DISCOVERY 



297 



earth and eclipsing the sun. The dark conical region 

 is that within which the sun is entirely hidden from 

 sight. This portion of the shadow is called the umbra. 

 Around the umbra is an enveloping cone, shown shaded 

 in Fig. I, with its vertices directed towards the sun, 

 the penumbra. To an observer within this region the 

 sun is partly hidden from view. As the apparent 

 path of the moon may pass from the north or south of 

 the line joining the earth and sun, the axis of its shadow 

 may pass from the north or south of the earth and 

 may not meet it at all. An eclipse of the sun is called 

 central when the shadow axis strikes any part of the 

 earth ; partial when only the penumbra falls upon 

 the earth. It is evident that an eclipse can be seen as 

 central only at those points of the earth's surface over 

 which tlie axis of the shadow passes. An eclipse is 

 total when the imibra actually reaches the earth ; 

 annular when it does not. These two cases are shown 

 m Figs. 2 and 3. In Fig. 2 the shaded portion repre- 

 sents the shadow of the moon striking the earth ; the 

 sun is enthely hidden within the region pp. In Fig. 3 

 within the region qq it is seen that a part of the sun 

 can be seen even when the moon is between it and 

 the earth. In this case at the moment of greatest 

 eclipse a narrow ring of sunlight is seen surrounding 

 the dark body of the moon. 



Eclipses fall naturally into groups or series, each of 

 which Professor Turner calls a " family." The move- 

 ments of the earth and moon with respect to each 

 other and to the sun are such that after about eighteen 

 5'ears and ten and one-third days the three bodies 

 come back almost to the same relative positions, so 

 that after this interval an eclipse merely repeats itself, 

 and continues to do so every succeeding eighteen 

 years. Any distinguishing feature of one eclipse, as, 

 for instance, an unusually long duration, recurs in all 

 the rest, and so is a characteristic of the whole family. 

 This period was kno\\ii to the ancients and called bv 

 :hem the Saros. 



The eighteen years and ten days that constitute the 

 Saros imply that the period contains five leap years. 

 If it contains only four, then the e.xtra odd davs will, 

 of course, be eleven, and if the period includes only 

 three leap years — as, for example, from 1897 to 1915 

 (igoo was not a leap year) — then the odd days will be 

 twelve. This point is not always stated clearly, with 

 the result that there is not a little popular confusion 

 about the additional days. 



The different families are distinguished on maps by 

 the tracks they trace out on the earth. In order to 

 be definite in speaking of the tracks, we will confine 

 our attention to the eclipses in which totality occurs. 

 The track of one eclipse does not fall on the same 

 part of the earth as did the previous one, for the odd 

 fraction, viz. one-third, of a day in the Saros causes 



it to shift one-third of the way round the globe. It 

 follows, therefore, that the fourth of such tracks will 

 have shifted three- thirds, or the whole, way round. 

 Similarly, the seventh will have gone round twice, so 

 that the first, fourth, seventh, etc., will all lie in the 

 same part of the earth, the interval between them 

 being a little more than fifty-four years. But although 

 these lie in the same part of the earth, yet it is found 

 that the fourth does not cover precisely the same line 

 as the first ; it has moved a little to the north or south, 

 as the case ma}' be. The seventh has moved a little 

 farther still, and so they go on until one of the Poles 

 is reached. When this happens, the family leaves 

 the earth entirely and so becomes extinct. 



Fig. 3. 



The life-history of a famih^ extends over something 

 like 1,260 years. The first member comes into e.xist- 

 ence when its penumbra first grazes the earth not far 

 from one of the Poles. Then there follow a dozen 

 partial eclipses, each larger than its predecessor. By 

 this time the central line has come in at the Pole and 

 we have a series of forty or fifty total or annular 

 eclipses. About eighteen of them will be total and 

 twenty-seven annular, the ratio of total to annular 

 being approximately two to three. In the middle of 

 the series the central line strikes the earth near the 

 equator ; it continues to progress in the direction in 

 which it has already been traveUing, and leaves the 

 earth near the Pole opposite to that near which it 

 entered. Then come another dozen partial eclipses, 

 and the family becomes extinct. (It must be stated 

 that the numbers given in this paragraph are only 

 approximate.) 



Chambers traces one such family. It began at the 



