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the most important astronomical event of the year 1920 was the publication of Dr. Brown's Tables of the Motion of the Moon. It is a stupendous work. It consists of three solid volumes, containing 673 large quarto pages, including 14 pages of prefatory matter, and, bound in paper covers, weighs 9 pounds. Dr. Brown spent altogether 29 years in improving the theory, computing the gravitational terms that enter into it, analysing the observations, preparing the tables, and seeing them through the press. Three Universities share the honour of this production, Cambridge which printed it, and Yale and Oxford which published it. The expense was met by appropriations from the income of the funds of the Winchester Observatory, U.S.A. A public lecture is not the occasion, and I am certainly not the person to appreciate the mathematical powers which the author has displayed, but I think it will generally be conceded that all that gravitational astronomy can perform has been brought to its perfection by Dr. Brown, and that the outstanding differences between Dr. Brown's places of the Moon and those obtained by observation are due to causes of which our gravitational theory does not take account. For the benefit of those who are not familiar with astronomical tables, I may explain that these tables do not give the position of the Moon, but merely facilitate the computation. With Hansen's tables, which have hitherto been used, it was estimated that a skilled computer could find one complete place of the Moon in a whole day's work. I hope