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The solar constant
Letter to the Editor T H E SOLAR C O N S T A N T article of Dr. T h e k a e k a r a published in Solar Energy, THE J a n . - M a r c h , 1965 I X , No. 1, pp. 7-20. interests me greatly, T h e scientific view appears to be t h a t more exact determin a t i o n of the i n t e n s i t y of t h e e a r t h ' s supply of solar r a d i a t i o n is necessary, T h e a u t h o r makes generous reference to the S m i t h s o n i a n work, b u t points out the uncertainties relating to the u l t r a violet and infrared regions of spectrum, not observed b y us. He also refers to the work of Stair a n d J o h n s t o n of 1955, saying: " T h e y a t t e m p t e d to eliminate some of the m a j o r sources of error in the S m i t h s o n i a n d a t a . " I would like to c o m m e n t on these two points, When, at Dr. Hale's i n v i t a t i o n , I was sent to M o u n t Wilson to observe the solar c o n s t a n t of radiation, in 1905, the Secretar),, Dr. S. P. Langley, said to me: " R e m e m b e r , Mr. Abbot, t h a t you are going not so m u c h to fix the exact value of the solar c o n s t a n t as to fix the range of its v a r i a t i o n . " T h e n with a twinkle in his eye he added: " I n fact I m a y say p r i v a t e l y to you t h a t I incline to prefer t h a t value of the solar c o n s t a n t which nearest approaches three calories." I am of the opinion t h a t we know the solar c o n s t a n t closely enough to realize t h a t it varies t h r o u g h a range of two percent in amplitude continuously. I have been satisfied with this present degree of c e r t a i n t y for the purpose of investigating the efl'ect of such v a r i a t i o n s on weather. As for a more exact value, 1 do not t h i n k it can be found from m o u n t a i n - t o p observations, with the atnmsphere lying between the observer and the sun. Even to get measures from m o u n t a i n tops as good as those of the Smithsonian observers is not practicable, because of the present and future travel of jet planes, and the prevailing rumors of wars to come. But,, if it is desirable to be more exact, it is necessary only to spend a few billion (or million) dollars and keep properly equipped satellites revolving a b o u t the e a r t h in a plane at right, angles to the line earth-sun. A b e t t e r value of the solar c o n s t a n t could be obtained in 24 hours from such a satellite t h a n m o u n t a i n tops could yield in a m o n t h . B u t to obtain the mean value of the solar c o n s t a n t m u s t require n m n t h s of satellite observ'~tion to eliminate harmonic a n d irregularly occurring v a r i a t i o n s of the i n t e n s i t y of the sun's radiation. These a m o u n t ordinarily to two percent in a m o n t h , and possibly at h m g intervals several percent, as appeared in 1922-1923. As for m o u n t a i n - t o p observations, a fine ground-work was laid before 1900. I t needed only i m p r o v e m e n t s in details to o b t ' d n values of the solar c o n s t a n t accurate enough to discover the sun's variations. The ground-work included: 1 Pouillet's pyrheliometer, AngstrSm's electrical compensation pyrheliometer, Langley's bolometer, and the German investigation of the hollow c h a m b e r as a complete absorber, published in the 1890 decade, 2 Pouillet's employment of the daily m a r c h of the sun's air mass to measure atmospheric losses, from which Pouillet, a b o n t 1876, published his value of the solar c o n s t a n t as 1.76 calories, 3 Langley's m a t h e m a t i c a l proof (amplified in Vol. 2 of Annals of the Astrophysical Observatory) t h a t s p e c t r u m measures are necessary, I t only remained for l a t e r observers to perfect measurem e n t s and details, The bolometer of Langley, even with photographic regist r a t i o n , was far too capricious. He told me t h a t , in his M o u n t W h i t n e y expedition, one observer read the position of the light spot on the galvanometer scale as fast as he could, while 166
a n o t h e r made desired exposures of the bolometer. The lightspot, he said, would often gallop off of a meter-long scale in one minute. His desired measures were ferreted out from a m u l t i t u d e of recorded figures. In 1895, when I reached Washington, the galvanometer spot usually drifted 20 centimeters or more across the photographic record during one run t h r o u g h the infrared s p e c t r u m from A to ~]. Only the great absorption b a n d s of oxygen, water vapor, a n d carbon dioxide were cert a i n l y shown. Dr. Kayser of Bonn was once f o r t u n a t e l y detained with me one afternoon a t our observing place in W a s h i n g t o n for an hour, during a violent thundershower. He told me of P a s e h e n ' s g a l v a n o m e t e r with a v e r y light suspended system. Mr. Fowle and I t h e n worked up the theory and construction of the coils a n d the needle system of the reflecting galvanometer. This we described in the Astrophysical Journal, 18, No. 1, J u l y 1903. I also devised and provided a c o n s t a n t t e m p e r a t u r e , mechanically operated, electrical balancing device for t h e bolometer. I made also a b e t t e r m o u n t i n g for the sensitive strips of the bolometer. Some years l a t e r I s u b s t i t u t e d a variable high-resistance s h u n t for balancing the bolometer and enclosed the sensitive strips in a high vacuum. These steps produced a b o u t 100-fold sensitiveness, greater s p e c t r u m resolution, and a h n o s t perfect freedom from drift and wiggle of the photographic record. I n short, we made the bolometer as easy to use as a tape measure. A n m l t i t u d e of s p e c t r u m lines were then well shown in our bolographic records. See the specimen bolograph exhibited in the G r e a t Hall a t the Smithsonian, and p r i n t e d on page 18 of m y book " A d v e n t u r e s in the World of Science". Pouillet's mercury-box pyrheliometer became the "Silverdisk P y r h e l i o m e t e r " , of which more t h a n 100 copies were made and s t a n d a r d i z e d and d i s t r i b u t e d on orders from all continents and m a n y islands. By long, well-diaphragmed, entrance-tubes, we limited the sky around the sun so greatly t h a t in our mount a i n - t o p observations the sky c o n t r i b u t i o n was less t h a n 1/~ percent of the sun's. We also modified the ~,ngstrSm compensation pyrheliometer so as to limit its sky exposure similarly. As t h a t i n s t r u m e n t is much easier to observe with t h a n the silver-disk pyrheliometer, we used it thereafter for our daily program of m o u n t a i n - t o p observation of the solar c n n s t a n t , a n d used the silver-disk i n s t r u m e n t only occasionally for comparisons to assure t h a t the :~ngstr6m retained its s c a l e Until a b o u t 1910 there was no s t a n d a r d i n s t r u m e n t to cornpletely measure the heat of the solar beam in c.dories. Eraploying the principle of the hollow chamber as an " a b s o l u t e l y black b o d y " we designed and constructed two instruments, the water-flow and the water-stir pyrheliometers. These are described in Volumes 2 and 3 of Annals of the Smithsonian Astrophysical O b s e r v a t o r y (See Fig. 4 and 8, pages 53 and 64, Volume 3). In each of t h e m the sun's rays were introduced t h r o u g h a tube provided with a series of diaphragms to limit sky exposure. A measured d i a p h r a g m a d m i t t e d the solar rays into a blackened chamber shaped like a test tube and to a conical receiver at the back. Two coils of wire, one on the back of the cone, the other in front of its leading edge, were provided for introducing electrical heating. The front coil was more f a v o r a b l y placed t h a n the rear coil to conduct heat to the chamber. Calorimetry was provided in the water-flow i n s t r u m e n t b y passing a s t r e a m of water spirally t h r o u g h the hollow wall of the c h a m b e r and the receiving cone, and measuring its rise of t e m p e r a t u r e , during insolation or electric heating, b y a Wheat.stone's bridge h a v i n g two coils a t the entrance, two at the exit. of the flowing stream. T h e water-stir pyrheliometer, used only for comparisons with the water-flow i n s t r u m e n t and a few silver-disk pyrheliometers, had a similar c h a m b e r and heating coils, b u t was surrounded b y a body of
Solar Energy
Smith. Misc. Coll., 145, No. 3, Pub. 4545, 1963). This result is
vigorously stirred w a t e r whose t e m p e r a t u r e rise was measured b y an electrical resistance t h e r m o m e t e r , I n 1913, Mr. L. B. Aldrich a n d I satisfied ourselves b y 65 trials involving b o t h s t a n d a r d pyrheliometers, a n d b o t h h e a t ing coils in each, t h a t electrical h e a t i n g would be fully t r a n s ferred to the w a t e r calorimeters. (See T a b l e 22, Annals, Vol. 3, page 68.) We preferred the water-flow i n s t r u m e n t as more c o n v e n i e n t for s t a n d a r d i z i n g secondary pyrheliometers. I t h a d one defect. Air bubbles or o t h e r irregularities in the w a t e r c u r r e n t caused such wiggles of the g a l v a n o m e t e r spot t h a t a large n u m b e r of repetitions of observations was required for t r u s t w o r t h y results. This difficulty was cured when V . M . Shulgin of Russia, a b o u t 1927, suggested using two chambers and dividing the flow of w a t e r equally between them, so t h a t the causes of i r r e g u l a r i t y would simultaneously occur, and the g a l v a n o m e t e r would not be affected. We immediately adopted his suggestion, a n d it worked beyond expectation. A t the same time we used the ~kngstrhm electrical compensation principle, h e a t i n g one c h a m b e r with the sun, the other simultaneously electrically, a n d a l t e r n a t i n g them. This change throws us b a c k to our proof made in 1913 t h a t electrical h e a t i n g is t o t a l l y t r a n s f e r r e d to the flowing water. All of our s t a n d a r d i zations of pyrheliometers since 1930 are done b y this method, Following Langley's direction of 1905 our chief concern has always been to discover and measure the variations of solar r a d i a t i o n as t h e y would be found outside the atmosphere, Hence we used every precaution and test to insure t h a t all of our publications of the solar c o n s t a n t were m a i n t a i n e d " o n the scale of 1913". W i t h the adoption of Shulgin's suggestion we found t h a t " t h e scale of 1913" was a b o u t 2 percent too high. B u t a b o u t 1935, studies convinced us t h a t we had made a n error of a b o u t 2 percent b y u n d e r e s t i m a t i n g the i n t e n s i t y of r a d i a t i o n in the u l t r a v i o l e t and infrared, not observed b y the spectrobolometer. One error balances the other so t h a t I still regard the m e a n value 1.946 as the best result of S m i t h s o n i a n solar-constant determinations, In 1923 we fully perfected the " s h o r t m e t h o d " of solarc o n s t a n t m e a s u r e m e n t . The f u n d a m e n t a l m e t h o d of Langley requires a series of sI)ectrum observations extending interm i t t e n t l y over several hours, in order to determine the transmission of the e a r t h ' s atmosphere t h r o u g h o u t the spectrum, We found t h a t a m e a s u r e m e n t of the radiation scattered by the atmosphere in a zone s u r r o u n d i n g the sun could give good measures of atmospheric t r a n s p a r e n c y at all wavelengths, if accompanied by measures of atmospheric h u m i d i t y and s u i t a b l y p r o g r a m m e d for each s t a t i o n occupied. Every new st,~tion requires to be "estt~blished" b y comparing " s h o r t m e t h o d " transmission values with Langley transmission for at least a year. All records can then be t a b u l a t e d from the beginning. Although f u n d a m e n t a l , the Langley m e t h o d c a n n o t be t r u s t e d for a n y individual day because the atnmsphere m a y have become clearer or less clear during the hours of observation. Only b y taking groups of solar c o n s t a n t s eraploying Langley transmissions can these errors be ironed out in the mean. The " s h o r t m e t h o d " , on the other hand, requires b u t ten minutes for observation a n d three or five determinations can be made and fully reduced in one day, while the Langley method involves hours of c o m p u t a t i o n for only one d e t e r m i n a t i o n . So from 1923 to 1952, all S m i t h s o n i a n solarc o n s t a n t measures were made b y the " s h o r t m e t h o d " , except occasional check measures on especially fine days by t h e Langley m e t h o d too. As shown b y 1992 comparisons of results of identical days from four m o u n t a i n peaks t h o u s a n d s of miles separated, the probable error of one (lay of solar c o n s t a n t work (lone at one s l a t ion is J~) of 1 percent. (See Table 1, page 13,
of course relative " o n the scale of 1913". I t does not imply t h a t we know the absolute mean value of the solar c o n s t a n t to b e t t e r t h a n one or two percent. I now t u r n to the criticisms of Stair and J o h n s t o n as rep o r t e d b y T h e k a e k a r a . T h e most serious is the s t a t e m e n t t h a t we "focused . . . the solar image . . . on the slit of the spectrog r a p h " . We did not. The direct reflection of the coelostat fell all over the slit ten centimeters tall. P e r h a p s Stair a n d J o h n ston were confused because we used a tower telescope image of the sun every day at M o u n t Wilson, between bolographs used for the solar constant, in order to measure the spectral distrib u t t o n of radiation over an east-west d i a m e t e r of the sun. (See Pub. 4545, Fig. 52). T h e y also imply t h a t the coeh)stat, b y t a r n i s h e d mirrors and change of reflection with angle*, gave varying transmission as to t o t a l and b y wavelength. We were experts, provided with facilities for silvering and polishing. We n e v e r allowed tarnishing. On m a n y occasions over ninny years we spent a day at M o u n t Wilson to determine the transmission of the entire optical train, in order to know the distribution of r a d i a t i o n outside the atmosphere. A collection of the results ou t h a t distribution is published b y Abbot, Fowle and Aldrich, Smith. Misc. Coll., 74, No. 7, Table 10, page 15, 1923. This work was done m a n y times in m a n y years and with prisms of white flint glass, ultraviolet crown glass, rock salt, quartz crystal. The weighted mean result is given in the table just cited. B o t h in the " s h o r t m e t h o d " and the Langley method of solar c o n s t a n t m e a s u r e m e n t the transinission of the optical t r a i n enters as of secondary importance. At m y request Miss Graves computed the solar c o n s t a n t for one fine d a y a t M o u n t Wilson, using two widely differing though reasonable tables of assumed i n s t r u m e n t a l transmission at 40 wavelengths. The two solar c o n s t a n t values found differed by only a small fraction of one percent. In fact the t a b u l a r values of i n s t r m n e n t a l spectral transmission can justly be regarded merely as weights applied to the different p a r t s of the spectrum, so t h a t the sum of 40 values measuring total radiation shall more t r u l y correspond with the pyrheliometer reading t h a n if the direct measures upon the spectrum bolograph had been added, w i t h o u t change, for the total area. We did our best for accuracy in the form of spectral distributton, keeping all optical parts in first-class condition, and measuring the i n s t r u m e n t a l transmission on ninny days as follows: For the coelostat we had four freshly silvered mirrors used a l t e r n a t e l y in pairs, a n d also as t a n d e m of four. F o r the s p e c t r o b o h m m t e r we used two complete spectrographs, a h e r n'~tely, and in tandem, observing altogether with the bolometer t h r o u g h o u t for the transmission of the optical systems. H a v i n g done this, we made use of the total i n s t r u m e n t a l spectral transmission values observed at M o u n t Wilson Io compute solar c o n s t a n t s not only there, b u t also where our o t h e r mountain s l a t i o n s were similarly equipped. P r o b a b l y few will care to pursue the subject of S m i t h s o n i a n s o l a r - c o n s t a n t measures further, b u t some m a y like a brief history of its ninny investigations from 1890 to 1952 and after. To them I m a y refer Publications 4222 and 4545, Smith. Misc. Coll., 181, No. 1, and 146, No. 3. There are also being published this year (the 75th a n n i v e r s a r y of the founding of the Smithsonian Astrophysical Observatory) Mrs. Jones' historical sketch, and m y own account, of the investigations 1903 to 1952. M u c h f u r t h e r detailed information is available in the An~mls, Volumes 1-7, and in some 200 papers by members of the staff. I would single out as of special interest Publications 4988, 4135, 4179. 4211, 4213, 4390, 4462, and 4471.
* I n " s h o r t - m e t h o d " work only ten minutes is required for one d e t e r m i n a t i o n .
C.G. Abbot Washington, l). C.
Vol. 9, No. 3, 1965
167
a n o t h e r made desired exposures of the bolometer. The lightspot, he said, would often gallop off of a meter-long scale in one minute. His desired measures were ferreted out from a m u l t i t u d e of recorded figures. In 1895, when I reached Washington, the galvanometer spot usually drifted 20 centimeters or more across the photographic record during one run t h r o u g h the infrared s p e c t r u m from A to ~]. Only the great absorption b a n d s of oxygen, water vapor, a n d carbon dioxide were cert a i n l y shown. Dr. Kayser of Bonn was once f o r t u n a t e l y detained with me one afternoon a t our observing place in W a s h i n g t o n for an hour, during a violent thundershower. He told me of P a s e h e n ' s g a l v a n o m e t e r with a v e r y light suspended system. Mr. Fowle and I t h e n worked up the theory and construction of the coils a n d the needle system of the reflecting galvanometer. This we described in the Astrophysical Journal, 18, No. 1, J u l y 1903. I also devised and provided a c o n s t a n t t e m p e r a t u r e , mechanically operated, electrical balancing device for t h e bolometer. I made also a b e t t e r m o u n t i n g for the sensitive strips of the bolometer. Some years l a t e r I s u b s t i t u t e d a variable high-resistance s h u n t for balancing the bolometer and enclosed the sensitive strips in a high vacuum. These steps produced a b o u t 100-fold sensitiveness, greater s p e c t r u m resolution, and a h n o s t perfect freedom from drift and wiggle of the photographic record. I n short, we made the bolometer as easy to use as a tape measure. A n m l t i t u d e of s p e c t r u m lines were then well shown in our bolographic records. See the specimen bolograph exhibited in the G r e a t Hall a t the Smithsonian, and p r i n t e d on page 18 of m y book " A d v e n t u r e s in the World of Science". Pouillet's mercury-box pyrheliometer became the "Silverdisk P y r h e l i o m e t e r " , of which more t h a n 100 copies were made and s t a n d a r d i z e d and d i s t r i b u t e d on orders from all continents and m a n y islands. By long, well-diaphragmed, entrance-tubes, we limited the sky around the sun so greatly t h a t in our mount a i n - t o p observations the sky c o n t r i b u t i o n was less t h a n 1/~ percent of the sun's. We also modified the ~,ngstrSm compensation pyrheliometer so as to limit its sky exposure similarly. As t h a t i n s t r u m e n t is much easier to observe with t h a n the silver-disk pyrheliometer, we used it thereafter for our daily program of m o u n t a i n - t o p observation of the solar c n n s t a n t , a n d used the silver-disk i n s t r u m e n t only occasionally for comparisons to assure t h a t the :~ngstr6m retained its s c a l e Until a b o u t 1910 there was no s t a n d a r d i n s t r u m e n t to cornpletely measure the heat of the solar beam in c.dories. Eraploying the principle of the hollow chamber as an " a b s o l u t e l y black b o d y " we designed and constructed two instruments, the water-flow and the water-stir pyrheliometers. These are described in Volumes 2 and 3 of Annals of the Smithsonian Astrophysical O b s e r v a t o r y (See Fig. 4 and 8, pages 53 and 64, Volume 3). In each of t h e m the sun's rays were introduced t h r o u g h a tube provided with a series of diaphragms to limit sky exposure. A measured d i a p h r a g m a d m i t t e d the solar rays into a blackened chamber shaped like a test tube and to a conical receiver at the back. Two coils of wire, one on the back of the cone, the other in front of its leading edge, were provided for introducing electrical heating. The front coil was more f a v o r a b l y placed t h a n the rear coil to conduct heat to the chamber. Calorimetry was provided in the water-flow i n s t r u m e n t b y passing a s t r e a m of water spirally t h r o u g h the hollow wall of the c h a m b e r and the receiving cone, and measuring its rise of t e m p e r a t u r e , during insolation or electric heating, b y a Wheat.stone's bridge h a v i n g two coils a t the entrance, two at the exit. of the flowing stream. T h e water-stir pyrheliometer, used only for comparisons with the water-flow i n s t r u m e n t and a few silver-disk pyrheliometers, had a similar c h a m b e r and heating coils, b u t was surrounded b y a body of
Solar Energy
Smith. Misc. Coll., 145, No. 3, Pub. 4545, 1963). This result is
vigorously stirred w a t e r whose t e m p e r a t u r e rise was measured b y an electrical resistance t h e r m o m e t e r , I n 1913, Mr. L. B. Aldrich a n d I satisfied ourselves b y 65 trials involving b o t h s t a n d a r d pyrheliometers, a n d b o t h h e a t ing coils in each, t h a t electrical h e a t i n g would be fully t r a n s ferred to the w a t e r calorimeters. (See T a b l e 22, Annals, Vol. 3, page 68.) We preferred the water-flow i n s t r u m e n t as more c o n v e n i e n t for s t a n d a r d i z i n g secondary pyrheliometers. I t h a d one defect. Air bubbles or o t h e r irregularities in the w a t e r c u r r e n t caused such wiggles of the g a l v a n o m e t e r spot t h a t a large n u m b e r of repetitions of observations was required for t r u s t w o r t h y results. This difficulty was cured when V . M . Shulgin of Russia, a b o u t 1927, suggested using two chambers and dividing the flow of w a t e r equally between them, so t h a t the causes of i r r e g u l a r i t y would simultaneously occur, and the g a l v a n o m e t e r would not be affected. We immediately adopted his suggestion, a n d it worked beyond expectation. A t the same time we used the ~kngstrhm electrical compensation principle, h e a t i n g one c h a m b e r with the sun, the other simultaneously electrically, a n d a l t e r n a t i n g them. This change throws us b a c k to our proof made in 1913 t h a t electrical h e a t i n g is t o t a l l y t r a n s f e r r e d to the flowing water. All of our s t a n d a r d i zations of pyrheliometers since 1930 are done b y this method, Following Langley's direction of 1905 our chief concern has always been to discover and measure the variations of solar r a d i a t i o n as t h e y would be found outside the atmosphere, Hence we used every precaution and test to insure t h a t all of our publications of the solar c o n s t a n t were m a i n t a i n e d " o n the scale of 1913". W i t h the adoption of Shulgin's suggestion we found t h a t " t h e scale of 1913" was a b o u t 2 percent too high. B u t a b o u t 1935, studies convinced us t h a t we had made a n error of a b o u t 2 percent b y u n d e r e s t i m a t i n g the i n t e n s i t y of r a d i a t i o n in the u l t r a v i o l e t and infrared, not observed b y the spectrobolometer. One error balances the other so t h a t I still regard the m e a n value 1.946 as the best result of S m i t h s o n i a n solar-constant determinations, In 1923 we fully perfected the " s h o r t m e t h o d " of solarc o n s t a n t m e a s u r e m e n t . The f u n d a m e n t a l m e t h o d of Langley requires a series of sI)ectrum observations extending interm i t t e n t l y over several hours, in order to determine the transmission of the e a r t h ' s atmosphere t h r o u g h o u t the spectrum, We found t h a t a m e a s u r e m e n t of the radiation scattered by the atmosphere in a zone s u r r o u n d i n g the sun could give good measures of atmospheric t r a n s p a r e n c y at all wavelengths, if accompanied by measures of atmospheric h u m i d i t y and s u i t a b l y p r o g r a m m e d for each s t a t i o n occupied. Every new st,~tion requires to be "estt~blished" b y comparing " s h o r t m e t h o d " transmission values with Langley transmission for at least a year. All records can then be t a b u l a t e d from the beginning. Although f u n d a m e n t a l , the Langley m e t h o d c a n n o t be t r u s t e d for a n y individual day because the atnmsphere m a y have become clearer or less clear during the hours of observation. Only b y taking groups of solar c o n s t a n t s eraploying Langley transmissions can these errors be ironed out in the mean. The " s h o r t m e t h o d " , on the other hand, requires b u t ten minutes for observation a n d three or five determinations can be made and fully reduced in one day, while the Langley method involves hours of c o m p u t a t i o n for only one d e t e r m i n a t i o n . So from 1923 to 1952, all S m i t h s o n i a n solarc o n s t a n t measures were made b y the " s h o r t m e t h o d " , except occasional check measures on especially fine days by t h e Langley m e t h o d too. As shown b y 1992 comparisons of results of identical days from four m o u n t a i n peaks t h o u s a n d s of miles separated, the probable error of one (lay of solar c o n s t a n t work (lone at one s l a t ion is J~) of 1 percent. (See Table 1, page 13,
of course relative " o n the scale of 1913". I t does not imply t h a t we know the absolute mean value of the solar c o n s t a n t to b e t t e r t h a n one or two percent. I now t u r n to the criticisms of Stair and J o h n s t o n as rep o r t e d b y T h e k a e k a r a . T h e most serious is the s t a t e m e n t t h a t we "focused . . . the solar image . . . on the slit of the spectrog r a p h " . We did not. The direct reflection of the coelostat fell all over the slit ten centimeters tall. P e r h a p s Stair a n d J o h n ston were confused because we used a tower telescope image of the sun every day at M o u n t Wilson, between bolographs used for the solar constant, in order to measure the spectral distrib u t t o n of radiation over an east-west d i a m e t e r of the sun. (See Pub. 4545, Fig. 52). T h e y also imply t h a t the coeh)stat, b y t a r n i s h e d mirrors and change of reflection with angle*, gave varying transmission as to t o t a l and b y wavelength. We were experts, provided with facilities for silvering and polishing. We n e v e r allowed tarnishing. On m a n y occasions over ninny years we spent a day at M o u n t Wilson to determine the transmission of the entire optical train, in order to know the distribution of r a d i a t i o n outside the atmosphere. A collection of the results ou t h a t distribution is published b y Abbot, Fowle and Aldrich, Smith. Misc. Coll., 74, No. 7, Table 10, page 15, 1923. This work was done m a n y times in m a n y years and with prisms of white flint glass, ultraviolet crown glass, rock salt, quartz crystal. The weighted mean result is given in the table just cited. B o t h in the " s h o r t m e t h o d " and the Langley method of solar c o n s t a n t m e a s u r e m e n t the transinission of the optical t r a i n enters as of secondary importance. At m y request Miss Graves computed the solar c o n s t a n t for one fine d a y a t M o u n t Wilson, using two widely differing though reasonable tables of assumed i n s t r u m e n t a l transmission at 40 wavelengths. The two solar c o n s t a n t values found differed by only a small fraction of one percent. In fact the t a b u l a r values of i n s t r m n e n t a l spectral transmission can justly be regarded merely as weights applied to the different p a r t s of the spectrum, so t h a t the sum of 40 values measuring total radiation shall more t r u l y correspond with the pyrheliometer reading t h a n if the direct measures upon the spectrum bolograph had been added, w i t h o u t change, for the total area. We did our best for accuracy in the form of spectral distributton, keeping all optical parts in first-class condition, and measuring the i n s t r u m e n t a l transmission on ninny days as follows: For the coelostat we had four freshly silvered mirrors used a l t e r n a t e l y in pairs, a n d also as t a n d e m of four. F o r the s p e c t r o b o h m m t e r we used two complete spectrographs, a h e r n'~tely, and in tandem, observing altogether with the bolometer t h r o u g h o u t for the transmission of the optical systems. H a v i n g done this, we made use of the total i n s t r u m e n t a l spectral transmission values observed at M o u n t Wilson Io compute solar c o n s t a n t s not only there, b u t also where our o t h e r mountain s l a t i o n s were similarly equipped. P r o b a b l y few will care to pursue the subject of S m i t h s o n i a n s o l a r - c o n s t a n t measures further, b u t some m a y like a brief history of its ninny investigations from 1890 to 1952 and after. To them I m a y refer Publications 4222 and 4545, Smith. Misc. Coll., 181, No. 1, and 146, No. 3. There are also being published this year (the 75th a n n i v e r s a r y of the founding of the Smithsonian Astrophysical Observatory) Mrs. Jones' historical sketch, and m y own account, of the investigations 1903 to 1952. M u c h f u r t h e r detailed information is available in the An~mls, Volumes 1-7, and in some 200 papers by members of the staff. I would single out as of special interest Publications 4988, 4135, 4179. 4211, 4213, 4390, 4462, and 4471.
* I n " s h o r t - m e t h o d " work only ten minutes is required for one d e t e r m i n a t i o n .
C.G. Abbot Washington, l). C.
Vol. 9, No. 3, 1965
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