A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Frank Reed
Date: 2021 Mar 21, 17:50 -0700
Late this afternoon I saw the Moon was very close to half full. Checking my USNO clone app, I see that it was just about 52% illuminated at that time. Then at the beginning of twilight with the Moon about 73° high, I noticed that the terminator, the line through the horns of the Moon, pointed almost exactly South (or North, if you face the other way). It was interesting to think through why this had to be the case tonight -- date very close to the equinox, Moon at an elongation (or lunar distance) or just over 90° from the Sun... It was an almost perfect case for this "terminator as compass" trick. Which raises the question: when would you have the worst case scenario for the half moon at sunset? Or more generally, for the half moon on the meridian? And what would you have to know to "fix" the compass created by the Moon's horns on other dates?