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sky, the ascending node at one point, and the descending node at the opposite point, then the sun will appear to us to pass each of these points in the course of a year. While the sun is passing one node the shadow of the earth will seem to be passing the other. It is only near these two times of the year that an eclipse of the sun or moon can occur. We may therefore call them eclipse seasons. They commonly last about a month; that is to say it is generally about a month from the time when the sun gets near enough to a node to allow of an eclipse until the time when it is too far past for an eclipse to occur. In 1901 the seasons were May and November.

If the moon's node stayed in the same place in the sky, eclipses would occur only some time during these two months. But, owing to the attraction of the sun on the earth and moon, the position of the nodes is continually changing in a direction opposite that of the motion of the two bodies. Each node makes a complete revolution around the celestial sphere in eighteen years and seven months. Hence in this same period the eclipse seasons will course all through the year. On an average they occur about nineteen days earlier every year than they did the year before. Thus it happens that in 1903 one season occurs in March and April and the other season in September and October. The change will keep going on until, in the year 1910, the season which in 1901 was in May will have gotten back to November, while the November one will have gotten back to May, each having passed through all the intermediate months, and the two