Calculation method of day and night length
1. Find the length of day and night according to the time of sunrise and sunset.
The day length of a place =( 12 hours-sunrise time) * 2;
The night length of a place =(24:00- sunset time) * 2;
Sunrise time: when the dawn line and the latitude line intersect at the meridian;
Sunset time: when the intersection of dark line and latitude line is on meridian.
2. Calculation of the length of day and night on the illumination map
According to the longitude (V = 15/ hour) of the solar arc and the night arc in the latitude circle where the point is located;
Solar arc+night arc = 360; Day length = day arc /V, night length = night arc/v; Day length+night length =24 hours.