January 1, 2004
An explanation of the Zodiac now and 2,000 years ago.
Before we embark on distinguishing a zodiacal sign from a zodiacal constellation, let's first of all define the zodiac. The zodiac is the section of the sky in which we find the Sun, Moon and the planets. We except the planet Pluto, which was discovered after the formation of the zodiac. As of now, the zodiac hasn't been changed to accommodate the most recently discovered planet.
The band of the zodiac is some sixteen degrees wide, extending about eight degrees north and south of what's called the ecliptic -- the Sun's path through the stars. More properly, the ecliptic is the projection the Earth's orbital plane into outer space. The ecliptic is depicted on many, if not most star charts.
The signs, unlike the constellations, are defined by the seasons. When the Sun stands right over the Earth's equator on the March equinox, this marks the first point of the sign of Aries. Some three months later, the Sun on the June solstice reaches its northernmost declination for the year at the tropic of Cancer. Six months after the March equinox, the Sun stands at the first point of Libra on the September equinox. Toward the end of the year, the Sun reaches its southernmost declination at the tropic of Capricorn on the December solstice. No matter the year, the signs are defined by the seasons, not by the backdrop of stars.
Over 2,000 years ago, signs and their namesake constellations more or less accompanied each other in the same section of sky. But they didn't match up perfectly even then, because signs represent equal thirty degree divisions of sky, whereas constellations vary in size. Since that time, the March equinox Sun has shifted about thirty degrees westward through the constellations of the zodiac. At the present time, the Sun is at the first point of Aries on the March equinox, but the stars of the constellation Pisces backdrop the March equinox Sun. At the June solstice, the Sun stands at the tropic of Cancer but in front of the constellation Taurus. On the September equinox, the sun resides at the first point of Libra but in front of the constellation Virgo; on the December solstice, the Sun stands at the tropic of Capricorn but in front of the constellation Sagittarius.
While signs neatly extend eight degrees north and south of the ecliptic, aligning with the borders of the zodiac, constellations do no such thing. The belt of the zodiac passes through some 24 constellations. Given that the zodiac extends eight degrees north and south of the ecliptic, an additional twelve constellations reside in part within the zodiac's belt of stars: Auriga, Canis Minor, Cetus, Corvus, Crater, Hydra, Ophiuchus, Orion, Pegasus, Scutum, Serpens and Sextans.
The borders of the constellations were drawn up and finalized by the International Astronomical Union in 1930. The Sun, which travels the straight and narrow road of the ecliptic, even passes through a thirteenth constellation -- Ophiuchus, the Serpent Bearer -- from about November 29 to December 17. What's more, the Sun barely touches upon the constellation Cetus, the Sea Monster, at the end of the first week in March.
In a golden field guide, I read that the Sun spends more time in the constellation Ophiuchus than in Scorpius right now, because of the precession of the equinoxes. I believe this is incorrect. I'm sure it's because the border between Scorpius and Ophiuchus has been redrawn since ancient times.
For a chart giving the dates of the Sun's passage within the constellations of the zodiac, click here.
copyright 2004 by Bruce McClure
Re-published on Astronomy Net with permission of the author.
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