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Gautama Siddha belonged to one of the three prominent Indian families of expert-astronomers settled in China during Tang dynasty (618-906 AD). Members of this family held important positions in the successive astronomical bureaus of Tang dynasty. Gautama, once President of the bureau, was assigned the task of writing down a compendium Datang Kaiyuan Zhanjing ( ) since he was very well acquainted with both systems, Chinese and Indian. He devoted one chapter out of hundred and twenty altogether, to Indian calendar-making method, popularly known as the Jiuzhili or Navagraha method. At the outset Gautama explained the decimal notation system as employed in arithmetical operations in India at that time. He recorded symbols for numbers one to nine and also zero. He went on to explain the division of earth’s orbit into 360 degrees as against more than 365 in the Chinese system and further proposed algorithms for calendar making and prediction of lunar and solar eclipses. Trigonometric functions were also officially introduced to China through this work.

Allusions to Ancient Indian Mathematical Sciences in an Early Eighth Century Chinese Compilation by Gautama Siddha