Page 454 - 89 history of astronomy_opt
P. 454
The frequency of the Aurora Borealis
was found by Wolf to follow the same pe-
riod. In fact, it is closely allied in its cause
to terrestrial magnetism. Wolf also collected
old observations tracing the periodicity of
sun-spots back to about 1700 A.D.
Spoerer deduced a law of dependence
of the average latitude of sun-spots on the
phase of the sun-spot period.
454
