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Da Yan Calendar
《大衍历》
D a Yan Cdlenddr was drafted by Zhang Sui ( 683一727，also named Seng Yixing)in 727，and was later sorted out by Zhang Shuo(667一730) and Chen Xuanjing.
《大衍历》，公元727年唐朝天文学家张遂(683一727，又名僧一行)修订，后经张说(667一730)和陈玄景整理成文。
The calendar consists of seven parts that elaborate on a variety of computa-Lion. There are another 12 articles dealing with some theoretical issues，mainly the articles Zhang Sui wrote in the calendar.
《大衍历》分七章，详细介绍了各种计算方法。此外，张遂还在《大衍历》中写了12篇相关理论问题的文章。
At the beginning of calendar making，in order to measure the position of stars in their orbits and get the law of movement, Zhang，in cooperation with his colleagues，made the bronze armillary sphere and ecliptic sphere，with which they gath-erect lots of astronomic data through practical ob-nervation，and carried out effective astronomic re-search. For instance，they remeasured the post-tion of over 150 stars and the degree between the 28 constellations and the celestial North Pole，hence concluding the stars were always in mo-tion. Based on this fad，Zhang inferred other stars on the celestial bodies were also movable，over-throwing the conclusion reached by his predeces-sors.Halley(1656一1742)，a British astronomer also Put foward a similar view-Point，but was over a thousand years later.
在《大衍历》修订初期，为了测量日、月、星辰在其轨道上的位置和掌握其运动规律，张遂等人制造了观测天象的“浑天铜仪”和“黄道游仪”。借助天文仪，通过实际观测，他们搜集了许多天文数据，有效地进行了对天文学的研究。例如，通过重新测定150多颗恒星的位置，多次测定二十八星宿距天体北极的度数，发现恒星是运动的。根据这个事实，张遂推断出天体上的恒星肯定也是移动的，于是推翻了前人的恒星不运动的结论。英国天文学家哈雷(1656 } 1742)也提出了恒星自己移动的观点，但比张遂的发现晚1000多年。
In revising the calendar, Zhang attached huge importance to practice. Togeth-er with Nan Gongshuo，he used a post to measure the shadow cast by the sun，and calculated the relations between the sun’s positions and the solar terms. Be-sides，Zhang also designed an astronomic instrument called Fujuyi for measuring the altitude of the North Pole in different places throughout the country.
在历法修订过程中，张遂非常重视实践。张遂和南宫说等人一起，用标竿测量日影，推算出太阳位置与节气的关系。张遂设计制造了“复矩仪”，用于测量全国各地北极的高度。
Between 724一725，Zhang led a large-scale project to identify the astro-geo-detic measurement of 13 places and，based on the results，concluded that the length of a degree of the meridian line was 351.27 li(131.3 kilometers)by the Tang measurement. This was the first measurement of the meridian ever done in the world.
公元724-725年，张遂组织了全国13个点的天文大地测量。从测量数据中，张遂得出了北极高度相差一度，南北距离就相差351里80步(合现代131. 3千米)的结论。这个数据就是地球子午线一度的弧长。唐朝测出子午线的长度，在当时的世界上还是第一次。
Beginning in 725，Zhang set out to revise the calendar, and finished the draft namely the Da Yan Calendar,before his death in 727. The new calendar was im-plemented in 728.
公元725年，张遂开始编订历法，至逝世前完成《大衍历》草稿。公元728年《大衍历》颁行。
An innovative calendar in Chinese history，Da Yan Calendar inherited the strong points of calendars of the previous dynasties，while making improvement on the shortcomings and defects，making a significant breakthrough in the calendar-making field. The calendar provided a deep insight into the asymmetrical move-ment of the sun and the moon. The most prominent point about it lies in the correct calculation with a new formula of the speed of the sun’s movement along the eclip-tic.
《大衍历》是一部具有创新精神的历法，它继承了前朝历法的优点和长处，对不足之处和缺点作了修正，因此，在历法制定领域取得了重要突破。《大衍历》对太阳和月亮的不均匀运动有很深见解，最突出的表现在，它运用新的公式比较正确地计算出太阳在黄道上运行的速度。
Well organized and logically deducted，Da Yan Calendar was more precise than its predecessors，and it served as a benchmark for calendar makers for many dynasties afterwards.
《大衍历》结构合理、逻辑严密，比以往各朝代历法更精确，成为后世历法编制的经典模式。
Da Yan Calendar was a leading calendar in the world at that time. In 717，Ja-pan sent Kibi Makibi to China to study astronomy，and when he went back to Ja-pan in 735，he brought with himself manuscripts of Da Yan Calendar，and the cal-endar was spread to Japan and used there for over a hundred years.
《大衍历》是当时世界上先进的历法。公元717年，日本派吉备真备来中国学习天文学，735年，吉备真备回国时带走了《大衍历》。于是《大衍历》便在日本广泛流传起来，作为历法使用了100多年。
