IN DESCRIPTIVE ASTRONOMY. 135 



Calculate the surface of a sphere whose radius is the dis- 

 tance of the earth from the sun, and then estimate what pro- 

 portion of that area the earth occupies. 



4 . Hoiv do we know the heat of the sun's rays at any 

 planet ? 



The intensity of the heat and light varies inversely as the 

 square of the distance. 



5. Can you give any other proof than that named in 

 the booh of the rotundity of the earth ? 



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

 curving form of the earth's surface. 



6. In what way is the force ivhich acts on a spinning- 

 top opposite to that ivhich produces precession ? 



Gravity, acting on the top, tends to draw C P (Fig. 40) 

 from 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. 



7. IVIiy 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 G-emini, 

 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. 



8. In Greenland, at what part of the year will the 

 midnight sun be seen due north ? 



At the summer solstice. 



9. Wlien is the moon seen high in the eastern sJcy in the 

 afternoon, long before the sun sets ? 



During the second quarter before it comes into opposition. 



10. WJiy is the Ecliptic so called ? 



Because eclipses always occur in or near it. 



