Glare and celestial visibility

Abstract

Glare is when a bright source of light hides a nearby fainter source and is caused by scattering of light within the atmosphere, the telescope, and the eyeball. In this paper I develop a model of the effects of glare on the visibility of astronomical sources as viewed with the human eye whether unaided or through a telescope. This model is tested and found to closely reproduce observations of lunar appulses, Galilean satellites, the Martian moons, well-known double stars, and lunar occultations. Glare calculations are then applied to a wide variety of situations of historical and astronomical interest. (1) Ancient Chinese lunar appulse reports have been used to determine the acceleration of the Earth's rotation. However, a detailed analysis shows that the results depend critically on the adopted weighting scheme and that the ancient Chinese reports contain too many errors to allow for a meaningful conclusion. (2) The Crucifixion eclipse of A.D. 33 April 3 (or any other partial lunar eclipse with less than almost-equal-to 70% coverage by the umbra) would not have appeared "blood colored" because of the eclipse, since scattered white light from the penumbra will always mask the faint red light in the umbra. (3) Contrary to a statement by Aristotle, the visibility of stars from the bottom of deep wells or chimneys is worse than the visibility under an open sky. (4) Venus can be barely visible at inferior conjunction under optimal conditions with no optical aid. (5) An observing strategy is proposed which may lead to the discovery of several sungrazing comets per year by observations with small ground-based telescopes. (6) The effects of glare on the visibility of stars in loose open clusters is small. (7) The conditions for the detection of double stars are derived. (8) The model suggests various procedures for observers afflicted with glare, of which the most important is that the magnification should be pushed to the maximum usable power. (9) Various challenges are presented for observers, including the creation of a worldwide network to discover sungrazing comets.