The classical constellations of western astronomy goes back to the time of Ptolemy. Almost until the 15th century only 48 constellations were on the list. The interesting fact is the presence of a 13th constellation on this Zodiac. There are only twelve in the Zodiac of today. Who dropped the 13th sign or the constellation? The Sun signs are broken per the modern constellation boundaries. 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.