Ādāna-Pradāna: What Came In, What Went Out
The twelve signs came in from the Greek world; Indian tables went out to Baghdad and to the Tang court in China
In 718 an astronomer of Indian descent, Gautama Siddha, translated the Navagraha calendar for the Tang emperor, writing zero as a dot. Follow knowledge both ways: the twelve rāśis and Greek words entering Sanskrit through the Yavanajātaka, Indian tables reaching Chang'an and Baghdad, and how each borrowing was credited, tested against the sky, and is still being re-examined today.
A Dot in the Emperor's Library
In the sixth year of the Kaiyuan reign, 718 CE, an astronomer named Gautama Siddha sat at work in Chang'an, the capital of Tang China. Chang'an was then one of the largest cities on Earth. Its walled wards ran in a neat grid, and its markets rang with traders from Persia, Central Asia and India. On Siddha's table lay a brush, an inkstone and sheets of paper. Beside them was a Sanskrit astronomy text, and an order from the emperor Xuanzong to put it into Chinese.
In Chinese he was called Qutan Xida. Qutan is how Chinese writes Gautama. His family had come from India generations before, and they had served the imperial astronomy office for so long that it had become the family trade. Siddha now headed that office.
The stakes were high. In Tang China the calendar belonged to the throne. A correctly predicted eclipse showed that heaven and the emperor were in step. A missed one embarrassed the whole court. The official calendar of the day had started to miss. Siddha was offering the emperor an Indian way of computing the sky. Would the court trust a foreign method with the most political numbers in the empire?
One small passage in his translation shows how carefully he worked. Explaining how Indians write numbers, he noted that each of the nine digits is made with a single stroke, and that "in every empty place, a dot is always put." That dot is zero, written into a Chinese book in the early 700s.
Nine Planets for the Tang Court
The text Siddha translated became the Jiuzhi li, the "Nine Seizers calendar". The name is a literal rendering of Navagraha, because graha means "seizer". The nine are the Sun, the Moon, the five visible planets, and Rāhu and Ketu, the two points where eclipses happen. Chinese astrology still calls these two Luohou and Jidu, Chinese spellings of the Sanskrit names.