ON THE TOTAL SOLAR ECLIPSE OP AUGUST 29, 188G. 
297 
IV. On the Accuracy required in Adjusting an Equatorial for Photographic 
Purposes during Total Solar Eclipses. By Arthur Schuster. 
Observers preparing for a total Solar eclipse have in general only a very moderate 
time at their disposal, and it is of great importance to them to settle beforehand to 
what degree of accuracy the adjustment of their instruments is to he carried. The 
time which is spent over adjustment in any one direction must necessarily be taken 
away from other more important matters, as there is never any lack of work on these 
occasions. 
If, for instance, a photographic picture of the Solar corona is to be obtained, it would 
be clearly waste of time to refine on the adjustments of the equatorial or the rate of 
the clock beyond the point at which the Sun’s change in declination would produce a 
visible effect. We shall see that this consideration limits the time of exposure for 
which the full advantage of the aperture of the lens can be realised, and tbis again 
will give us a limit beyond which it would be unnecessary to adjust the equatorial. 
We might, indeed, take the Sun’s apparent motion into account, and point the 
instrumental axis, not to the pole, but to some point near it, which might easily be 
determined by calculation. We shall see, however, that the available time of exposure 
of a 4-inch lens is quite sufficient for our present requirements, and that it is therefore 
unnecessary to make allowance for the Sun’s change of declination. 
When the instrument is nearly adjusted, the relation between the true declination 
of a star 8 and the apparent declination 8 ' is given by— 
8 = 8’ — y cos (r — 8), 
where y is the angle between the true pole and the instrumental pole, r is the hour 
angle of the star, and 8 the hour angle of the instrumental pole. If 8 is constant the 
change of apparent declination from bad adjustment in a short time t is found from 
the above equation to be — sin (t — 8), which will, numerically, always be smaller 
than yt. If the time t, measured in minutes of time, is p, we can write for this 
maximum change— 
44 X 10-^ py. 
We find in this way that a change of apparent declination of one second of arc per 
minute could be produced if 
y = 3’ 49". 
Now, tbe change of the Sun’s declination may be, and during the last eclipse w^as, 
nearly one second of arc per minute.''' It will be unnecessary, therefore, to spend any 
* During the total phase of the late eclipse, owing to the low altitude of the Sun (18° 45'), the 
apparent change of altitude due to change of refraction was about 2^ seconds of arc; but the change in 
declination due to I’efraction was small, and generally the effects of refraction may be neglected. 
MDCCCLXXXIX.— A. 2 Q 
