Few people know that the term “Zodiac” actually refers to the band of constellations the sun appears to travel through on its yearly journey, as viewed form Earth. Even fewer realize that the commonly known mid month to mid month time frame that is traditionally used for calculating the sun sign (the one people refer to when asking your sign) is incorrect because it is about 1,500 to 2,000 years out of date.
The Tropical chart most astrologers today use was created by Claudius Ptolemaeus, known in the west as Ptolemy, who wrote one of the most important texts on astrology called Tetrabiblos, almost 1,850 years ago. Since his calculations were published the earth’s alignment has shifted by about 25 degrees, due to natural rotational cycle changes. Due these natural changes and the calendar changes we have made, such as the adoption of the Gregorian calendar, the signs of the zodiac are out of sync with the modern calendar.
The Sidereal chart was created by an Irish astrologer named Cyril Fagan in 1944, and was more in line with the actual constellations and eastern astrology, though not nearly as popular with western astrologers.
It has been said that modern astrology actually should include a 13th sign named Ophiuchus, the Serpent Bearer. This sign would have fallen between the constellations of Scorpio and Sagittarius, from November 30 - December 17. This information is inaccurate, but not because of the common answer in the west of “the constellations have nothing to do with the zodiac” (Which by the way is the battle cry of the ill informed fool, ignorant as to the origins of his professed art).
The reason Ophiuchus should not even be considered as a sign is a question of history and politics. The immediate predecessors to the modern astronomers enjoyed naming constellations after themselves or favorite things. So much so that by the 19th century the night sky had become filled with overlapping constellation boundaries and often contradictory names as different schools of astronomy prepared their own versions of the star maps. To fix this problem, names and boundaries were officially reassigned to 88 constellations by the International Astronomical Union in 1930, completely covering the entire sky. The only reason that Ophiuchus falls into the zodiac band is that when astronomers redefined the constellation boundaries, it made the sun travel through a tiny part of the huge constellation. Astrology is primary concerned with the original boundaries of the constellations which divide the zodiac into twelve houses, as they are viewed from earth.
Below we present the Tropical, Sidereal and modern constellation dates for each sign. The precise dates of the signs in the Sidereal chart and the constellations will vary slightly from year to year.