134 ANSWERS TO PRACTICAL QUESTIONS 



97. What is the apparent diameter of the sun ? 



The mean apparent diameter of the sun is 32'. 



98. How can a sailor find Jtis latitude and longitude at 



sea? 



(See Astronomy, p. 280.) 



99. How many miles on the solar disk represent a sec- 

 ond of apparent diameter ? 



V on the solar disk equals 450.3 miles. YOUNG. "The 

 spider-line used in a large telescope will cover a portion of the 

 sun's surface \ of a second in breadth, or hide a strip over 100 

 miles wide." 



100. At what latitude will there be twilight during the 

 entire midsummer night ? 



(See Manual, p. 114, Question 23.) 



"When the sun crosses the meridian at midnight, its distance 

 below the horizon is greater than when the sun is at any other 

 part of its diurnal path. If, therefore, the depression of the 

 sun bellow the horizon at midnight be not greater than 18, the 

 sun will, during the entire night, be within 18 of the horizon, 

 and hence the twilight will be continuous. It will be noticed in 

 solving all the problems connected with twilight (as, for ex- 

 ample, Question 23, p. 114, and Question 27, p. 124), that the 

 result will be slightly changed if the exact amount of the 

 obliquity of the ecliptic (23 27' 15") be used, instead of the 

 ordinary statement, 23|. 



ADDED QUESTIONS AND ANSWERS. 



1. Did Tycho Srahe have a telescope ? 



No. Galileo invented the telescope. 



2. Suppose one should watch the sky, on a winter's 

 evening, from 6 P.M. to 6 A.M., what portion of the celes- 

 tial sphere would he see ? 



All that is ever seen in his latitude. 



3. How do we find tvhat proportion of the sun's heat 

 reaches the earth ? 



