ALTAZIMUTH SOLAR OBSERVATIONS. | Jao 
astronomy’ that if A’ denote the apparent, and A the geocentric 
azimuth, reckoned east or west from the elevated pole, p’ the radius 
vector of the point at which the observations are made, ¢ and ¢’ 
its astronomical and geocentric latitudes, and z’ the equatorial 
horizontal parallax, then 
sin (A’— A) =p’ sin z’ sin (f — ¢’) sin A’ sec A........(23) 
in which h is the true geocentric altitude. 
' The term ¢-—¢’, the so-called “angle of the vertical,” has a 
maximum value of about 704” at latitude 45°, if we accept Clarke’s 
last values for the dimensions of the terrestrial spheroid.2 The 
greatest value of z’ is about 9”, so that supposing p’ to be unity, 
A’ to be 90°, the correction can never be more than 0°’0307 sec h; 
consequently, as in all altazimuth observations for meridian / is 
never great, the quantity is negligible. Denoting it by a’, its 
mean value is 
= - 0°’0302 sin 2¢ sin A’ cosec ¢......... (24) 
The negative sign denotes that it is always to be subtracted from 
the azimuthal angle reckoned from the elevated pole. 
The parallax in zenith distance or altitude, is on the contrary 
always sensible, except in the case of very small theodolites. 
According to Newcomb® the value of the equatorial horizontal 
parallax for the earth’s mean distance from the sun is 8:°"848 ; 
and according to Clarke the polar semiaxis is about 333 less than 
the equatorial, consequently the polar horizontal parallax is about 
9-030 less than the above quantity. 8-’84 may therefore be taken 
as a general mean value for the entire surface of the earth, which 
would correspond very nearly to its proper value for a latitude of 
30°. Since the semidiameter S of the sun, given for each day in 
any ephemeris, varies exactly as the parallax—that is to say, both 
vary reciprocally as the earth’s distance from the sun’s centre— 
a ets 
;: 1 See “he am tees and Practical Astronomy, Vol. 1., p. 113, 
th Editio 
2 Vide « sida p. 319, Edit 1880. 
$ Washiagton Observations 1865, Appendix II., p. 29. 
