1 2,1)00 times the diameter of the earth, creates the effect of reducing 

 the world to a mere celestial speck, changing the field of view virtu- 

 ally from its surface to its center. 



Parallax is the difference between the apparent altitude of a 



celestial body as observed from the earth's surface or from its center. 

 In solar observations it is equal to the angular \alue of the earth's 

 radius as viewed from the sun, or H". c >4 in the hori/on and 

 nothing in the zenith. In solar observations for the meridian, it is 

 a negligible quantity, but if a correction were to be applied it would 

 always be plus where refraction is minus. It may be found by 

 multiplying the Solar paialiax, a^ L;M :; al>\r, h\ the cosine of the 

 observed altitude. 



The Zenith is in the plane of the obser\er's meridian at the 

 point Z, marking a position where the direction of a plumb line, 

 jMolonged upward, would pierce the celestial sphere. 



The North Pole, P, is as far removed from the zenith as the 

 earth appears to be inclined on its axis. The sun, in his apparent 

 diurnal path, rising at M and setting at T, will, at each moment in 

 the day, occupy a certain relationship with respect to the fixed 

 points, Z and P. It follows then, that the spherical triangle, Z 1 ' 

 one that is constantly changing in proportions, depending upon the 

 sun's position above or below the equator and the hour angle, or 

 his position in his daily path. In the morning the angle Z^P i-. 

 constantly diminishing until at noon, XII, the spherical triangle 

 lias become a straight line. Between XI and I the angle 7.^ 

 too small to be accurately solved by this process. On the other 

 hand, from sunrise up to about 8 o'clock, atmospheric refra 

 makes the calculations rather uncertain. Between 8 and 11 a 

 and 1 and 4 p. m. is the best time of day for solar observation. 



The Sun's Declination is his angular distance above or below 

 the celestial equator as measured from the center of the earth. 1- 

 is regarded as positive when north and negative when south. The 

 declination is at the vernal and autumnal equinox on March 21st 

 and September 23rd when the sun crosses the equator. The hourK 

 change in declination is nearly a minute of arc at these dates. On 

 June 22nd and December 22nd in the summer and winter sol- 

 the declination has increased to approximate!] 2.^ 27' when the 

 hourly change is nearly zero. 



The Sun's Polar Distance is equal to W plus or minus the 

 apparent declination, depending upon whether south or north of the 

 equator. This is the angle the solar telescope makes with the pio 

 longation of the polar axis, when pointed at the sun. 



The Latitude is the angular distance of any point on the 

 earth's surface, like E, from the equator, Q; or it may be otlu- 

 expressed as the declination of the zenith. The angular >.: 

 the zenith from the pole is to the co-latitude of the 



place of observation, or 90 r ' L. The degrees of latitude run 

 parallel with the equator at varying dist.r miles 



apart, depending upon the rotundity or oblateness of the zones 

 tiasrrsed. 



161 



