88i.l and its Effect on Astronomical Observations . 509 
similar arc in the opposite direction. Consequently during 
January the right ascension of the Pole Star will decrease 
rapidly. During the course of the pole from p to Q, during 
April, the right ascension will be nearly constant, and about 
April 1st — when a line from the star to the pole is tangential 
to the curve then traced by the pole — the right ascension at 
that date will not vary. Observation, as recorded in the 
“Nautical Almanac/’ agrees with minute accuracy with the 
above details. 
Again, about June 18th the right ascension of the Pole 
Star will increase rapidly, whilst about the end of Odtober 
— when the arc from the Pole Star to the pole is tangential 
to the curve — the right ascension will be again a constant. 
Observation proves this to be the case. The change in polar 
distance of the Pole Star between June and December can 
also be taken off scale, and the value found to correspond 
with observation. 
We can now test the accuracy of this curve by another 
important star, viz., 8 Ursas Minoris. The direction of this 
star is also plotted on the diagram, its right ascension being 
about 18 h. 10 m. During January the pole moving from M 
to n moves diredtly away from 8 Ursae Minoris. Conse- 
quently the polar distance of this star will increase during 
January by the length of the arc M N : the length of this arc 
by scale taken off the diagram will be found about g"' 5 , and 
the recorded increase in polar distance of this star — found 
after years of observation, and recorded in the “ Nautical 
Almanac ” — is g"*2 for the month of January. 
By examining the curve on diagram, the following ques- 
tions can be answered : — 
1. When will the polar distance of this star remain con- 
stant during a few days ? — When the pole is near the 
point p, at which time the pole is tracing an arc at 
right angles to the arc then joiningthe pole and the star. 
2. When will the polar distance of this star decrease most 
rapidly ? — When the pole is moving from R to s ; that 
is, during June, at which date the polar distance will 
decrease by the amount of the arc R s. 
Examine the recorded observations in the “ Nautical 
Almanac,” and these two fadts agree with the results of 
observation. 
3. When will the polar distance of this star remain a 
constant during the latter portion of the year ? — 
When the pole is at v on Odtober 1st, at which date 
the pole is moving at right angles to the arc then 
joining the pole and the star. 
