《梦溪笔谈·象数一》
所以在西天印度历法中，罗睺、计都两星都是依据其逆行来设定运算的，此二星便是我们现在的黄道和白道的两个交点。交初点称为 罗睺 ，交中点称为 计都 。 
故西天法罗睺、计都，皆逆步之，乃今之交道也。交初谓之 罗睺 ，交中谓之 计都 。 
Therefore, in the Western Indian calendar, Rahu and Ketu are both set according to their retrograde operation, and these two stars are the two nodes of our ecliptic and the white road. The initial intersection is called Rahu, and the midpoint is called Ketu.