明制简仪上地平日晷时间刻线的偏差与地理经度差修正问题的考证

A Textual Research about the Deviation of the Scribed Scale Lines on the Horizontal Sundial Attached to the Abridged Armilla Built during the Ming Dynasty and the Correction for the Longitude Difference

在著名的明制简仪(现今保存于南京紫金山天文台)上有一架按西法建造的地平日晷。对于此日晷来历,百余年来的大多数学者都认为是清代加上去的。此晷面上的刻线有一个迄今未解的重大疑问:晷盘时间刻线的午正线方向相对于方位盘刻线的子午线北侧为何有着一个向东二度左右的偏离角?文章对这一问题作了多方面的考证,认为:这一偏离角正相应于南京和北京的地理经度之差,是当时的设计制造者有意为之的。由于在整个明代所用的官方标准时间并非北京的地方时间(顺天之数),而是南京的地方时间(应天之数),为了在北京的观测能在此日晷刻线上直接读取相应于南京的地方时时刻,必须作两地经度差的改正。而这一向东二度左右的偏离角,正是相应于南京和北京经度差的改正角。在明代末期,中国天文学家也已经了解了关于地理经度和经度差的某些概念。由此,可以认为该地平日晷最迟在明代末期已经建造于北京了。

There is a horizontal sundial built according to Western astronomy as an attachmentof the Abridged Armilla now preserved by the Purple Mountain Observatory. The Abridged Armilla isa famous ancient Chinese astronomical instrument built during the Ming Dynasty. As for the past his-tory of this horizontal sundial, most scholars over the last 100 years believed that it was added duringthe Qing Dynasty. There remains a major unresolved question about the scribed scale lines on thesurface of this horizontal sundial. Between the scale lines for the high noon and the meridian there isan angle of deviation. The high noon line is deviated eastwards from the meridian line for about 2 de-grees. The doubt is why there is such a deviation angle？ With a comprehensive textual research on this doubt, the authors conclude that the deviation an-gle of about 2 degrees corresponds to the difference between the geographical longitudes of Nanjingand Beijing cities, and it must be designed by the maker intentionally. The authors note that duringthe whole Ming Dynasty, the official China standard time was Nanjing local time （named as ＂Time ofYing-tian＂ ） rather than Beijing local time （named as ＂Time of Shun-tian＂ ）. In order to directlyand conveniently readout Nanjing local time through the scribed scale lines on this sundial in the cityof Beijing, the longitude difference of the two cities must be taken into consideration when buildingthe sundial. Such a longitude difference is just in accordance with the deviation angle of about 2 de-grees as expressed by the scribed scale lines on above-mentioned sundial. During the last stage of the Ming Dynasty, the Chinese astronomers had already known some-thing about geographical longitude and set up the concept of time difference between locations of dif-ferent geographical longitudes. So it is reasonable to conclude that this sundial must have been builtin Beijing during the last stage of the Ming Dynasty rather than during the Qing Dynasty.