he Earth accomplishes a revolution on
her axis, and therefore the length of the sidereal day, which is 3
minutes 56 seconds shorter than the mean solar day, is invariable. In
this motion of the Earth we have a time-measuring unit which may be
regarded as absolutely correct.

The Earth completes a revolution of her orbit in 365-1/4 days. In this
period of time she accomplishes a journey of 580 millions of miles,
travelling at the average rate of 66,000 miles an hour. The change of
the seasons, and the lengthening and shortening of the day, are natural
phenomena, which occur as a consequence of the Earth's annual revolution
round the Sun. Precession is a retrograde or westerly motion of the
equinoctial points, caused by the attraction of the Sun, Moon, and
planets on the spheroidal figure of the Earth. By this movement the
poles of the Earth are made to describe a circular path in that part of
the heavens to which they point; so that, after the lapse of many years,
the star which is known as the Pole Star will not occupy the position
indicated by its name, but will be situated at a considerable distance
from the pole. These motions, Milton says, unless attributed to the
Earth, must be ascribed to several spheres crossing and thwarting each
other obliquely; but the Earth, by rotating from west to east, will of
herself fetch day, her other half, averted from the Sun's rays, being
enveloped in night. Thus saving the Sun his labour, and the 'primum
mobile,' 'that swift nocturnal and diurnal rhomb,' which carried all the
lower spheres along with it, and brought about the change of day and
night.

Milton's allusion to the occurrence of natural phenomena in the Moon
similar to those which happen on the Earth is in keeping with the
opinions entertained regarding our satellite, Galileo having imagined
that he discovered with his telescope continents and seas on the lunar
surface, which led to the belief that the Moon was the abode of
intelligent life.

... and other suns, perhaps,
With their attendant moons, thou wilt descry
Communicating male and female light.--viii. 148-50.

Milton in these lines refers to Jupiter and Saturn, and their
satellites, which had been recently discovered; those of the former by
Galileo, and four of those of the latter by Cassini. The existence of
male and female light was an idea entertained by the ancients, and which
is mentioned by Pliny. The Sun was regarded as a masculine star, an