SUN-SPOTS. 
171 
SUN-SPOTS. 
The story of the dark spots that may often be 
seen on the surface of the sun might well fill an 
interesting book. 'When the sun is near the horizon 
the spots may sometimes be seen through a smoked 
glass. 
They first appear at the eastern limb of the sun 
as a narrow dark streak. This gradually widens, 
until it becomes a more or less circular s]3ot in the 
center of the disk. It continues westward, growing 
narrower again, to the very edge of the disk. About 
thirteen days after 
a spot disappears, 
it frecpiently reap¬ 
pears at the east¬ 
ern limb, changed 
but little, if any. 
Occasionally the 
same spot may be 
observed even three 
times. 
From the mo¬ 
tions of the spots, 
it has been con¬ 
cluded that the sun 
rotates upon its axis once in twenty-five and one-third 
days. From these spots we are also able to deter¬ 
mine the poles and equator of the sun. 
Chinese observers noticed spots on the sun three 
centuries before the Christian era, long before any one 
Fig. 85.—Ordinary sun-spot as observed 
June 22, 1885. 
