130 



KNOWLEDGE. 



[July 1, 1892. 



20°. Indeed, of 79 groups only four were as far from it as 

 lat. 18". Spots in their " teens " were becoming decidedly 

 scarcer, whilst spots on or close to the equator were 

 relatively more common. 



It will be noticed that this change in position accom- 

 panied a marked decline in the numbers and areas of the 

 groups. In 1886 there were 128 groups observed, and the 

 average daily extent of the spotted area of the sun was 

 450 millions of square miles. In 1887 there were only 

 79 groups, and the average daily extent of the spotted area 

 had shrunk to 210 millions of square miles. In 1888 the 

 number of groups had gone down to 51, and the mean 

 daily spotted area to just half what it had been the previous 

 year ; and this shrinkage had been caused for the most part 

 by the disappearance of high latitude spots, so that the 

 mean latitude of the spotted area was now only 7^°. 

 North of the equator, the only group that exceeded 10° in 

 latitude lay in latitude 11° ; in the southern hemisphere, 

 which was the more active, there was one group at 12°, 

 two at 13°, and one at 15°, but all the others were under 

 10° ; so closely were such groups as still appeared huddled 

 together towards the equator. 



But on the last day but one of the year, one little faint 

 insignificant spot broke out in quite a different region from 

 these. True, it was gone before the morrow. Its mandate 

 might have been like that of Macbeth's visions, to "come 

 like shadows, so depart." But none the less, small, faint, 

 and fleeting as it was, it was the harbinger of a mighty 

 change in the general aspect of the sun. 



The next year, 1889, showed a yet further decline in the 

 solar activity. Only 32 groups were observed, and the 

 mean daily spotted area was now only 90 millions of 

 square miles ; and during the first half of the year, so 

 marked was the decay of the high latitude spots, that not 

 one instance was observed of the mean position of a group 

 exceeding 10°. 



But on June 29th the harbinger spot of the previous 

 December was followed by a second, quite as small and 

 faint, and nearly as evanescent, for it lasted only two days, 

 but in a higher latitude still, 40'3°. Other groups 

 followed it. A group was seen m S. lat. 24° in July ; 

 a fine group in S. lat. 20° ran its course during three 

 several appearances, in August, September, and October ; 

 and in all, counting these three apparitions of this group 

 as three chfl'erent groups, twelve groups were seen with 

 latitudes exceeding 20° during the last half of the year, 

 whilst nine appeared in the equatorial region. The spots 

 during the last six months of lH89 were therefore divided 

 into three distinct zones — a northern zone, comprising 

 spots with latitudes of 20° and upwards ; an equatorial 

 zone, embraced between latitudes 10° N. and S. ; and a 

 southern zone, with spots of latitude 20° and upwards. 

 But the zone 10° to 20° on either side of the equator was 

 wholly barren. 



In 1890 the sun began to show clear signs of reviving 

 activity. With this revival of activity came a further decline 

 in the number and area of spots in the equatorial zone, and 

 a very rapid increase in those of high latitude. Only four 

 groups out of the fifty-five were found in the former region, 

 and these were but small. With October, 1890, the old 

 series practically came to an end, and since then the high 

 latitude spots have held the entire field. Several groups 

 have been seen between 30° and 35°, and two out of every 

 three groups were situated in the 20° to 30° zones. 



But as time goes on, these tend ever to seek lower and 

 lower latitudes. In 1891 there were two spot zones, one 

 north and the other south of the equator, and the 

 equatorial region was entirely barren ; but as the cycle 

 progresses and as spots in positions as high as 28° or 30° 



become rarer, so spots close to the equator become more 

 common, till at length the state of affairs will again be as 

 it was in 1886 and 1887, and there will be practically but 

 one spot zone, and that close to the equator. 



We have, therefore, three phases of the spot-cycle. 

 I. Before minimum ; spots few and diminishing ; one spot 

 zone, viz., that close to the equator. II. After minimum ; 

 spots few but increasing ; three spot zones, viz., an equa- 

 torial zone, and a zone of high latitude in either hemi- 

 sphere, separated from the equatorial zone by a broad 

 barren belt. III. At or near maximum ; spots numerous ; 

 two spot zones, one north and one south of the equator, 

 the equatorial region itself being entirely barren. It 

 follows, then, that the two barren belts of the second period 

 are on the average the most prolific for the cycle as a 

 whole, for they become the seats of spot activity just at the 

 time when sunspots are largest and most numerous. 



There is, therefore, a steady and continuous tendency 

 ever to seek lower and lower latitudes from the beginning 

 of a cycle till its end. The first small and faint spots of 

 the new cycle break out in the very highest latitudes 

 attained. In this sense the new period begins at once 

 with its utmost vigour, and during the years in which it 

 runs its course is simply undergoing a continual decline. 

 As to the numbers and areas of the spots, the case is very 

 different. For some three years there is a most marvellous 

 and rapid increase, followed by two or three years of great 

 activity. Then comes a decline, which is, however, slower 

 and less striking than the increase. Finally, for about a 

 year, we have on the sun the last feeble relics of the old 

 cycle, together with the first heralds of the new one. Or 

 we may put it another way. The sunspot cycle is about 

 eleven years on the average, but each particular spot-cycle 

 takes about twelve years to entirely run its course, so that 

 for a year the two cycles run concurrently. 



The following little table will illustrate this for the last 

 spot-cycle, that of 1879-1890 :- 



The mean area is expressed in millions of square miles, 

 and for the years of overlapping, 1879, 1889, and 1890, 

 only the spots proper to this cycle have been included. 



This was no special behaviour of a single cycle. It is 

 the usual typical behaviour. If we take the cycle 1856- 

 1867, and divide it into periods of fifteen rotations on the 

 average, we find the mean spot latitude of the successive 

 periods will run as follows - :—32^ 26-7°, 21-3°, 17-8°, 

 17°, 14-2°, 12-1°, 10-4°, 10-3°, 9-2°, 8-2°, 8°. Of these 

 twelve periods the first two were coincident with the last 

 two periods of the earlier cycle, with mean latitudes 7-8° 

 and 5° ; and the last two, with the earliest periods of a 

 new cycle, latitudes 81 " and 22'. Similarly, this cycle, 

 1867-1878, ran from 31° and 22° to 22-6°, 18-8°, 16-2°, 

 13-8°, 11-2°, 11-4°, 10-3°, 9-2", 7-0', and 5-4°; and these 

 last two periods again were coincident with the first two 

 of the cycle of which we have already treated, that of 

 1878-1889, as the last two periods of that cycle in its turn 

 were coincident with the first two of the cycle through 

 which we are now passing. 



* The figures are those of Dr. Spoerer (dsfr. Nach., No. 2565), 

 who more thin any otlier ob.^erver his brought these rehitious under 

 tlie notice of astronomers: 



