The earth goes round the sun once in a year, the moon goes round the earth once in a month, and sometimes the three bodies are in one straight line. In this case the intermediate body--earth or moon--deprives the other, wholly or partially of the light from the sun, thus causing an eclipse. If the orbits of the earth and moon were in the same plane, an eclipse would happen every time the moon was new or full; that is to say, at every conjunction and every opposition, or about twenty-five times a year. But the plane of the moon's orbit is inclined to the plane of the earth's orbit at an angle of about 5°, and so an eclipse only occurs when the moon is in conjunction or opposition and is at the same time at or very near one of the nodes--that is, one of the two points where the plane of the earth's orbit intersects the moon's orbit. If the moon is in opposition, or "full," then, under these conditions, an eclipse of the moon takes place, and this is visible at all places where the moon is above the horizon at the time. If, however, the moon is in conjunction, or "new," it is the sun that is eclipsed, and as the shadow cast by the moon is but small, only a portion of the earth's surface will experience the solar eclipse. The nodes of the moon's orbit are not stationary, but have a daily retrograde motion of 3´ 10·64´´. It takes the moon therefore 27{d} 5{h} 5{m} 36{s} (27·21222{d}) to perform a journey in its orbit from one node back to that node again; this is called a Draconic period. But it takes the moon 29{d} 12{h} 44{m} 2·87{s} (29·53059{d}) to pass from new to new, or from full to full, _i. e._ to complete a lunation. Now 242 Draconic periods very nearly equal 223 lunations, being about 18 years 10-1/3 days, and both are very nearly equal to 19 returns of the sun to the moon's node; so that if the moon is new or full when at a node, in 18 years and 10 or 11 days it will be at that node again, and again new or full, and the sun will be also present in very nearly its former position. If, therefore, an eclipse occurred on the former occasion, it will probably occur on the latter. This recurrence of eclipses after intervals of 18·03 years is called the Saros, and was known to the Chaldeans. We do not know whether it was known to the Hebrews prior to their captivity in Babylon, but possibly the statement of the wise king, already quoted from the Apocryphal "Wisdom of Solomon," may refer to some such knowledge. Our calendar to-day is a purely solar one; our months are twelve in number, but of purely arbitrary length, divorced from all connection with the moon; and to us, the Saros cycle does not readily leap to the eye, for eclipses of sun or moon seem to fall haphazard on any day of the month or year.