SUNLIGHT AND ITS MEASUREMENT 153 



action probably plays a considerable part in producing short- 

 wave radiation without necessitating temperature increases of 

 the magnitudes required by the radiation laws mentioned 

 above, it seems clear that the actual temperature of the sun 

 may. be far above or far below the figure given. However, 

 it also seems clear that changes — even local — in the sun's tem- 

 perature or translocations of chemically active material, both 

 of which occur, may alter the sun's spectrum both in total 

 energy and in distribution of energy. The terrestrial signifi- 

 cance of these solar changes cannot be stated as yet but it 

 appears 4 that there are two general processes operating; long 

 term variations (measured in months or years) in the effective 

 temperature of the sun, altering the total energy and its spectral 

 distribution; short term changes (perhaps measurable in days) 

 in transparency of the outer envelopes of the sun which probably 

 affect chiefly the shorter waves. 



Since the intensity of radiation received at the earth from 

 any given luminous point on the sun varies inversely as the 

 square of the distance intervening it follows that the energy 

 incident upon a given area of the earth's surface will vary with 

 the time of year (i.e., with changes in the earth's distance from 

 the sun) from this cause alone to produce a maximum difference 

 of about five per cent. Accordingly, astro-physicists alter their 

 radiation figures to those that would be correct if the earth's 

 orbit was a circle with radius equal to the average distance 

 from the sun to the earth's present orbit. Since the total 

 energy content of the sun's spectrum must vary with changes 

 in the sun, the "solar constant" (the number of 15° calories 

 received as radiation from the sun upon an area of 1 sq. cm. 

 in one minute at equatorial sea level with the sun 90° above all 

 points on the horizon and the earth at its mean orbital dis- 

 tance from the sun if all the earth's atmosphere were removed) 

 should vary likewise, as is, indeed, the case. 4 The mean solar 



4 Abbot, C. G., F. E. Fowle and L. B. Aldrich, On the distribution of radiation 

 over the sun's disk and new evidences of solar variability. Smithsonian Misc. 

 Coll. 66: No. 5. 1916. 



