However, due to precession, the Earth has moved on its axis since then, and the dates used to identify the signs no longer correlate to the backdrop constellations that give them their names. In reality, the timeline has migrated by one sign to the west. That is to say, depending on the mathematical division of the year, zodiac sign dates roughly correspond to the position of the sun in the constellations of the signs preceding them today. (The fixed character of the signs is also why the Minnesota Planetarium Society’s 2011 argument that a 13th zodiac sign, Ophiuchus, should be added now, failed to result in a major astrological shift.)
“Previously, astrologers looked at where the sun was in relation to background constellations in general, and that almost exactly matched up with the signs of the zodiac defined by Ptolemy,” Odenwald adds.
Astrologers now base their calculations and predictions on where the planets and the sun are in relation to the 12 fixed signs, rather than where they are in relation to the constellations. According to astrology, if the sun is in the sign of Sagittarius on your birthday, you are a Sagittarius.
Is there any truth to the zodiac signs?
Ancient astronomers envisioned the zodiac constellations as having unique patterns that reflect the shapes of animals and humans. As illustrated, the zodiac constellations form an imagined belt in the sky that stretches about eight degrees above and below the ecliptic plane.