Encke's comet, which is one of the
three 'interior comets', completes its course in 1200 days, but from the
form and position of its orbit it is as little dangerous to the earth as
Halley's great comet, whose revolution is not completed in less than
seventy-six years (and which appeared less brilliant in 1835 than it had
done in 1759): the interior comet of Biela intersects the earth's orbit, it
is true, but it can only approach our globe when its proximity to the sun
coincides with our winter solstice.
The quantity of heat received by a planet, and whose unequal distribution
determines the meteorological variations of its atmosphere, depends alike
upon the light-engendering force of the sun; that is to say, upon the
condition of its gaseous coverings, and upon the relative position of the
planet and the central body.
p 44
There are variations, it is true, which, in obedience to the laws of
universal gravitation, affect the form of the earth's orbit and the
inclination of the ecliptic, that is, the angle which the axis of the earth
makes with the plane of its orbit; but these periodical variations are so
slow, and are restricted within such narrow limits, that their thermic
effects would hardly be appreciable by our instruments in many thousands of
years.
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