“From heaven he has sent fire into my bones,” wrote Jeremiah, and it’s clear from the context--replete with extravagant metaphors for all sorts of suffering--that this was not a good thing. Perhaps the poor old boy suffered from arthritis, in addition to the moral, cultural and historical agonies that regularly fell to the lot of an Old Testament prophet.
But there’s another way to read his evocative sentence: it’s a simple physical reality that the heavens, every second of every day, freely shower the Earth with the life-giving energy that allows us all to exist. The true manna of this time—of all time—is sunlight.
(Of course, we need the plants to render it edible for us—to get it 'into our bones.' We need the whole fantastically intricate mechanism—or organism—of a functioning ecology. But the plants need, first of all, not to freeze. And it is “fire from heaven” that keeps them warm enough to perform the miracle of photosynthesis. All the other miracles of biology follow.)
In this Hub, and its followups, we’ll be taking a quick look at just a few of the many researchers who have sought to understand this ‘fire from heaven.’ One of the first was Horace Benedict de Saussure (February 17, 1740 – January 22, 1799), a Swiss mountaineer, traveler and scientific observer. His 1767 work with the ‘heliothermometer’ was later described by the French mathematician, physicist and administrator, Joseph Fourier:
It is to the celebrated traveller, M. de Saussure, that we are indebted for a capital experiment, which appears to throw some light on this question.
The experiment consist in exposing to the rays of the sun, a vessel covered with one or more plates of glass, very transparent, and placed at some distance one above the other. The interior of the vessel is furnished with a thick covering of black cork, proper for receiving and preserving heat. The heated air is contained in all parts, both in the interior of the vessel and in the spaces between the plates. Thermometers placed in the vessel itself and in the intervals above, mark the degree of heat in each space. This instrument was placed in the sun about noon, and the thermometer in the vessel was seen to rise to 70º, 80º,100º,110º, (Reaumur,) and upwards. The thermometers placed in the intervals between the glass plates indicated much lower degrees of heat, and the heat decreased from the bottom of the vessel to the highest interval.
The effect of solar hear upon air confined within transparent coverings, has long since been observed. The object of the apparatus we have just described, is to carry the acquired heat to its maximum; and especially to compare the effect of the solar ray upon very high mountains, with what is observed in plains below. This experiment is chiefly worthy of remark on account of the just and extensive inferences drawn from it by the inventor. It has been repeated several times at Paris and Edinburgh, and with analogous results.
De Saussure was able to obtain temperatures of up to 240º Fahrenheit with his apparatus. Most importantly for his research, there was little difference between heating at greater or lesser altitudes, showing that the solar intensity did not differ. De Saussure also experimented with practical use of his apparatus: “Fruits. . . exposed to this heat were cooked and became juicy.” His heliothermometer was also the first solar oven.
Around the time that Fourier was considering De Saussure’s work, the English astronomer Sir John Herschel was seeking to extend it. In 1825 he invented an instrument he termed the “actinometer.” Its purpose was to measure “the direct heating power of the sun’s rays.”
It was a finicky thing, and difficult to use, as one modern re-enactor of Herschellian observation has described at some length. Even Herschel himself had trouble with it at times; his 1838 South African observations turned out to be useless due to calibration problems. (Luckily, his main research objective—cataloging astronomical objects of the Southern Hemisphere—wasn’t affected. And as a bonus, his photographic images of South African flowers, accurately colored by his wife, were published and retain considerable scientific interest to this day. As you can see below, they are also quite simply beautiful images.)
Like De Saussure before him, Herschel too experimented with culinary uses of solar ‘hot boxes’:
As these temperatures [up to 240º F] far surpass that of boiling water, some amusing experiments were made by exposing eggs, meat, etc. [to the heat inside the box], all of which, after a moderate length of exposure, were found perfectly cooked. . . [On] one occasion a very respectable stew of meat was prepared and eaten with no small relish by the entertained bystanders.
On another occasion Herschel cooked an egg:
Which burned Peter’s [his son’s] fingers as if fresh from the pot. It was done as hard as a salad egg and I ate it and gave some to my wife and six small children that they might have it to say they had eaten an egg boiled hard in the sun in South Africa.
Coincidentally, it was also in 1838 that French physicist Claude Pouillet (February 16, 1790-June 14, 1868) completed various series of measurements using both his version of the actinometer and an instrument of his own, the “pyrheliometer.” The latter was no more convenient to use than the actinometer, as Pouillet’s description makes clear:
. . .the pyrheliometer is held in the shade, but very near the place where it is to receive the sun: it is placed so that it looks towards the same extent of sky, and there, for four minutes, its warming or its cooling is noted from minute to minute; during the following minute it is placed behind a screen, and then adjusted so that on removing the screen at the end of the minute, which will be the fifth, the solar rays strike it perpendicularly. Then, during five minutes, under the action of the sun, its warming, which becomes very rapid, is observed from minute to minute, and care is taken to keep the water incessantly agitated; at the end of the fifth minute the screen is replaced, the apparatus withdrawn into its first position, and for five minutes more its cooling observed.
Inspired by the terrestrial heat budget that Fourier had constructed, Pouillet set himself an ambitious research program:
The object of this memoir is--1) the quantity of solar heat which falls perpendicularly, in a given time, on a given surface: 2) the proportion of this heat which is absorbed by the atmosphere in the vertical passage: 3) the law of absorption for different obliquities: 4) the total quantity of heat which the earth absorbs from the sun in the course of a year: 5) the whole quantity of heat which is emitted at each instant by the whole surface of the sun: 6) the elements which must be known in order to ascertain whether the mass of the sun cools gradually from century to century, or whether there is a cause destined to reproduce the quantities of heat which incessantly escape from it: 7) the elements which would allow its temperature to be determined: 8) the absolute quantity of heat emitted by a body whose surface, temperature and radiating power are known: 8) the laws of cooling of a body which loses its heat without receiving any: 9) the general conditions of temperature of a body protected by a diathermanous covering analogous to the atmosphere: 10) the cause of the cooling of the high regions of the air: 11) the law of that cooling: 12) the temperature of space: 13) the temperature which would be observable everywhere on the surface of the earth if the sun’s action was not felt: 13) the elevation of temperature which results from the solar heat: 14) the relation of the quantities of heat which the earth receives from the sun, and from space or all the other celestial bodies.
The first item—determination of the solar constant—was highly successful. Pouillet’s value was within about 10% of the correct value—excellent indeed for a first attempt. Of note was Pouillet’s analytic technique: realizing that the solar constant must be (relatively) fixed, but atmospheric absorption would differ each day due to changing atmospheric conditions; and that the latter (given sufficiently stable weather conditions) would increase as a quadratic function of the angle of the sun, Pouillet was able to tease out a simple formula to separate the two values. In this way he was able to dodge the complicating effects of atmospheric absorption.
His estimate of the sun’s temperature was less successful. Lacking the Stefan-Boltzman Law, he relied upon an empirical relation developed by Pierre Dulong, Pouillet’s predecessor at l'École Polytechnique, that did not hold up well at Solar temperatures. As a consequence, Pouillet calculated a Solar temperature of 1750º C, only about a third of the correct value.
Particularly interesting was the investigation of his first and twelfth research objectives, for here he deployed the actinometer at night. Here he found that even with the bulk of the Earth screening off the sun he could still measure ‘fire from heaven.’ It could be only two things: the ‘temperature of space’—for Pouillet, like Fourier before him, supposed that the wash of radiation from a Universe full of stars must endow space with some definite temperature—or the atmosphere itself, emitting radiation downward. Pouillet’s analysis convinced him that the temperature of space must be much too low to account for his observations. A downward-radiating atmosphere it must be, then.
Claude Pouillet had just discovered what would later be called “sky radiation,” “back radiation” or (the preferred version) “downwelling infrared radiation”—often “DLR,” or “DLW.” Henceforth, there would be two streams of “fire from heaven” to be studied: daytime’s direct radiation, and the indirect radiation originating within the atmosphere.
Here too there is a curious oblique Biblical resonance: the story of the escape of Israel from Egypt describes how the Israelites were guided in the wilderness: “And the LORD went before them by day in a pillar of a cloud, to lead them the way; and by night in a pillar of fire, to give them light. . .” (Exodus 13:21) The natural “fire from heaven” reverses this supernatural pattern: by day the ground and sea receive fiery direct sunlight, while (as we shall see) a significant fraction of the nocturnal “fire” is provided by cloud.
(It must be admitted for clarity, though, that DLR is not really nocturnal; it occurs at all times of day. But early on, daytime observations of DLR were technically not very practical.)
Measurements and analyses in these veins would continue, as would the development of better instruments—for example, the original form of the actinometer reached its culmination in the instrument of Jules Violle (November 16, 1841-September 12, 1923.) Violle, too, would measure the solar constant, making observations at Mont Blanc in 1881 and arriving at an estimate of 2.54 calories per minute per square centimeter.
But science was changing, becoming an increasingly international--and also relatively more American--phenomenon. Accordingly, in 1884 American astronomer and aeronautical pioneer Samuel Langley (August 22, 1834-February 27, 1906) would make an influential determination of the solar constant.
Langley was doubtless inspired by the work of previous researchers in the area, and like Herschel found the power of solar radiation quite fascinating. In 1881 he made an expedition to Mt. Whitney to measure solar radiation.
He reported in a note, “The Mount Whitney Expedition,” (Nature, August 3, 1882) that he had also made trials of the good old ‘hot box’:
As we still slowly ascended . . . and the surface temperature of the soil fell to the freezing point the solar radiation became intenser. . .near the summit the temperature in a copper vessel, over which lay two sheets of plain window glass, rose above the boiling point of water, and it was certain that we could boil water by the solar rays in such a vessel among the snow fields.
In 1884, after much work in ‘reducing’ the data, and applying necessary corrections , Langley had his answer: the solar constant (expressed in modern units) was 2.903 kiloWatts per square meter.
It wasn’t as close as Violle’s figure, itself too high; Pouillet’s estimate, referred to by Langley in the Nature note, eventually turned out to be the best. But Langley’s heritage goes beyond his initial number for the solar constant. In the words of his pupil and successor, Charles Greeley Abbot (May 31, 1872-December 17, 1973):
The eminent astronomer Dr. Samuel Pierpont Langley, third Secretary of the Smithsonian Institution, at Dr. George E. Hale's invitation sent me to Mt. Wilson Observatory in 1905 to observe the radiation of the sun. Langley suspected that this might be variable, and that its variation might be a cause of weather changes. If the suspected solar variations proved periodic, they might lead to long-range weather forecasts of great value for agriculture and water supply.
Langley instructed me that my objective was not the most exact determinations of the solar constant of radiation but rather the discovery of solar variation. In 1905 there was not even an approximate knowledge of the intensity of solar radiation, in free space as it exists outside the earth's atmosphere; and no instruments existed for detecting or measuring solar changes with a sufficient degree of accuracy.
The first part of Secretary Langley's instruction required me to procure or design and construct the best instruments and means for determining the intensity of the sun's radiation in free space at the earth's mean solar distance. Thereafter I was to make sufficient determinations to show whether this radiation varies appreciably. If it was found to vary periodically, I was to discover the laws of such variation. The second suggested objective, if such solar variation was found, was to determine what effects it produces on weather.
Langley suspected, correctly, that “solar constant” was actually a misnomer. Though the solar fire burns with remarkable evenness, it is not perfectly constant.
Perhaps even more significant--in the practical and sociological senses at least--was the date of Dr. Abbott’s words: for he wrote them in 1966, 60 years after Langley’s death, as an introduction to a paper on solar variation and weather. Langley, with Abbott’s able continuance, had created a research program that would long outlast him. It’s probably fair to say that the best results we associate with Langley’s name are more directly due to his abilities as mentor, leader and administrator than those as researcher, considerable though the latter were.
Highly characteristic was Langley’s instruction to “. . . procure or construct the best instruments and means.” Langley frequently looked for better technological means to achieve his scientific ends. And, though electricity was not a new discovery—Pouillet, for instance, had done significant work on the analysis of electric circuitry—Langley was to his fingertips a child of the dawning Electric Age.
So it was natural that in 1878 Langley invented the bolometer, an electric instrument to measure radiation. In the words of Wikipedia, it
. . .consisted of two platinum strips covered with lampblack. . . Electromagnetic radiation falling on the exposed strip would heat it and change its resistance. By 1880, Langley’s bolometer was refined enough to detect thermal radiation from a cow a quarter of a mile away. This instrument enabled him to thermally detect across a broad spectrum, noting all the chief Fraunhofer lines. He also discovered new atomic and spectral absorption lines in the invisible infrared portion of the electromagnetic spectrum.
The new instrument accompanied pyrheliometers and actinometers, as well as telescopes, on Langley’s 1881 odyssey from Pittsburgh to the summit of Mount Whitney. Along the way, it survived the mercies of railroad baggage handling, a 120-mile mule-train desert traverse in temperatures as high as 110º F, and an ascent covering 16 miles of rugged wilderness and ending at 15,000 feet above sea level. (So much for the ‘ivory tower!’)
The bolometer, continually refined and developed, would remain a prominent member of the observational arsenal—today bolometers are a mainstay in investigations of the cosmic background radiation.
Abbott and his team played an important role in early bolometer development, as well as the program of research for which it had been created:
The solar constant method of Langley required nearly three hours of observation to measure the spectral transmission. Only the best cloudless days were suitable for such observations, and often atmospheric changes during several hours would make the solar-constant values too high or too low. Only a comparatively few days were good enough to be used, and only the means of groups of measures could be depended on for accuracy. A new method was imperative.
My able staff included assistants F. E. Fowle and L. B. Aldrich, and Andrew Kramer, instrument maker. Together we designed five types of pyrheliometer including the water-flow standard; also, the two-mirror coelostat for reflecting a fixed solar beam into the laboratory; the vacuum bolometer, sensitive to a 10-millionth of a degree of temperature, and the special galvanometer to record its indications; and finally, the pyranometer to measure radiation from an area of sky. All these new instruments and many adjuncts to them were built in the Smithsonian shop. . .
In 1919, A. F. Moore of our Calama [Chile!] staff showed me a lot of measurements he had made with the pyranometer of the sky radiation from a narrow zone just outside the sun. We found that when plotted against Langley's method, spectral transmission curves could be drawn for 40 wavelengths, 0.34-2.5 At, so that we could substitute pyranometer sky measures requiring less than ten minutes for spectrobolometer measures requiring two to three hours. By 1923 we had perfected this short method, which was useful even on days of some cumulus clouds. It could yield three solar-constant measures, or even five, in one forenoon. We established this empirical "short method" at all our arid mountain stations and accumulated over 9000 days of solar-constant mean daily measures between 1923 and 1952. Of course, we frequently made "Langley method" measures on fine days, to check in groups the accuracy of the short method.
Abbot found that one of Langley’s corrections was mistaken, in part accounting for the 1884 overestimate of the solar constant. As the high-altitude measurements continued to accumulate, the resulting values for the solar constant varied between 1.322 and 1.465 kiloWatts per square meter. These values are quite close—closer, finally, than Pouillet’s estimate from 1838.
The revision wasn’t received tamely; another prominent American researcher of the day, Frank. W. Very, wrote in 1911:
The reformation of the methods for obtaining the value of the constant introduced by Forbes, given a solid rational basis by Langley, and further improved on the mathematical and observational sides by Crova and Savelief, has apparently been relinquished at the present time in favor of methods which do not differ essentially from those of Pouillet. The elaborate researches of Abbot and Fowle, while purporting to be a continuation and perfection of Langley's methods, are in reality a complete abandonment of the essential principle which was admirably stated by Langley in his paper ''On the Amount of the Atmospheric Absorption”. . .
The value of approximately 2 calories which is given in the second volume of the Annals of the Astrophysical Observatory of the Smithsonian Institution is to all intents and purposes the same as that of Pouillet; for if the positive corrections which are known to be required for the latter, on account of the defects of the water pyrheliometer, are applied, we shall have a value of the solar constant differing very little from that which is now offered to us as an improvement on Langley's result, but which is quite the reverse. The methods used in the Annals have all the characteristics of the original Pouillet model, including the admirable agreement of the separate measures and the small apparent probable error of the final result. The fallacy attending such methods may be recognized when it is known that actual reliable measures of solar radiation may be made within the atmosphere which exceed the supposed value outside the atmosphere, as I shall show in a subsequent paper.
Very goes on to put forth a number of apparently convincing arguments, which, however, have since been proven incorrect. The controversy continued for several years. Abbott responded to Very’s criticisms by turning once again to technology. David Devorkin writes:
Still harried by critics, however, Abbot turned to balloonsondes to reach greater heights. Collaborating with the Weather Bureau and Signal Corps, with Anders Knut Angstrom, who had been in residence for several years, and with the help of his chief assistant Loyal B. Aldrich, Abbot flew special pyrheliometers on balloons. He created a new type of robotic pyrheliometer out of parts from standard Weather Bureau meteorgraphs that was fully automatic and self-recording. Automatic techniques for meteorological observations from balloons were well developed by then. But Abbot was the first to use such automata in America for astronomical measurements.
Abbot's instruments, built by Andrew Kramer, were marvels of sophistication and planning. They were flown by Aldrich from the California coast in 1913 and 1914, and some of the balloonsondes reached over 25 kilometers; at least one of them returned clear evidence for thermometric and barometric variations that confirmed his terrestrial extrapolations and allowed him to determine the value of the solar constant at the top of the Earth's atmosphere. This technical feat, requiring the cooperation of the Weather Bureau and the Signal Corps, quieted criticism of the Smithsonian value for the solar constant. It helped to affirm Abbot's reputation and established the modern range for the solar constant.
Abbott seems to have gone ‘a bridge too far’—he became convinced that the variations he thought he was detecting actually determined weather to a significant degree—witness the title of his 1966 report: “Solar Variation, A Weather Element.” In fact, his analysis had been subject to serious statistical criticism as early as 1925, calling into question the wide variation which he championed. Though his public bravado in defense of solar variation had been matched with quieter efforts to bolster his cause with better observations, the issue would continue to simmer quietly for the rest of Abbott’s life. (He died at the age of 101, toward the end of 1973.)
But on October 24, 1979, the Nimbus 7 satellite was launched into a ‘near-Polar sun-synchronous orbit,’ and among the experiments aboard was the first Earth Radiation Budget experiment (ERB).
By the following year data was coming in. As it turned out—and despite the elaborate care and ingenuity Abbott and his team had lavished upon the problem of eliminating atmospheric influences upon observed solar radiation--the real variation is much smaller than Abbott had argued.
Today, satellites continue to observe the ‘fire from heaven’—the latest program is Solar Radiation & Climate Experiment—SORCE, launched in 2003. The quest is not so much for an unchanging value of the solar constant, but for a reliable timeseries showing the evolution of solar radiation over time. Due to the limited lifetimes of satellites, and the existence of measurements made by different instruments under differing conditions, there is no single accepted timeseries; partisans battle over the merits of the ACRIM and PMOD reconstructions of Total Solar Irradiance (TSI.)
But there is nonetheless a generally-accepted number for the solar ‘constant’—1.361 kiloWatts per square meter, down slightly from the pre-SORCE value of 1.366. That number, it would appear, is the primary measure of the ‘fire by day.’
But what of the ‘cloud’—the ‘fire’ reradiated from the atmosphere?
That is too much story to tell here—indeed, I’ve only given a very brief and incomplete sketch of the scientific study of the ‘fire by day.’ The story of the indirect 'fire' is yet more complicated, and bears more directly on present controversies about climate change. But that story--the story of the ‘cloud by night’—is told in next Hub in this series.
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Doc Snow (author) from Camden, South Carolina on August 17, 2011:
Let me know what you think--comments, corrections, questions and random puns are all welcome.