TH E 



AMERICAN JOURNAL OF SCIENCE 



[FOURTH SERIES.] 



Art. XII. — The ^7 -Day Auroral Period and the Moon; 

 by H. Helm Clayton. 



Several years ago my attention was called to a tendency 

 of auroral displays to recur at intervals of 27 days. This is 

 illustrated by the following cases taken from the records of the 

 Blue Hill Meteorological Observatory : 



Table I. 



27 

 27 

 27 



1892 1893 1895 

 Dec. 23 98 Dec. 29 9q Jan. 19 



1893 1894 l Feb. 14, li 

 Jan. 20 9fi Jan. 27 Jan. 2 Mar. 14 

 Feb. 15 Vt Feb. 4 Feb. 22, 23 Zb " 3 ! Apr. 10 

 Apr. 11 °° "5 "4 

 May 7 ~ b 



This table gives all the auroras observed between the first 

 and last dates in each of the series. In these instances the 

 recurrence of auroras at intervals of approximately 27 days 

 seems too well marked to arise merely from chance coincidence. 

 For example, in the last series there are four auroral displays at 

 exact intervals of 27 days and no intermediate displays. The 

 probability of a casual arrangement of the auroral displays in 

 this manner is inversely as the cube of 27, or as 1 to 19,683. 

 That is, out of 19,683 auroral displays one chance arrangement 

 of this kind might be picked out. But during three succes- 

 sive winters, as shown by the above table, there were auroral 

 displays in series with an interval of 27 days plus or minus 

 one day in each case, and with an average interval very slightly 

 exceeding 27 days. The chance of this being merely an acci- 



A.M. Jour. Scl— Fourth Series, Vol. V, No. 26. — Februart, 1898. 

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