Look toward the southeast around 9:30 p.m. to see a large, important constellation that does not get as much respect as it deserves — Ophiuchus the Serpent Holder.
Ophiuchus is supposed to be a man holding a large snake represented by Serpens Caput (head) on its western end and Serpens Cauda (tail) on its eastern end. Ophiuchus is a misshapen oval with lines of stars (the Serpens) on either end.
Ophiuchus is most deserving of our attention because it and Serpens Cauda are contiguous to the western edge of the Milky Way. Ophiuchus has reasonably bright stars and contains many delightful binocular and telescopic objects favored for viewing and study by astronomers. It is one of the 13 constellations that cross the ecliptic, the apparent path in the sky that the sun follows on its yearly journey.
Most of the planets are close to the ecliptic, and their paths around the sky are confined to the narrow zone known as the zodiac, which stretches about 9 degrees on either side of the ecliptic.
The 12 main constellations through which the ecliptic and the sun and planets pass are the zodiacal constellations: Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpius, Sagittarius, Capricornius, Aquarius and Pisces.
The “signs of the zodiac” were established several thousand years ago when astrology and astronomy were closely related. However, modern astronomers no longer follow the tenets of astrology, and the two disciplines have gone their separate ways.
Formal constellation names and boundaries were standardized by the International Astronomical Union (IAU) in 1933 to prevent confusion. In this process, the zodiac gained a 13th constellation, Ophiuchus, because its modern boundary overlaps the zodiac.