My goal is to produce a binary map indicating whether the moon is visible at a certain time of day with GRASS GIS. (i.e. A mountain could be blocking the view of the moon.) r.sunmask would be the perfect module. Pretend the moon is the sun and just provide the azimuth and altitude. However the module runs too slowly. r.sun is then the recommended substitute. However r.sun does not provide for direct azimuth/altitude input, just the time of the day. I could somehow do a funky backwards calculation of the time when the sun would match the azimuth/altitude of the moon, but I'm not sure that would work 100% of the time.
What is a good way to accomplish this task?
p.s. Is web Mercator (EPSG:3857) suitable for this task or do I need a different projection?