196 



NA TURE 



[June 20, 1901 



A LONG PERIOD SUNSPOT VARIATION.^ 

 TT has long been known, and Dr. Rudolf Wolf of Zurich was 

 the first to draw attention to it, that the length of a sunspot 

 period is only in the mean eleven years, and that the real length 

 of any one period might differ from this value by as much as ± 

 two years. Another fact of observation is that the times of 

 maxima do not occur a constant number of years after a pre- 

 ceding minimum, and Dr. Wolf determined the mean interval 

 as 4'5 years. The minimum also follows the maximum in a 

 mean interval of 6'5 years. 



It has further been noticed that the intensity of each period, 

 i.e. the total amount of spotted area included between one 

 minim um and the next, was not constant. Dr. Wolf held that 



these quantities indicated a certain periodicity, and at first 

 suggested a period of 1 78 years, and later 55 '5 years, or a period 

 extending over five eleven-year periods (11 'i x 5 = 55'5). 



The present investigation was limited to the interval of time, 

 namely, 1833-1900, over which systematic observations of the 

 sun's surface have been regularly made, and as Dr. Wolfs 

 relative numbers agree well with the actual facts of observation 

 over this period, these numbers have been employed. 



The important magnetic results obtained by Mr. William 

 Ellis served as a check on the w'hole work, since he has shown 

 that the curves for the magnetic elements are in almost exact 

 accord with those of the sunspots. Any variations determined 

 from the sunspot curves should, then, have their counterpart in 

 the magnetic curves. 



1 Abstract of a paper, "The Solar Activity 1333-1900," read before the 

 Royal Society on May 23, by Dr. William J. S. Lockyer. 



NO. 1 65 I, VOL. 64] 



Fig. I will give the reader an idea of each sunspot curve from 

 minimum to minimum for the period above mentioned. 



They are so arranged in order of date that each individual 

 curve can be examined separately. The times of succeeding 

 minima are arranged vertically under each other, so that any 

 variation as regards acceleration or retardation of the following 

 maxima, and any inequality in the length of the period minimum 

 to minimum can be seen at a glance ; each of these epochs is 

 indicated in the figure by a short arrow with the corresponding 

 dates. 



DeaUng with the inequalities of the interval minimum to 

 inaximum, it is found that there is a regular variation having a 

 period of about thirty-five years. Curve B, Fig. 2, shows this 

 variation, the abscissa; representing the time element and the 

 ordinates the intervals minimum to maxi- 

 mum plotted at the epochs of the minima. 

 Dealing with the intervals minimum to 

 maximum of the magnetic curves in a 

 similar way, the result obtained is shown 

 in the same figure, curve c. 



Both these curves thus indicate that • 

 there is some law at work which introduces 

 a secular variation by retarding periodic- 

 ally the sunspot maxima in relation to the 

 preceding minima. The actual epoch of 

 maximum relative to the preceding mini- 

 mum oscillates about the mean value 4*12 

 years, its greatest amplitude being in the 

 mean about O'S year. 



Another point of great importance is 

 that when the epoch of maximum spotted 

 area follows in the shortest interval of time 

 after a minimum, the spotted area for the 

 whole period is greater than at any other 

 time. 



Thus, if the spotted area included be- 

 tween consecutive minima be summed up 

 for each period, and these values, used as 

 ordinates, be plotted at the epochs of 

 minima, as done previously, and the curve 

 inverted, curve D Fig. 2 is the result. It 

 will be noticed that this curve is very 

 similar to the two immediately above it, 

 and .shows a period of about the same 

 length, namely, about thirty-five years. It 

 may be here remarked that the value for 

 the total spotted area for the period i833'9 

 to i843'5, the earliest value in point of 

 time dealt with, is not quite in harmony 

 with the other values. There seems, 

 however, sufficient evidence to indicate 

 that the small value may be due to the fact 

 that the observations were not then made 

 quite on a uniform plan. That the maxi- 

 mum of 1836 was a great one, and only 

 equalled by that of 1S70, is well known. 



The discussion of these observations thus 

 leads to the important conclusion that 

 underlying the ordinary sunspot period oj 

 about eleven years there is another cycle of 

 greater length, namely, about thirty-five 

 years. 



This cycle not only alters the time Oj 

 occurrence of the maxima in relation to the 

 preceding minima, but causes changes in the total spotted area 

 of the sun from one eleven-year period to another. 



A glance at Fig. i will show that the length of the period 

 minimum to minimum seems to alternate, the magnitude of 

 these alternations becoming smaller. An attempt was made_ to 

 see if any law could be traced, but although there was a variation 

 suspected in the length of both the magnetic and sunspot periods 

 (reckoning from minimum to minimum), which increases and 

 decreases in alternate eleven-year periods from a mean value, 

 the observations do not extend over a sufficient interval of time 

 to allow a more definite conclusion to be drawn. 



It is generally conceded that the spots on the surface of the 

 sun are the result of greater activity in the circulation in the 

 solar atmosphere, and therefore indicate greater heat and, there- 

 fore, light. This being so, the curve representing the spotted 

 area may be regarded as a light curve of thesun. i 



