Knowing that the rekiho (method of making calendars) used by the Qing dynasty (the Jikenreki; known in English as the Chinese calendar) was, in fact, based on Western calendrical methods introduced by Jesuit missionaries, Yoshimune TOKUGAWA, the then Seii taishogun (literally, great general who subdues the barbarians), decided to adopt a new calendrical system by introducing Western astronomy with all Christian influence removed, and in accordance with Yoshimune's wishes, Masayoshi NISHIKAWA and Noriyoshi SHIBUKAWA began preparations to reform the calendar.
清朝の暦法（時憲暦）が、実はイエズス会の宣教師が伝えた技法を採用していることを知った征夷大将軍徳川吉宗は、キリスト教色を排除した形で西洋天文学を取り入れた暦法を採用したいと考えるようになり、その意向を受けた西川正休・渋川則休らの手によって改暦の準備が始められた。