The zodiac signs as we presently know them were devised around the year 500 BCE in Mesopotamia based on an earlier model of eighteen signs. Within a few centuries the Greeks were deeply involved in the study of astronomy and astrology. Hellenistic astrology, which was the foundation for later European and Islamic traditions of astrology, was largely produced in Alexandria in Egypt starting around the second century BCE. Alexander died in 323 and Ptolemy took control of Egypt. The Ptolemaic Kingdom (305–30 BCE) ruled over Egypt until it came under Roman domination after the death of Cleopatra (69–30 BCE). The Romans subsequently took a deep interest in astrology and in the late Republic of the first century BCE it served as an exotic and alternative system of divination in competition with traditional Roman divination (augury and so forth). Although the chronology is somewhat unclear, between the second to fifth centuries CE, Hellenistic astrology was introduced to India and in various ways blended with the native systems of religious lore and astrology based on the twenty-seven or twenty-eight nakṣatra-s (lunar stations).
The scientific astronomy of the Greeks was likewise introduced in these centuries. The tradition of Indian jyotiṣa produced eminent figures like Āryabhaṭa (b. 476) in the Gupta dynasty, whose work on astronomy entitled Āryabhaṭa-siddhānta circulated throughout even the Iranian Sāsānian dynasty (224–651). It seems, however, that Buddhist institutions did not participate much, if at all, in the development of Indian astronomy. Buddhist Mount Meru cosmology, particularly that outlined in Abhidharma literature, is unscientific and based on authoritative statements in scripture. The world is conceived of as a flat disc with four continents of different shapes surrounding an hourglass-shaped Mount Meru with the sun and moon circuiting around it propelled by winds (for some details on this see here).
Later on around the early eleventh century when the Kālacakra literature was being produced (the Śrī-kālacakra tantra and its commentary the Vimalaprabhā), Buddhist authors demonstrated knowledge of advanced observational astronomy. The Śrī-kālacakra (ninth section of chapter one) discusses astronomy for instance. It describes the corruption of siddhānta-s (astronomical treatises), which the commentary identifies as those of Brahma, Sauram, Yamanakam and Romakam. The former two are Indian, but so far as I know, not Buddhist. It seems in any case there were no notable specifically Buddhist schools of astronomy. The latter two mean Yavana (Ionian or “Greek”, or later meaning other foreign cultures) and Roman, which highlight their foreign origins. The Kālacakra also uses the tropical zodiac rather than sidereal zodiac, which is significant because originally it was only late Hellenistic traditions of astronomy that used the tropical zodiac while Indians continued using the sidereal model (see Edward Henning's article here). This use of the tropical zodiac in the early eleventh century in India could possibly indicate an Islamic source for that element in light of the vibrant tradition of astronomy in Baghdad and other such centers of learning which Indian traditions were aware of. Islamic learning was not at all remote from India in those years.
Although Indian Buddhist institutions had limited interest in astronomy for most of their history, they still took an interest in astrology. There are plenty of early Buddhist texts that display a passive belief in astrological determinism, which is a topic of a paper I recently wrote (it is presently under review for publication). Astrological determinism is the belief that events and qualities of people are somehow influenced or signaled by celestial bodies. The belief that certain days are auspicious and conducive to some favorable outcome is an example of this.
The Buddhist poṣadha (sangha gathering to recite the precepts and carry out business) occurs according to the Indian lunar (nakṣatra) calendar on specific days of the cycle which are deemed favorable (such as the full and new moons). Although one might assume it was merely a convenient way to keep track of time, the Mahāsāṃghika-vinaya (translated into Chinese in the early fifth century) has the Buddha stating that a specific day “agrees with the nakṣatra-s” which is effectively electional astrology (selecting a time to do something based on astrological considerations). The Mahāsāṃghika-vinaya – even in the early fifth century when the Chinese monk Faxian 法顯 picked up a copy in Pāṭaliputra (modern Patna) – was considered in ancient India to be the oldest recension of the vinaya and some modern scholarship agrees that this is likely true. That would mean the early Buddhist sangha believed in astrology or at least a system of electional astrology based on the nakṣatra calendar. Perhaps even the Buddha himself believed in astrology.
There were therefore few ideological or philosophical obstacles in Buddhism to adopting elements from foreign systems of astrology, such as the twelve zodiac signs, from around the sixth century onward. As we discussed earlier in an earlier post (see here), it seems the teacher of Śubhakarasiṃha 善無畏 (637–735), a certain Dharmagupta of Nālandā, was the original human author behind the Mahāvairocana-sūtra, an early text in the tantric tradition. Śubhakarasiṃha's commentary on the text briefly mentions the twelve zodiac signs or houses, but goes into no details. The *Garbhadhātu-maṇḍala associated with the text does however depict these figures around the perimeter and they are understood as deities, albeit minor ones.
The concept of star worship was by no means alien to Buddhism as the aforementioned Mahāsāṃghika-vinaya has an invocation of nakṣatra deities. I tend to think that the practice of astral magic was actually native to Magadha originally. Early Brahmanism on the other hand had a low opinion of astrologers and forbid them from attending sacrifices. The Manusmṛti (chapter three) has the following code:
162. A trainer of elephants, oxen, horses, or camels, he who subsists by astrology, a bird-fancier, and he who teaches the use of arms, ... (all these) must be carefully avoided.
Nevertheless, the importance of observing astrological considerations is highlighted:
277. He who performs it on the even (lunar) days and under the even constellations, gains (the fulfilment of) all his wishes; he who honours the manes on odd (lunar days) and under odd (constellations), obtains distinguished offspring.
Although astrologers might have been disparaged, the validity of astrology itself was not questioned. There are some examples in Buddhist literature of astrology's validity being attacked, but in general most of the texts that I have surveyed indicate a passive belief in astrology despite the monastic prohibitions against practicing astrology.
The zodiac signs as they were depicted in China are preserved in an important document in Japan, the Taizō zuzō 胎藏圖象, which visually represents the deities of the *Garbhadhātu-maṇḍala. These representations are based on those brought to Japan from China by Enchin 圓珍 (814–891). He copied them in 855 in Chang’an at Qinglong-si 青龍寺, a center of learning for esoteric Buddhism. It is believed that these icons were first produced by Śubhakarasiṃha. The icons therefore have been recopied several times by Japanese and Chinese hands, but assuming they were faithful to the originals, we perhaps have a set of zodiac icons as they were generally envisioned by Śubhakarasiṃha, who represents the late seventh century Nālandā tradition of Buddhism, though at the same time we must concede that the icons as we presently have them show Central Asian and Chinese influences. One might even imagine that Śubhakarasiṃha had the icons in some manuscript from India and then asked a local artist to reproduce them. Not being an art historian myself, I will not make any judgments about this and will just present them here.
1. Aries - Meṣa
2. Taurus - Vṛṣabha
3. Gemini - Mithuna
4. Cancer - Karkaṭa
5. Leo - Siṃha
6. Virgo - Kanyā
7. Libra - Tulā
8. Scorpio - Vṛścika
9. Sagittarius - Dhanus
10. Capricorn - Makara
11. Aquarius - Kumbha
12. Pisces - Mīna
The depiction of Capricorn as a Makara is interesting. Monier-Williams defines makara as follows:
m. a kind of sea-monster (sometimes confounded with the crocodile , shark , dolphin &c ; regarded as the emblem of kāma-deva [cf. mokara-ketana &c below] or as a symbol of the 9th arhat of the present avasarpiṇī ; represented as an ornament on gates or on head-dresses).
As I mentioned earlier, the zodiac signs were treated as deities and there are also mantras for addressing them collectively with other astral deities. It should be noted that they were minor figures. However, it is interesting that in Buddhist literature they are regarded as deities alongside the planets because in the Greco-Egyptian tradition of astral magic, so far as I know, only the planets are regarded as gods (this was carried over into Latin which is why we still in English say Mercury, Venus, Mars, Jupiter and Saturn).
In Hellenistic astrology, the zodiac houses serve as domiciles which planets rule over, but in the associated magical tradition, at least as it is preserved in extant papyri, I am unaware of zodiac signs being treated as sentient gods. The nakṣatra-s had already long been regarded as sentient gods for many centuries in Magadha, so transforming the zodiac signs into such figures was perhaps a natural progression.