1. A Solar eclipse occurs whenever the sun, moon and earth have their centres so nearly in line that the moon hides some part, or all, of the sun from an observer on the earth’s surface. In the former case the eclipse is partial, in the latter total. The relative motions of the three bodies are extremely complicated; but as knowledge of the apparent positions of the sun and moon is of importance for safe navigation, these are regularly computed in advance and published by the Admiralty in the Nautical Almanac. Since the Nautical Almanac tables are the data for eclipse calculations, it is essential to understand the method there employed for recording the positions of the heavenly bodies, and the meanings of a few technical terms.