Precession also causes the equinox and solstice points to drift across the backdrop of stars, including the constellations that cross the zodiac. The zodiac is a line in the sky that one gets by extending the plane of the solar system (all planets except Pluto orbit on approximately the same plane) into space. Astrologers extend the zodiac zone about 8 degrees above and below that line for purposes of noting planetary alignments and such. That's about 16 times the width of the full moon.
Tropical astrology, the type used in most newspaper horoscopes, uses the solstice points of the first millennium. Since then, precession caused the astronomical solstice points to drift almost a full sign.
The dates of the Sidereal zodiac, practiced by Indian astrologers, adjust to account for precession. This adjustment means that a person’s sign under Sidereal astrology, which may not match the sign of the Tropical zodiac, more closely matches the actual position of the stars.
Both Tropical and Sidereal astrology divide the zodiac into 12 equal segments and assign each segment to a sign based on the constellation that is now or was in that general region of the sky. However, the actual constellations are not uniform sizes, and the amount of space on the zodiac is not equal. While astrology allocates 30 degrees for each sign, the constellations occupy more or less than that. In addition, a thirteenth constellation, Ophiuchus, lies on the band of the zodiac, a fact noted by Ptolemy in the second century and generally ignored by most astrologers.
“Zodiac” is good for 18 points in Scrabble. The word comes from the Greek words for “animal” and “circle.”
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