322 G. H. KNIBBS. 
meridian of the observer the result on the apparent declination 
at the same moment being zero. For a star whose distance from 
the equator, i.c. whose declination is 6, and whose hour angle is 
T, the effect will be, in right ascension da say, 
da, = 0*0214 cos ¢ sec 6 cos 7.......... (19) 
and in declination 
dS = 0°"321 cos dsin 6 sin 7............ (20) 
the former of which can become considerable only for a star near 
the pole, and must always be small for a rapidly moving star. 
The effect of the diurnal aberration on the azimuth and zenith 
distance of a star may readily be derived from these last equations: 
it is :— 
dA = 0°321” cos ¢ cos A cosec ¢ (21) 
d¢ = 0°321” cos ¢ sin A cos ¢ (22) 
in which the angle A should be reckoned from the north line. 
These quantities are to be added to azimuths and zenith distances 
computed from the star places given in an ephemeris. The above 
expressions, though not rigorously exact in the case of the sun, 
are sensibly so, since the difference between the sidereal and solar 
apparent rotations is very small. The diurnal aberration may 
always be neglected with instruments that do not read to within 
1”, and is perhaps always negligible in the case of the sun because 
of the large uncertainty of the refraction and the indifferent 
definition of the sun’s limb. Nevertheless the calculation of the 
correction is only the work of a minute, and it may at least serve 
to decide the last figure in the expression of the final result in 
whole seconds. 
8. Correction for parallax.—Since an ephemeris to be generally 
applicable gives only the geocentric positions of celestial objects 
it is necessary to reduce the results of observations made at 4 
point on the earth’s surface also to their geocentric values. This 
reduction, called the correction for parallax, affects theoretically 
‘both the altitudes and azimuths as given by observation, because 
of the spheroidal form of the earth. Turning first to the correc 
tion for parallax in azimuth, it is shewn in treatises on spherical 
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