What is the “scientific” basis for the apparently accepted view that the moon cannot be sighted with naked eye if it younger than 13 hours or so?
Scientific basis is that in 13 hours, the angular separation of the moon from sun is between 7.2 to 8.5. The angular separation (elongation) is the most important factor in moon’s visibility besides several other factors combined. At the angle 7, no sun light reflected from the moon can come to the earth meaning that the crescent is not formed to see from earth (This is due to the mountains on the surface of the moon that block the sunlight coming to the earth – Danjon effect). Between 7.2 and 8.5 the crescent is invisible to the eyes, because the brightness of this thin crescent is less than the sky brightness on the horizon. At elongation less than about 7.5 even telescopes do not pick the thin crescent. This is the discussion for naked eye versus telescope or binocular sightings. The published record for moonsighting with bare eyes shows that no one has ever seen the crescent of less than 15.4 hours old [See “Records of Young Moon Sightings, Quarterly Journal of Royal Astronomical S
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