Prowling in used bookstores can yield gratifying surprises. I’ve always had an eye for the musty treasures in the history of astronomy, but this book was special. With a stiff and elaborate marbled cover, Volume 2 of the 1908 Annals of the Astrophysical Observatory of the Smithsonian Institution was first of all huge – coffee table size – and in excellent condition after a century of life. Its big glossy pages, full-page charts, and fine illustrations were carefully stitched together – a grand old book of heft and dignity. Written with unusual clarity, the book’s entire focus was sun. It was for the time the most comprehensive study of solar radiation of any I’d come across; historically relevant too, in my opinion, to contemporary climate and even exoplanet concerns. It was inscribed by the director of the Smithsonian Institution, “C. G. Abbot” with F. E. Fowle. Abbot was of course Charles Greely Abbot, about whom several of my previous posts are concerned.
So, you know now how I met him, in the pages of that intriguing book.
Part I of the book – the meat of it – caught my particular interest. It is called, Determination of the Intensity of the Solar Radiation outside the Earth’s Atmosphere, Otherwise Termed ‘The Solar Constant of Radiation’. It reports on the progress made as of 1908 on a multi-year Smithsonian Institution-sponsored project ultimately to determine the surface temperature of the sun. It was a small but important chapter in the still-young field of solar astrophysics, when the windows of astronomical research were being opened to the stars in a new and powerful way, through spectroscopy.
The PDF accompanying this post follows the attachment of the previous post; it concludes with the mathematics Abbot used to derive the solar constant. You’re welcome to work through it, but if you feel it’s a bit too technical, the gist of what he did is summarized below.
Abbot used two instruments to find the solar constant. He measured the total heat energy received on Earth from the sun with a ‘pyrheliometer’ – a specialized thermometer with a blackened silver disk aimed at the sun – through varying zenith angles as the sun moved across the sky. Its measures the varying amounts of solar radiation as it passes through greater or lesser lengths of atmosphere. Abbot knew that estimating the sun’s temperature would be guesswork without first understanding the properties of the blanket of air that surrounds the earth: “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.”
The second instrument was an improved version of the ‘spectro-bolometer’ invented in 1880 by Abbot’s old boss, Samuel Langley. This ingenious device was simply a wire whose resistance would change as radiation of a particular wavelength would be directed on it after passing through a prism. Once the whole spectrum is passed across the wire, a “bolograph” showing the energy-intensity curve over the solar spectrum would be obtained. Since it could thus measure radiation intensities at different wavelengths of the solar spectrum, and at different zenith angles, Abbot could map a radiation curve of the sun’s output, plotting intensity versus wavelength for each selected zenith angle. It would be an improved version of Langley’s pioneering diagram (in our first post) that resembled a beached whale.
In practice, Abbot measured solar radiation wavelength-by wavelength over 44 different wavelengths: not high-resolution by modern standards, but groundbreaking observational research for the day. After correcting intensity data with the atmospheric absorption data for all 44 wavelengths, Abbot mathematically extrapolated his information to find what those values would be outside the atmosphere. (Such is the simple elegance of mathematics!) Of course, as in any experimental endeavor, there were instrumental issues to consider, such as absorption of radiation by the bolograph, the varying sensitivities of bolometers with temperature, Earth’s location in its orbit, and other factors. After correcting for all this, Abbot summed up the radiation data across all the sampled wavelengths, and now finally arrived at the value of the extra-atmospheric solar constant. It was a heroic effort!
Abbot’s best value in 1908 for the mean solar constant was 2.01 calories per square centimeter per minute, or about 1403 watts, too high against modern values. Later in 1911 he concluded that his coefficients of atmospheric transmission were all about 1.4 percent too small. These adjustments plus changes in instruments and instrument sensitivity calculations led him to derive a mean solar constant as of 1911 of 1.922 calories (per square centimeter per minute). By 1915, with accumulating observations and refinements of method, this number would come down to 1.93, close to the current value of 1.95 (1361 watts per square meter).
In a co-authored paper that year to the Proceedings of the National Academy of Sciences, Charles Greely Abbot revealed that from 1903 to 1914 he and his colleagues had made nearly a thousand determinations of the intensity of solar radiation outside the atmosphere, from Washington DC, from Bassour, Algeria, and from Mounts Wilson and Whitney in California. In a 1922 paper in the journal Nature that number was up to 1,244 observations, some of which also included from Calama in Chile. It was an exhaustive multi-decade effort to pin down a crucially important constant important to the habitability of life on earth.
Abbot did not make grand paradigm-shifting discoveries, but rather worked steadily over the years to improve his instruments and observations. He was an exemplar of the in-the-trenches scientific sleuth who could not let go of a subject until he mastered every detail and accounted for every source of error. He exhibited a vitality and thoroughness that science in every field needs to advance its work.
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