restrial
climates. Mr. Hopkins, when treating of this theory, remarked, that so
far as we were acquainted with the position of the stars not very remote
from the sun, they seem to be so distant from each other, that there are
no points in space among them, where the intensity of radiating heat
would be comparable to that which the earth derives from the sun, except
at points very near to each star. Thus, in order that the earth should
derive a degree of heat from stellar radiation comparable to that now
derived from the sun, she must be in close proximity to some particular
star, leaving the aggregate effect of radiation from the other stars
nearly the same as at present. This approximation, however, to a single
star could not take place consistently with the preservation of the
motion of the earth about the sun, according to its present laws.

Suppose our sun should approach a star within the present distance of
Neptune. That planet could no longer remain a member of the solar
system, and the motions of the other planets would be disturbed in a
degree which no one has ever contemplated as probable since the
existence of the solar system. But such a star, supposing it to be no
larger than the sun, and to emit the same quantity of heat, would not
send to the earth much more than one-thousandth part of the heat which
she derives from the sun, and would therefore produce only a very small
change in terrestrial temperature.[208]

_Variable splendor of stars._--There is still another astronomical
suggestion respecting the possible causes of secular variations in the
terrestrial climates which deserves notice. It has long been known that
certain stars are liable to great and periodical fluctuations in
splendor, and Sir J. Herschel has lately ascertained (Jan. 1840), that a
large and brilliant star, called _alpha_ Orionis, sustained, in the
course of six weeks, a loss of nearly half its light. "This phenomenon,"
he remarks, "cannot fail to awaken attention, and revive those
speculations which were first put forth by my father Sir W. Herschel,
respecting the possibility of a change in the lustre of _our sun
itself_. If there really be a community of nature between the sun and
fixed stars, every proof that we obtain of the extensive prevalence of
such periodical changes in those remote bodies, adds to the probability
of finding something of the kind nearer home." Referring then to the
possible bearing of such facts on ancient r