蔡元定(1135~1198年),字季通,号西山,南宋时期建阳(今属福建)人;精于天文、地理、吕律、象数,著作有《律吕新书》、《大衍详说》等;为朱熹“四大弟子(蔡元定、黄于、刘爚、陈淳)”之首。关于蔡元定从学朱熹的时间,尚有争议。当今学者方彦寿先生所著《朱熹书院与门人考》对此作了考辨,采刘爚在《西山先生墓志铭》所言“乾道间,见文公于崇安,遂师事焉”的说法。~①换言之,蔡元定在1165年或稍后从学于朱熹。蔡元定的《律吕 Cai Yuan Ding (1135 ~ 1198), the word quarter pass, Xishan, Southern Song Dynasty Jianyang (now Fujian) people; good at astronomy, geography, Lu law, elephant, the book “Lu Lu new book,” “ ”; For Zhu Xi “ four disciples (Cai Yuan Ding, Huang Yu, Liu Yong, Chen Chun) ”first. There is still controversy about the time when Cai Yuandeng learned Zhu Xi. Today’s scholar Fang Yan Shou’s “Zhu Xi College and the door test” made a test of this, mining Liu Yong in the “epitaph” Mr. Xishan said “Road, see the public in Chong’an, then teacher Yan.” statement. In other words, Tsai Yuan-ting learned from Zhu Xi in 1165 or later. Cai Yuan Ding’s "Law Lu