The rumours on the existence of the 13th Horoscope, the Ophiuchus (蛇夫座) constellation, have been around since the early 20th century. This rumours arise with some astrologers acknowledge its appearance existence behind the sun within the IAU [Inaugural General Assembly in Rome in 1922] constellation boundaries (Occur between November 29 and December 17); some other astrologers even identify a 14th Horoscope, the Cetus (The Whale) constellation, as part of the Horoscope Constellation.
With all said, its a MYTHS to count the Ophiuchus or the Whale constellation in the Horoscope Constellation Cycle.
Truths
The Horoscope philosophy structure is so complex, in-depth and structured that to add in the 13th and/or the 14th in the Horoscope Constellation Cycle, typically is like going against the law of nature and balance.
The followings are 2 of the major considerations on why the Ophiuchus/Whale constellation are not to be part of the Horoscope Constellation Cycle.
Numerology
"12" is the number of times/cycles. For instances,- 12 symbolise the cycle of time in both the Western and Chinese philosophy
(e.g. 12 number cycles in a clock, month, and year; multiple of 12 to calculates phase of time) - The 12 Chinese Zodiac: a classification scheme that assigns an animal and its reputed attributes to each year in a repeating 12-year cycle.
Elementalism
The universe/nature is balanced by a set of methodology. For instances, the 4 nature element of Wind, Water, Earth, and Fire; the 5 formation of the universe by Metal, Wood, Water, Fire, Earth; the 3 realm of nature via Sky, Earth, Spirit; and etc. Likewise, the the Horoscope Constellation Cycle must be able to divide by such sets of methodology in order to allow balance by the law of nature.
- thus having a 13th horoscope, such odd prime number cannot be divided equally by a balanced set of methodology.
(e.g. the 12 horoscope is divided into 4 nature element of Wind, Water, Earth, and Fire, as well as 3 psychology formations of Cardinal, Fixed, and Mutual signs)
Lastly, to prevent anymore confusion, it will be best for the readers to fix on the ancient philosophy (by the 12 horoscope); as such ancient philosophy have more substantial meaning as compared to the new concept (having the 13th & 14th horoscope) where there are only limited references to support its argument.