ROTARY MOTION. The sun, all the primary planets, and their satellites,
as far as known, rotate from west to east. Each rotation constitutes a
day for the rotating body. The central line of rotary motion is called
the axis of rotation, and the extremities of the axis are called the
Poles.
REVOLUTION AROUND THE SUN. All the primary planets and asteroids revolve
around the sun in the direction of their rotation, that is from west to
east; and the planes of the orbits in which they revolve coincide very
nearly with the plane of the sun’s equator. One revolution around the
sun constitutes the year of a planet.
All the satellites, except those of Uranus and perhaps Neptune, also
revolve from west to east.
Most of the comets revolve around the sun in very irregular and
elongated orbits, only a few having their entire orbit within the
planetary system. Some so move that after having entered our system and
made their circuit around the sun, they seem to leave it, never to
return.
[Illustration: The Egyptian Planisphere containing the Zodiacal signs,
with the Southern constellations, according to Kircher.]
[Illustration: The Egyptian Planisphere containing the Zodiacal signs,
with the Northern constellations, according to Kircher.]
Since the orbits of the planets are in most cases not far removed from
the plane of the ecliptic, they are to be seen in a comparatively narrow
belt of the heavens called,
THE ZODIAC. The belt of the sky which occupies 8° on each side of
the ecliptic is called the Zodiac, and it is within this belt that
the moon and the chief planets confine their movements, as none of
their orbits is inclined to that of the earth by more than 8°. The
Zodiac, which circles the celestial sphere, is divided into twelve
signs each of which occupies 30°, and roughly coincides with a
constellation. The following lists give the signs of the Zodiac,
with the seasons in which the sun passes through each of them:
Spring: Aries the Ram; Taurus the Bull; Gemini the Twins.
Summer: Cancer the Crab; Leo the Lion; Virgo the Virgin.
Autumn: Libra the Balance; Scorpio the Scorpion; Sagittarius the
Archer.
Winter: Capricornus the Goat; Aquarius the Water-bearer; Pisces the
Fishes.
Owing to the precession of the equinoxes, the signs of the Zodiac do
not now correspond with the constellations of which they bear the
names. Thus the sign Aries, in which the sun is seen on March 21st
as it passes the vernal equinox, with which the solar year begins,
is now in the constellation of Pisces, and in the course of the next
23,000 years it will move steadily backward through the
constellations until it returns to the Ram, where it stood when its
name was first given to it.