THE rarity and striking character of total eclipses of the sun naturally predisposes astronomers to their study. Eclipses have revealed a remarkable feature in the movement of the moon, and successive observations are year by year increasing our knowledge of the sun's surface and surroundings. In the eighteenth century Halley, from the study of records of early eclipses, inferred a secular acceleration in the moon's motion. Twenty years ago Cowell confirmed this, and discovered a small secular acceleration of the sun's motion. His results have been further verified and amplified by Fotheringham. It has been shown by Taylor and Jeffreys that these results can be quantitatively explained by a slowing of the earth's rotation caused by the friction of the tides in certain shallow and constricted seas.