Background and career

The Sui Dynasty was integral for uniting China, so it was a good time for learning. But when Li was sixteen the Sui fell, and the Tang rose. Nevertheless, the Tang did not harm the conditions for education. Indeed, it rather strengthened it. The Imperial Academy's math teaching was formalized. He was appointed into the Imperial Astronomy Bureau as an advanced court astronomer and historian, in 627. Once several years had passed, he then was promoted to deputy director of the Imperial Astronomy Bureau in 641, and even director in 648. He was given these titles because the Chinese calendar of the era, despite that it had only been used for several years, was already having accuracy problems in predicting eclipses. In fact, Li was appointed partially because of his critique of the Wuyin calendar. Wang Xiaotong had been chosen to study the problem earlier. This was a very important job because of the Chinese belief in the Mandate of Heaven. So if one altered the calendar, that person would have some control over the connection between the heavens and the emperor.

Astronomy and calendar

In 665, Li introduced a reform calendar. It was called the Linde calendar. It improved the prediction of planets' positions and included an 「intercalary month.」 That is similar to the idea of a leap day. It would catch up a lunar year to a solar year because twelve lunar months are 1.3906 days short of one solar year. It was added every three years. The Linde calendar is the most prominent accomplishment of Li.

Li wrote a document complaining about the use of outdated equipment in the Imperial Astronomy Bureau, so he was commanded to construct a new armillary sphere. He completed it in 633. His construction had an additional third ring as opposed to the more common design of only two rings.

Mathematics

Li added corrections to certain mathematical works. Examples of this are in Nine Chapters on the Mathematical Art by Liu Hui. He demonstrated that the least common multiple of the numbers two, three, four, five, six, seven, eight, nine, ten, eleven, and twelve was 27720, the answer was flawed in the original text. Yet another instance of this was in Zu Geng's work about the area of a circle. Li gave 22/7 (3.1428571428571428571428571428571) instead of three as a better approximation of what we know now as pi. He began each annotation with the words 「Your servant, Chunfeng, and his collaborators comment respectfully on…」 Li did write some mathematical works of his own, little is known about them. They are usually dismissed as unimportant in comparison to his other accomplishments. With Liang Shu and Wang Zhenru, he wrote Shibu Suanjing (十部算经) in 656. These were ten mathematical manuals submitted to the emperor.

Literary works

Li contributed to the Book of Sui and Book of Jin, which cover the history of the Sui and Jin dynasties. He wrote about the discoveries in astrology, metrology, and music. These are the official histories of the periods. The book Massage-Chart Prophecies (Tui bei tu) is generally credited to Li. The book is a collaboration of attempts to predict the future using numerology. Therefore, Li is often thought of as being a prophet. The book gets it title from a poem near the end, discussing how much time it would take to tell the story of thousands of years, it would be better to take a break and enjoy a massage. Circa 640, Li wrote the Ganying jing 感應經 to elaborate the concept of ganying cosmic correspondence. Li wrote a book discussing the importance of astrology in Chinese culture called Yisizhan in 645. This is around when he would have been working on the Linde calendar. Yet another of is works is Commentary on and Introduction to the Gold Lock and the Flowing Pearls. In this book he describes Taoist customs that was probably part of influence from his father.

生平

父親李播是隋時的高唐縣尉，後放棄官職為道士，道號黃冠子，著有《天文大象賦》。

李淳風早年學道於天台山，最早是隋煬帝的司監官。他通曉天文星象，曆法以及數學。

一日，唐太宗得到一本秘讖，上面說：「唐中弱，有女武代王。」，李淳風預知武后將稱帝，並誅殺唐氏子孫殆盡。太宗欲殺之，李淳風諫言：「不可。陛下若留，雖皇祚暫缺，而社稷延長。陛下若殺之，當變為男子，即損減皇族無遺矣。」太宗遂止。。

劉餗的《隋唐嘉話》記載淳風能預測日蝕，唐太宗頗不信其言，李淳風說：「有如不蝕，則臣請死之。」一日唐太宗等不至日蝕，於是對李淳風說：「我放你回家，和老婆孩子告別。」李淳風在牆上zh:划; zh-hans:划; zh-hant:劃了一條標記說：「尚早一刻。等到日光照到這裡時，日食就出現了。」後來果「如言而蝕，不差毫髮」。一日李淳風與張文收同皇帝出遊，有狂風從南面迎來，李淳風認為在南面五里處一定有人在哭，張文收則認為那裡有音樂聲。後來果然有一送葬儀隊經過。

貞觀七年（633年），《舊唐書》、《新唐書》皆記載李淳風通過改進傳統渾儀，創造性的在赤道環上增加了黃道環和白道環，研製出當時絕無僅有的「三重環」渾天儀，名為渾天黃道儀。貞觀十五年（641年）任太史丞，撰研究渾天儀的《法象志》。

貞觀二十二年（648年）任太史令，奉詔註釋《算經十書》。李淳風還編撰有《推背圖》、《典章文物志》、《秘閣錄》、《乙巳占》等書。

李淳風還指出《戊寅元曆》的錯誤。唐高宗時，李淳風以劉焯的《皇極曆》為據，編成《麟德曆》。麟德二年（665年），改用李淳風的《麟德曆》。