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So far we have virtually ignored the Solar System with its planets and their satellites. We need only one fact for the moment, which is that the Solar System is almost two-dimensional. It is as if the Sun, the Earth, the Moon and all the other planets are within a very large and very thin old fashioned phonographic record. When you look at a flat object from the side it appears as a line.
Since all of the above-mentioned objects remain almost in a plane, they too also appear from the Earth as if they are always travelling along a line. This line is the ecliptic. The objects of the Solar System are too close to the Earth to appear as fixed points on the celestial sphere, but they always move along the ecliptic. The ecliptic crosses the celestial equator at two points at an angle of 23 ½ degrees. This angle is formed because the Earth's axis is not exactly perpendicular to the plane of the Solar System but is tilted 23 ½ degrees.
Even in ancient times, people were aware of the ecliptic and attributed particular significance to the constellations found along it. The constellations found along the ecliptic are named Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpio, Sagittarius, Capricorn, Aquarius, Pisces and Aries, and are collectively known as the zodiac. The rhyme that is used to help memorize the zodiac goes likes this:
The Ram, the Bull, the Heavenly Twin,
And next the Crab, the Lion stands,
The Virgin and the Scales,
The Scorpion, the Archer, the Sea-goat,
The Man who pours the water out,
The Fish with glittering tails.
It's not much of a poem, but has become the standard. The word zodiac comes from the Greek and means a circle of animals. All the constellations of the zodiac represent living things except Libra, the Scales.
The zodiac is one of the most important ingredients in the pseudo-science of astrology. Until recent times astrology was considered a respectable science, as was alchemy. Even Johannes Kepler, the great seventeenth-century scientist who discovered the basic laws of planetary motion, was not above casting a horoscope; he may even have believed in them for all we know. Leaving aside the question of whether a person’s “sign” has any real influence on his life, let us first figure what a person born under Aquarius really is. An Aquarius is someone born between January 21 and February 21, for example January 30. During the 24 hours of January 30, all 12 constellations of the zodiac encircled the Earth. Their position depends not only on the date but what time our Aquarius was born. The date January 30 is associated with Aquarius because at noon on that day the Sun is in the constellation Aquarius. Well, almost there. The dates that are used for astrology are based on calculations made about 2,100 years ago.
Every year the point at which the ecliptic meets the celestial equator slips back about 0-014 degrees. This is nothing to worry about; it just means that the Earth’s axis wobbles around every 27,500 years or so. Some astrologers get around this problem by claiming that Aquarius does not refer to the constellation but to the area in the sky that is near the constellation. If you still want to consult an astrologer, try to keep him honest. If he says something like, “now is the time to take risks because Jupiter in Scorpio,” and last night you looked up and saw that Jupiter was on the other end of the sky from Scorpio, you can at least find another astrologer who knows the sky as well as you. You can also mention that the gravitational of all the stars and galaxies in the universe combined together exerted less gravitation force on you during your birth than the doctor who delivered you.
It may be worthwhile to read this chapter, or better yet, to look at the sky and then reread it. The celestial sphere and sidereal time are not ends unto themselves, but means to help us grasp the way the stars move through the sky. Remember that because the Earth’s axis points towards Polaris, all the stars circle it once every sidereal day. The best way to get a feel for the sky’s motion is to pick a star and watch it slowly circles the heavens. Until then you may know the dry facts about our planet’s motion, but only when you take the time to look upward at the stars can you truly understand it.