《周髀算经》光程极限数值来由新探

<周髀算经>中光程极限167000里这一概念和数值在整个<周髀算经>宇宙体系中占有重要的地位.唐如川和陈文熙认为日出和日落时分,太阳离周城的距离就是光程极限,但在具体推算上还存在一些问题.如果考虑<周髀算经>中春秋分与实际春秋分的时间差(<周髀算经>中的春秋分按照四分法计算得来),以及由于地球轨道近日点的变化(1.7度/世纪)所导致的春秋分时间的变动,则可得到比较精确的结果,从而使这一数据的来由得到比较合理的说明.

A New Inquiry on the Value of Ultimate Light Distance in Zhou Bi Suan Jing

According to Zhou Bi Suan Jing (Arithmetical Classic of the Gnomon and the Circular Paths),the sun could illuminate an area only 167 000 li in diameter.This conception and numerical value occupied a very important position in the cosmic system of Zhou Bi Suan Jing. The cause of the above value is one of the most important issues in the study of Zhou Bi Suan Jing over the years.Tang Ruchuan and Chen Wenxi probed this issue in the perspective of rationality and held the view that at sunrise or sunset the distance between the sun and Zhoucheng is the ultimate light distance.But as problems existed in inference,they did not obtain a satisfactory result. If the time error between the equinox in Zhou Bi Suan Jing and in reality and the alteration of the time of equinox caused by the change of perihelion(1.7 degree/c) are taken into consideration,we may obtain a comparatively accurate result.So the cause for the data of ultimate light distance can be explained reasonably.