457 
rotatory Motion of the Stars on their Axes. 
The rotations of the sun and moon, and of several of the pla- 
nets, become visible in a telescope by means of the spots on 
their surfaces ; the remote situation and smallness of the 5th 
satellite of Saturn, leave us without this assistance ; but what 
we can no longer perceive, with our best optical instruments, 
we now supply by rational arguments. The change in the 
light of the satellite proves the rotation ; and the rotation 
once admitted, proves the existence of spots, or less luminous 
regions on its surface, which at setting off were only hypothe- 
tical. In the same manner a still more extended similarity 
between the sun and the stars offers itself, by the spots that 
now must also be admitted to take place on their surfaces, as 
well as on that of the sun. 
To return to the difficulty which has been started, it may be 
further urged, that there are some reasons to surmise that the 
34 Cygni is a periodical star of 18 years return;* and that 
other stars seem very slowly to diminish their lustre, and may 
probably recover it hereafter. 
In answer to this, I remark that it will not be necessary to 
remove objections to the rotatory motion of the stars, inferred 
from their very slowly changeable lustre, till they come pro- 
perly supported by well ascertained facts. Many causes in the 
physical construction of the stars may occasion an accidental 
and gradual increase or decay of brightness, not subject to any 
regularity in its duration. But when settled periods can be 
ascertained, notwithstanding they should be of the most ex- 
tended duration, it will not be difficult to find other causes to 
explain them, without giving up the rotatory motion. When 
the biography of the stars, if I may be allowed the expression, 
* Phil. Trans, for the year 1786, Part I. page 201. 
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