《明史·志·卷十三》
求第一南北差：纬度差。将日月合朔时太阳所在的宫分及午夜至合朔的时间代入数据表内，同上。取纬度差，按比例法求取，就是第一南北差。
求第一南北差以合朔时，太阳宫及子正，至合朔时入立成内，取纬差，依比例法求之，为第一南北差。
Find the first north-south difference: The difference in latitude. Convert the longitude of the sun at new moon and the time from midnight to new moon into a palace and minute and substitute these values into the data table. Take the difference in latitude and use the proportional method to calculate this difference. This is the first north-south difference.