The Earliest Visible Phase of the Moon

Abstract

I AM glad that Dr. Fotheringham in the interesting paper which appeared in the Classical Quarterly (April, 1920, pp. 97–8) adhered to the view that ‘Caesar calculated the new moon for January 1 [45 B.C]…and that this calculation determined the inaugural day of the Julian calendar.’ As the object of my brief note, on which he commented, was merely to show that Groebe had failed to prove that the day in question was January 2, I have only a few questions to ask. But first, in justice to Judeich, I ought to say that his calculations, which Dr. Fotheringham notices, were made with the help of the assistant-astronomer attached to the observatory of the University of Strassburg. While Dr. Fotheringham admits that I was right in maintaining that the new moon of January 2, 45 B.C. (1.26 a.m.) was not visible on the evening of that day, he holds that Groebe was ‘fairly entitled to say’ that the new moon of March 24, 58 B.C. (4.40 p.m.), was visible on March 25. Dr. Fotheringham may be justified in saying that it ought ‘in normal [or abnormally fine ?] weather to have been visible that evening’; but, as I observed in my note, Groebe affirmed that in calculating the time of visibility of the crescent we should accept the mean of the Babylonian estimates, 36 hours—less, I should have added, in the early spring and the winter, more in the summer and autumn. Was he entitled to deduct 9 hours from the mean for an observation made not in the clear atmosphere of the East, but in Switzerland? Dr. Fotheringham tells us that ‘the shortest interval between a new moon and the observation of the moon by Schmidt's naked eye comes’ not, as Groebe said, to 29, but ‘to 25.7 hours.’ But forty-eight of Schmidt's forty-nine observations were made at Athens, where the atmosphere is clearer than in Switzerland. The great difference in visibility which a clear atmosphere makes must strike everyone who goes from this country or from Switzerland to the East or to Northern Africa. I realized it for the first time when I was exploring in Tunisia before the war. Has Dr. Fotheringham or any other trustworthy observer ever seen with the naked eye in an atmosphere no clearer than that of Geneva a moon not more than 27 hours old? If so, is it not remarkable that, as Dr. Fotheringham has said, ‘Hitherto it has been the practice to assume that [in the early spring or the winter and in favourable weather ?] the moon becomes visible on the first evening when she is more than 30 hours old at sunset’?