GrENERAL comparative considerations of a whole class of
cosmical bodies must here precede their individual descrip-
tion. These considerations refer to the 22 principal planets
and 21 moons [satellites, or secondary planets'] which have
been discovered up to the present time, not to the planetary
bodies in general, among which the comets whose orbits have
been calculated are alone ten-fold more numerous The
planets possess, upon the whole, a feeble scintillation, inas-
much as they shine by the reflected light of the Sun, and
their planetary light emanates from disks. ( Cosmos, vol. iii.,
p. 76.) In the ash-colored light of the Moon, as well as in
the red light of its obscured disk, which is seen with great in-
tensity between the tropics, the Sun's light undergoes, in
reference to the observer upon the Earth, a twice repeated
change in its direction. Attention has been already directed
elsewhere* to the fact that the Earth and other planets pos-
sess in themselves a feeble power of emitting light, as h
specially proved by some remarkable phenomena upon that
portion of Venus which is turned away from the Sun.
We sha,ll consider the planets according to their number,
the sequence of their discovery, their volumes compared eithel
with each other or with their distances from the sun ; ac-
cording to their relative densities, masses, periods of rotation,
degrees of eccentricity, the iitclina.tions of their axes, and
characteristic differences within and beyond the zone oftht

' Coimot, vol. i., p. 201, and note p. 202.
