THE VISIBILITY OF STARS WITHOUT OPTICAL AID

Abstract

It is generally stated that the faintest stars readily visible to the unaided eye under ordinary conditions are of the sixth visual magnitude. To judge from the limiting magnitudes of the majority of the well-known naked-eye catalogs as well as from common experience, this statement is, indeed, substantially correct. The faintest stars in Ptolemy's Almagest, for example, are of magnitude 5.4; those in al Sufi's catalog are of magnitude 5.6. The more modern catalogs have somewhat fainter limits. The faintest stars in Argelander's limnometria Nova are, on the average, of magnitude 5 . 7, and those in the Atlas Coelistis Novus of Heis (who was noted for his keen eyesight) are of magnitude 6.1. Houzeau, in his Uranometrie Generale, has recorded stars of magnitude 6.4, and Gould, who many times remarked about the exceptionally clear atmosphere at Cordoba, states that persons of ordinarily good vision could see stars of the seventh magnitude. He has recorded in his Uranometria Argentina stars of magnitude 7.2. K. Lundmark1 has remarked that he can see M33, which has a total magnitude of 6.8, and P. Meesters,2 from a comparison of the Harvard Variable Star Charts with the sky, finds that he can see white stars of magnitude 6.8 and yellow stars of magnitude 6.9. While the faintest stars visible to these latter observers are