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 [15] 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.
KEPLER’S CELEBRATED LAWS OF
PLANETARY MOVEMENTS
The laws under which the planets move were discovered through the genius of John Kepler, and are known as Kepler’s Laws of Planetary Motion. Kepler derived these laws from observation only, but Newton first explained them by showing that they were the necessary consequences of the laws of motion and the law of universal gravitation.
Kepler’s First Law states: “The earth and the other planets revolve in ellipses with the sun in one focus.”
Kepler’s Second Law states: “The radius vector of each planet moves over equal areas in equal times.”