The motions of the sun and moon are so arranged that at the end of a given interval of time they return into almost precisely the same position, with regard to each other and to the earth, as they held at the beginning of that interval. “Almost precisely,” but not quite precisely. There will be a slight outstanding difference, which will gradually increase in successive intervals, and finally destroy the possibility of the combination recurring, or else lead to combinations of a different character. Thus the daily difference between the movement of the sun and of the stars leads the sun back very nearly to conjunction with the same star as it was twelve months earlier, and gives us the cycle of the year. The slight difference in the sun’s position relative to the stars at the end of the year, finally leads the sun back to the same star at the same time of the year, _viz._, at the spring equinox, and gives us the great precessional cycle of 25,800 years. So, too, with eclipses. Since the circumstances of any given eclipse are reproduced almost exactly 18 years and 11 days later, this period is called an _Eclipse Cycle_, to which the ancient astronomers gave the name of _Saros_;(77) and eclipses separated from each other by an exact cycle, and, therefore, corresponding closely in their conditions, are spoken of as being one and the same eclipse. Each _Saros_ contains, on the average, about 70 ± eclipses. Of these, on the average, 42 ± are solar and 28 ± are lunar. Since the _Saros_ is 11 days (or, more correctly, 10.96 days) longer than 18 years, the successive recurrences of each eclipse fall 11 days later in the year each time, and in 33 _Sari_ will have travelled on through the year and come round very nearly to the original date. But as the _Saros_ does not reproduce the conditions of an eclipse with absolute exactness, and as the difference increases with every successive return, a time comes when the return of the _Saros_ fails to bring about an eclipse at all. If the eclipse be a solar one before this takes place, a new eclipse begins to form a month later in the year than the old one, and becomes the first eclipse of a new series.