An introduction to astronomy: designed as a text-book for the use of students in college. By Denison Olmsted ...

REFRACTION. 3 5
observed the altitude of the sun's center at Paris to be 50 0' 14".
The latitude of Paris being 48~ 50' 10", and the sun's declination at that time being 15~ O' 25": Requiirecl the refraction.
By spherical trigonometry, Zx will be found equal to 850 10'
8"; consequently, the true altitude was 4~ 49' 52". Now to
50 0' 14", the apparent altitude, 9" must be added for parallax,
and we have 5~ 0' 23'", the apparent altitude corrected for
parallax. Hence, 5~ 0' 23"-4~ 49' 52"=10' 31", the refraction
at the apparent altitude 5~ 0' 14".*92. By these and similar methods, we could easily determine the refraction due to any elevation above the horizon,
provided the refracting mediumn (the atmosphere) were always
uniform. But this is not the fact: the refracting power of the
atmosphere is altered by changes in density and temperature.t
Hence, in delicate observations, it is necessary to take into the
account the state of the barometer and of the thermometer, the
influence of the variations of each having been very carefully
investigated, and rules having been given accordingly. With
every precaution to insure accuracy, on account of the variable
character of the refracting medium, the tables are not considered as entirely accurate to a greater distance from the
zenith than 74~; but almost all astronomical observations are
made at a greater altitude than this.
93. Since the whole amount of refraction near the horizon
exceeds 33', and the diameters of the sun and moon are severally
less than this, these luminaries are in view both before they
have actually risen and after they have set.
94. The rapid increase of refraction near the horizon is
strikingly evinced by the ovalc figure which the sun assumes
when near the horizon, and which is seen to the greatest advantage when light clouds enable us to view the solar disk.
Were all parts of the sun equally raised by refraction, there
* Gregory's Ast., p. 65.
t It is said that the effects of humidity are insensible; for the most accurate
experiments seem to prove that watery vapor diminishes the density of air in the
same ratio as its own refractive power is greater than that of air. (New Encyc.
Brit., iii., 762.)