Young-Moon Geometry
In Pearce, Arizona, Matt BenDaniel recorded the Moon at dawn on October 15, 2001, only 30.8 hours before new. He took the 4-second exposure on Kodak E200 film with a 130-millimeter Astro-Physics refractor at f/6.7.
On the morning of August 13, 1931, French astronomer André Danjon observed a Moon only 16h 12m before new with a 3-inch refractor. Much to his surprise, the thin crescent appeared to extend only 75° to 80° along the Moon’s limb — considerably less than the expected 180° (halfway around). When Danjon compiled many other observations of this “deficiency” effect, he came to a remarkable conclusion: Whenever the Moon is 7° or less from the Sun, there can be no visible crescent at all!
Danjon believed that mountains and other roughness along the lunar limb must be blocking some of the sunlit surface that would otherwise be seen, thereby clipping off the ends of the crescent. Bradley E. Schaefer (Louisiana State University, Baton Rouge) has modeled the crescent's perceived length by including physiological factors and atmospheric extinction. In any event, Danjon’s 7° limit should actually be revised to 7.5°, according to a 1998 study by Louay J. Fatoohi and his colleagues at the University of Durham.
What’s interesting is that a hard-and-fast Danjon limit of 7.5° still gives would-be record breakers a certain amount of leeway. Four other astronomical factors play a significant role in whether a crescent of minimum age can be seen:
The Moon should be at perigee (the near point of its elliptical orbit around Earth), for it will then draw away from the Sun most quickly and reach an elongation of 7.5° at the youngest possible age.
The Moon should be near its greatest ecliptic latitude, +5½° or –5½°, further adding to its elongation angle.
A nearly new Moon is most likely to be seen from a place on Earth where it has roughly the same azimuth as the setting (or rising) Sun, for the Moon is then highest above the observer’s horizon.
Those seeking to break a record should get as high above sea level as possible, Schaefer stresses, where the air is less hazy and the sky darker.
Care to make your own crescent-hunting attempt? Whether you are after a record or simply want to enjoy one of nature’s most enchanting spectacles, visit the South African Astronomical Observatory Web site. They maintain a large database with links to related sites and free software for exploring visibility circumstances.
