56 THE HABITAT 



and it is least at the zenith. \The absorption, and, consequently, the relative 

 intensity of sunlight, can be determined at a given place for each hour of 

 any sunshiny day by the use of chart 13. This chart has been constructed 

 for Lincoln, and will serve for all places within a few degrees of the 40th 

 parallel. The curves which show the altitude of the sun at the various times 

 of the day and the year have been constructed by measurements upon the 

 celestial globe. Each interval between the horizontal lines represents 2 de- 

 grees of the sun's altitude. The vertical lines indicate time before or after 

 the apparent noon, the intervals corresponding to 10 minutes. If the rela- 

 tive intensity at Lincoln on March 12 at 3:00 p.m. is desired, the apparent 

 noon for this day must first be determined. A glance at the table shows that 

 the sun crosses the meridian on this day at 9 minutes 53 seconds past noon at 

 the 90th meridian. The apparent noon at Lincoln is found by adding 26 

 minutes 49 seconds, the difference in time between Lincoln and a point on 

 the 90th meridian. When the sun is fast, the proper number of minutes is 

 taken from 26 minutes 49 seconds. The apparent noon on March 12 is thus 

 found to fall at 12:37 r-M., and 3:00 p.m. is 2 hours and 23 minutes later. 

 The sun's altitude is accordingly 36. If the intensity of the light which 

 reaches the earth's surface when the sun is at zenith is taken as i, the table of 

 the sun's altitudes gives the intensity at 3:00 p.m. on March 12 as .85. 



For places with a latitude differing by several degrees from that of Lin- 

 coln, it is necessary to construct a new table of altitude curves from the 

 celestial globe. It is quite possible to make a close approximation of this 

 from the table given, since the maximum and minimum meridional altitude, 

 and hence the corresponding light intensity, can be obtained as indicated 

 above. For Minnehaha, which is on the 105th meridian, and for other places 

 on standard meridians, i. e., 60, 75, 90, and 120 W., the table of apparent 

 noon indicates the number of minutes to be added to 12 noon, standard time, 

 when the sun is slow, and to be subtracted when the sun is fast. The time at 

 a place east or west of a standard meridian is respectively faster or slower 

 than the latter. The exact difference in minutes is obtained from the dif- 

 ference in longitude by the equation, iS^'^i hour. Thus, Lincoln, 96 42' 

 W. is 6 42' west of the standard meridian of go" ; it is consequently 26 min- 

 utes 49 seconds slower, and this time must always be added to the apparent 

 noon as determined from the chart. At a place east of a standard meridian, 

 the time difference is, of course, subtracted^ 



The actual differences in the light intensity from hour to hour and day to 

 day, which are caused by variations in the sun's altitude, are not as great 

 as might be expected. For example, the maximum intensity at Lincoln, 

 June 22, is .98 ; the minimum meridional intensity December 22 is .73. The 

 extremes on June 22 are .98 and .33 (the latter at 6:00 a.m. and 6:00 p.m. 



