Lunar crescent visibility

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The visibility of the thin crescent Moon is an important problem for the calendars of many societies, both ancient and modern. With roughly 1 × 109 people of the Islamic faith following the Islamic calendar, this problem is likely to be the one (nontrivial) problem in astronomy that has the greatest impact on our modern world. In the past decade, great advances have been made in the observation and theory of crescent visibility. This paper reports recent observations and analyses. New records have been set for the youngest Moon, with confident sightings at 15-0 hr by John Pierce with unaided vision and at 12-1 hr by Jim Stamm with telescopic assistance. These records can be significantly broken under optimal conditions. Various prediction algorithms are tested with the 294 collected individual observations plus the 1490 observations from the five Moonwatches. The age and moonset-lag criteria are found to be poor, the altitude/azimuth criteria can make a confident prediction only one-quarter of the time, while the best predictor by far is the modern theoretical algorithm.

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Astronomy and Geophysics

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