4CO 



NATURE 



[May 26, 192 1 



the sun's equator. As the cycle progresses they 

 become most numerous in successively lower lati- 

 tudes, while at the present phase of the cycle they 



Fig. I. — Photograph of the sun's disc on M«y 13d. qh. G.M.T. Original scale 7 J in 

 to sun's diameter. By kind permission of the Astronomer Royal. 



are found most frequently in latitude io°. Very 

 large groups on the equator are rare, however, 

 and the present one is 

 the largest which has 

 appeared in this position 

 during the last half- 

 century. Looking at 

 Fig. I, it is necessary to 

 bear in mind that at the 

 middle of May the north 

 end of the sun's axis of 

 rotation is about 21° 

 west of the north point, 

 and that the sun's equa- 

 tor passes 2^° north of 

 the centre of the disc. 

 Spots are carried by the 

 sun's rotation from the 

 east to the west limb 

 in about thirteen days. 

 The centre of this 

 group of spots was 

 nearest the centre of 

 the disc on May i4d. 

 i6h., when it was 

 within 3°. It was then 

 most nearly in line with 

 the earth, but the mag- 

 netic storm commenced 

 twenty-seven hours earlier, 

 visible to the naked eve 

 NO. 2691, VOL. 107] 



thirteen days it could have been seen with the tele- 

 scope. Its area averaged i / 1 500th part of the sun's 

 surface, or about eight times the area of the earth, 



but it was only half as large as the group 



of spots of March, 1920. 



(b) Sun-spots are nearly always to be 

 found in continual change, especiallv for 

 a few days following their formation, and 

 it is evident from earlier photographs 

 that this group developed after April 25, 

 and wasj therefore, of recent origin. It 

 was first seen at the east limb of the sun 

 on May 8 as a long, irregular spot, nearly 

 80,000 miles in length, with two principal 

 nuclei; by May 13 these had become the 

 centres of two spots 7° apart in longitude, 

 while the mid-portion of the original spot 

 had changed into a cluster of small ones. 

 It is unusual for a large spot to split up 

 in this manner. Pairs of spots often 

 appear, but they have generally been 

 evolved from two small nuclei. On the 

 photograph of May 9 it will be noticed 

 that the spot is surrounded by bright 

 faculae. The amount is unusually small 

 for so large a spot, but by the time the 

 group had reached the west limb of the 

 sun the area of faculae had extended con- 

 siderably. 



(c) The magnetic storm commenced 

 suddenly at i3h. lom. on May 13. 

 At this time the leading spot was 

 iii° east of the sun's central meri- 

 dian and the following spot 19°. The greatest 

 intensitv of the disturbance was about 5h. on 



Fig. 2. — Original scale ^\ in. to sun's diameter. Note tie foieshortening of the spot near the sun's limb. By 

 kind permission of the Astronomer Royal. 



The spot group was 

 for eight out of the 



May 15, at which time the following spot was 

 3° past ' the central meridian, while a 



