sun, acting on the 

 bulging mass of the earth's equator, tends to draw C P toward 

 the perpendicular. 



1 1 . Why is the Tropic of Cancer placed where it is ? 

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



seen directly overhead. 



12. Why is the Tropic of Cancer so called? 



When named, the sun was probably in that constellation at 

 the time of the summer solstice. Now, owing to the preces- 

 sion of the equinoxes, the sun is in the constellation Gemini, 

 and to be exact, it should be called the Tropic of Gemini. It 

 is still, however, the sign Cancer, as before. The same reason- 

 ing applies to the Tropic of Capricorn, which is now in the 

 constellation Sagittarius. 



13. In Greenland ', at what part of the year will the mid- 

 night sun be seen due north ? 



At the summer solstice. 



14. How do we know that the moon has little if any atmos- 

 phere? 



Because when the moon occults a star, there is no refraction 

 of the star's true place. 



15. When is the moon seen high in the eastern sky in the 

 afternoon, lotig before the ?un sets ? 



