300 



DISCOVERY 



whilst that of 1330 was the shortest, lasting 4 sees, 

 short of a minute. 



An interesting point in connection with eclipses, 

 both lunar and solar, is the times at which they occur 

 in anv particular year. These are grouped into two 

 chief divisions, separated by about six months, and 

 known as the " Eclipse Seasons." Eclipses can occur 

 onlj' when the sun is near a node,^ and this happens 

 only twice in the year. Hence we have the general 

 rule that in any year there are two dominant eclipses 

 separated by about six months, whilst others may occur 

 a fortnight before or a fortnight after them. 



This might seem to imply that the eclipse seasons 



Fig. 6.— TOTAI, eclipse OK 11111. SHOWING CORON'.\. 

 Reproduced, by permission, from the ''Monthly Notices " of Ih: Royal Astronomical 



Society. 



are in the same months year by year, an implication 

 contrary to experience. The nodes are continually 

 changing their positions, with the effect that the 

 eclipse seasons come about eighteen or twenty days 

 earlier in succeeding years. This can be seen if we 

 tabulate the eclipses for a year or two. 



Year. Date. 



1 



Mean. 

 April 15 



Oct. 8 



I Mar. ID 



J 



Sept. 



The maximum number of eclipses that can occur 

 near any one mean date is three ; there cannot 



' The " nodes " are the points in which the orbit of the 

 moon cuts the plane of the ecliptic, or the orbit of the earth. 



normally be more than six in a year, a number not 

 often reached. Occasionally, however, there may be 

 seven, but only if the eclipse seasons come either in 

 January and July, when there may be overlapping 

 from the succeeding year, or in June and December, 

 when there may just come in an extra one from the 

 previous year. 



This greatest possible number, seven, was reached 

 in 1917, and will be reached again in 1935. It was 

 reached once in the nineteenth century. According 

 to civil time, 1805 had seven, but as the first occurred 

 at I a.m. on January i, it would count as in 1804 

 according to astronomical reckoning, in which the 

 day is regarded as beginning at noon. But as against 

 this, 1823 had six according to the civil reckoning, 

 and seven according to the astronomical, the last 

 occurring at 8 o'clock on the morning of New Year's 

 Day 1824, a time astronomers would call 20 o'clock 

 on December 31, 1823. We may, therefore, count 

 either 1805 or 1823 as having had seven eclipses, 

 according to the system we adopt, but we cannot 

 fairly count both. 



It may be mentioned in passing that this dual 

 system of reckoning is to terminate at the end of 

 1924. The astronomical New Year's Day of 1925 

 will commence at midnight, and thus conform to civil 

 time. 



There seems to be no period of years which will 

 bring round the recurrence of seven eclipses with 

 certainty. There is a tendency to 112, as, for example, 

 from 1805 to 1917. Adding another 112 yeais we 

 have 2029, which misses the seven only by a few 

 hours. Then, of course, the Saros might sometimes 

 apply, as from 1917 to 1935, but we cannot state this 

 with accuracy as being generally true. 



The eclipse season also determines the direction of 

 the path of the total eclipse upon the earth. In 

 March it runs from S.W. to N.E., and in September 

 from N.W. to S.E. In June the track is a curve, 

 running first to the N.E. and then bending round tO' 

 S.E., and in December it runs S.E. first and bends 

 round to the N.E. In intermediate months we have 

 a compromise between these extremes. 



BIBLIOGRAPHY 



In 1887 Oppolzer published at Vienna his monumental work 

 entitled Canon der Finsternisse (Canon of Eclipses) , which con- 

 tains the approximate elements of all eclipses (8,000 solar and 

 5,200 lunar) between 1207 B.C. and a.d. 2162, with charts 

 for all annular and total eclipses of the sun. 



In vol. xlv of the Monthly Notices of the Royal Aslronom-cal 

 Society, Maguire has an article on the total eclipses in the British 

 Isles, together with a map. 



Most te.xtbooks on astronomy contain much information 

 on this subject, the article on " Eclipses " in the Encyclopesdia 

 Brilannica being particularly good. 



An excellent popular account is given in Professor H. H. 

 Turner's A Voyage in Space (S.P.C.K. 6s.). 



