The classical constellation set of western astronomy goes back to the time of Ptolemy. Only 48 items were on this set almost until the 15th century. Interestingly, there were thirteen constellations along the Zodiac of these earlier times. Of course, we only have twelve constellations in the Zodiac of today. Who dropped the 13th sign of the Zodiac?
Sun Signs and Constellation boundaries
Astrologers in the west name Sun signs by the constellations along the Zodiac. Therefore, the boundary among these constellations is important. However, astronomers have changed these boundaries recently. What does it mean for Sun signs? Sun Signs were set at a time when astronomers recognized only 48 constellations. IAU (International Astronomical Union) accepted 88 constellations in 1922.
Check the story below about the development of constellation boundaries. Sun signs were broken during the time of Ptolemy because of the 13th constellation.
What can be the reason for the 13th sign on Ptolemy’s Zodiac? Let us turn to the East for an answer. In the past, the astronomers of the Indian peninsula were experts in using the rotations of the Moon as a marker for months. The idea of a lunar month is popular in India even today. The Panchang calendar shows both the lunar and solar months. The length of an year is longer than a count of the days in twelve lunar months. A 13th month therefore shows up periodically, once in 3-4 years. However, NASA attributes the dropping of the 13th sign to a story from Babylonia.
The 13th sign is Ophiuchus, the snake bearer. The following figure shows the lengths of Zodiac months per the constellation boundaries per IAU (International Astronomy Union). No boundaries existed between 48 constellations during the time of Ptolemy. Explorers added new constellations until the 18th to increase the count of constellations to 88.
Bharani Nakshatra and Zend Avesta