6o8 



NA TURE 



[November 4, 1922 



Solar Radiation and its Changes. 1 



WHEN one reflects upon the wide knowledge 

 gained by astronomers concerning stellar 

 and nebular radiation and variability, it at first seems 

 surprising that variation in the visible radiation 

 emitted by the sun has been discovered only recently 

 and with much difficulty. Not until the second decade 

 of this century could the fact be regarded as estab- 

 lished, as a consequence of simultaneous determina- 

 tions of the " solar constant " made by the staff of 

 the Astrophysical Observatory of the Smithsonian 

 Institution, at two stations so widely separated as 

 Bassour in Algeria and Mount Wilson in California. 

 The solar constant is, of course, the estimated value of 

 the intensity of total solar radiation, in calories per 

 square centimetre per minute, at a point just outside 

 the earth's atmosphere, i.e. before suffering absorp- 

 tion in transmission to the earth's surface. 



The main difficulty arises from the necessity of 

 making practically absolute determinations of the 

 solar (onstant, because the sun's proximity to us 

 renders it sufficiently unique and solitary in the day- 

 light sky to prevent that comparison with many and 

 similar neighbours which is the foundation of our 

 knowledge of stellar variability. No real progress 

 towards such knowledge regarding the sun could be 

 made until it became possible to determine and allow 

 for the radiation absorbed in the earth's atmosphere. 

 Failing this, even long series of simultaneous observa- 

 tions of the changes in the solar radiation, as received 

 at different points on the earth's surface, are of little 

 use, because any features common to two stations may 

 arise from some common terrestrial cause. A striking 

 example of this was recorded by the Smithsonian 

 observers during the fifty days on which observations 

 were made both at Bassour and at Mount Wilson. On 

 June 6, 1912, a great volcanic eruption occurred at 

 Mount Katmai in Alaska ; on June 19 the sky became 

 slightly turbid in Bassour, and a day or two later also 

 at Mount Wilson. The milkiness rapidly increased 

 till in July and August a thick haze overspread the 

 whole sky and cut off more than 20 per cent, of the 

 sun's direct radiation at noonday ; yet after applying 

 properly determined corrections, normal and accordant 

 values of the solar constant were obtained at Bassour 

 and Mount Wilson during the above period. 



The variability discovered in the sun's radiation is of 

 two kinds ; irregular variations occur over periods of 

 a few days or weeks, amounting to a small percentage 

 of the whole intensity, while small variations of longer 

 period are found, showing some correlation with the 

 periodicity of sunspot activity. Considering how 

 marked are the changes in solar-spottedness, and in the 

 accompanying magnetic and auroral phenomena upon 

 the earth, it is remarkable how small are the variations 

 in the main solar radiation. Many attempts have 

 1 >een made to connect the sunspot cycle with meteoro- 

 logical changes likely to depend on the solar radiation 

 reaching the earth, but with very doubtful success — 

 the terrestrial factors which share in determining the 

 weather and crops are too complicated, and it now 

 appears that the long-period changes in the solar 



1 "Annals of the Astrophysical Observatory of the Smithsonian In- 

 stitution." Volume iv. Bv C G. Abbot, F. E. Fowle, and L. B AHnh. 

 Pp. xii + 390. (Washington, 1922.) 



NO. 2766, VOL. I JO] 



radiation are themselves very small. Hence the) were 

 bound to remain undetected till direct methods and 

 appropriate instruments were devised which made 

 possible a frontal attack upon the problem. The 

 method which has proved successful is due to S. P. 

 Langley, and the spectro-bolometer which he invented 

 (in 1880) is one of the chief instruments employed ; but 

 many improvements and additions both of method 

 and in the instrumental equipment have been since 

 made, and largely by the staff of the Smithsonian 

 observatory under its director Dr. C. G. Abbot. 



Langley's method is, briefly, as follows : Absolute 

 measurements of the total solar radiation reaching the 

 earth's surface are made with an instrument (the 

 pyrheliometer) which indicates the heat energy ab- 

 sorbed by a blackened silver disc exposed to the 

 radiation. It is claimed that the error of a single 

 reading with this instrument is less than i per cent., 

 and inter-comparison of pyrheliometers over periods 

 of several years shows that the scale is free from 

 secular changes exceeding i per cent. Such absolute 

 observations are made at frequent intervals during a 

 forenoon or afternoon, with the sun at different alti- 

 tudes from 15° upwards ; the measurements vary on 

 account of the varying absorption as the radiation 

 passes through a greater or lesser length of atmosphere. 



The correction for the absorption is obtained with 

 the aid of the spectro-bolometer, which consists 

 essentially of a wire on which radiation of a particular 

 wave-length is directed, after passing through a suitable 

 prism. The resulting rise of temperature in the wire 

 is measured by the change in its electrical resistance, 

 and by passing the whole available spectrum over the 

 wire a " holograph " showing the energy-intensity 

 curve over the solar spectrum is obtained. Such holo- 

 graphs, corresponding to different successive altitudes 

 of the sun, show the absorption in all parts of the 

 spectrum during the passage of radiation through 

 lengths of the atmosphere proportional to the secants 

 of the sun's zenith distance ; their comparison makes 

 it possible to correct each holograph for the absorption. 

 In certain regions of the energy-curve where powerful 

 selective absorption occurs by water and other atmo- 

 spheric vapours, it is assumed that the absorption bands 

 are absent outside the atmosphere, the curve being 

 completed by interpolation between adjacent parts 

 of the curve on either side. 



While highly sensitive, the holograph gives relative 

 rather than absolute measures of solar radiation, and 

 the scale of the uncorrected holograph is obtained by 

 comparing its area with the value of the total radiation 

 as measured by the pyrheliometer. The holograph 

 corrected for absorption then gives the value of the 

 solar constant. It is estimated that the probable error 

 of an ordinary daily determination of the latter is from 

 0-2 to 0-3 per cent, as regards the relative values from 

 day to day, i.e. omitting the probable error of the 

 pyrheliometer scale value. The whole daily error 

 should therefore be well below i per cent, under good 

 conditions, though at times irregular or systematic 

 errors of larger magnitude may occur. 



The absolute value of the solar constant, determined 

 from 1244 observations, mainly at Mount Wilson 



