In the same year an English amateur astronomer, Richard Christopher Carrington (1826-1875), commenced a series of solar observations which led to some remarkable discoveries. From observations on the spots, Carrington discovered that while the Sun’s rotation was performed in 25 days at the equator, it was protracted to 27½ days midway between the equator and the poles. In 1858 Carrington demonstrated the fact that spots are scarce in the vicinity of the solar equator, but are confined to two zones on either side, becoming scarce again at thirty-five degrees north or south of the equator. Contemporary with Carrington was Friedrich Wilhelm Gustav Spörer (1822-1895), who was born in Berlin in 1822 and died at Giessen, July 7, 1895. He commenced his solar observations about the same time as Carrington, and independently discovered the Sun’s equatorial acceleration. From observations at his little private observatory at Anclam in Pomerania, continued at the Astrophysical Observatory in Potsdam, Spörer demonstrated a remarkable law regarding sun-spots. This law is thus described by a well-known astronomer: “The disturbance which produces the spots of a given sun-spot period first manifests itself in two belts about thirty degrees north and south of the Sun’s equator. These belts then draw in toward the equator, and the sun-spot maximum occurs when their latitude is about sixteen degrees; while the disturbance gradually and finally dies out at a latitude of eight or ten degrees. Two or three years before this disappearance, however, two new zones of disturbance show themselves. Thus, at the sun-spot minimum there are four well-marked spot-belts,—two near the equator, due to the expiring disturbance, and two in high latitudes, due to the newly beginning outbreak.” These remarkable discoveries, which resulted from the investigations of Schwabe, Carrington, and Sporer, are a brilliant example of what may be done by amateurs in astronomy.
At the time when Carrington and Spörer were pursuing these researches, the spectroscope came into use as an astronomical instrument, and since 1859 solar astronomy has been almost entirely spectroscopic. Before we can rightly understand the principles of spectroscopic astronomy, we must go back to the life and work of its founder—Joseph von Fraunhofer.
The son of a poor glazier, Joseph von Fraunhofer was born on March 6, 1787, at Straubing, in Bavaria. His father and mother having died when their son was quite young, the boy, on account of his poverty, was apprenticed to a looking-glass manufacturer in Munich named Weichselberger, who acted tyrannically, keeping him all day at hard work. Still the lad borrowed some old books, and spent his nights in study. Young Fraunhofer lodged in an old tenement in Munich, which on July 21, 1801, collapsed, burying in its ruins its occupants. All were killed but Fraunhofer, who, though seriously injured, was dug out from the ruins four hours later. The distressing accident was witnessed by Prince Maximilian Joseph, Elector of Bavaria. He became interested in Fraunhofer, and presented him with a sum of money. Of this he made good use. He was already interested in optics, and he bought some books on that subject, as well as a glass-polishing machine. The remainder of the money served to procure his release from his tyrannical master, Weichselberger.
Fraunhofer became acquainted with prominent scientists at Munich, who provided him with books on optics and mathematics. Meanwhile the young optician occupied his time in shaping and finishing lenses. In 1806 he entered the optical department of the Optical and Physical Institute of Munich, and the following year, when only twenty years of age, was appointed to the chief post in that department. In 1814 he commenced his investigations with the prism, which have made his name famous.
Newton had found that, in passing through a prism, white light is dispersed into its primary colours, making up the band of coloured light known as the solar spectrum. But he failed to recognise the existence of dark lines in the spectrum. Casually seen in 1802 by William Hyde Wollaston (1786-1828), an English physicist, these lines were first thoroughly examined by Fraunhofer. Allowing light from the Sun to pass through a prism attached to the telescope, he was amazed to find several dark lines in the spectrum. By the year 1814 he had detected no less than 300 or 400 of these lines. Fraunhofer named the more prominent lines by the letters of the alphabet, from A in the red to H in the violet. They are now known as the Fraunhofer lines. At first he was much perplexed regarding the nature of the dark lines. He suspected that they might be an optical effect, depending on the quality of the glass used, and he tried different prisms, but the lines were still to be seen. Then he turned his prism to bright clouds to see if they were visible in reflected sunlight, and he found that they were. He examined the Moon and again perceived them, as moonlight is merely reflected sunlight; and they were also conspicuous in the spectra of the planets. It was thus proved that these lines were characteristic of sunlight, whether direct or reflected. It was, however, still possible that they might be caused by the passage of the rays of light from the celestial bodies through the Earth’s atmosphere. In order to test this theory, Fraunhofer examined the spectra of the brighter stars. He found that the lines visible in the solar spectrum were not to be seen in the spectra of the stars, thus proving that the lines were not merely an atmospheric effect. Each star, Fraunhofer observed, had a different spectrum from both the Sun and from other stars. These spectra were also characterised by numerous dark lines, much fainter than those in the solar spectrum.
Although he ascertained the existence of the dark lines in the Sun’s spectrum, Fraunhofer never really found out what they represented. As Miss Giberne expresses it, “Although he now saw the lines he could not understand them: he could not read what they said. They spoke to him indeed about the Sun, but they spoke to him in a foreign language, the key to which he did not possess.” However, he expressed the belief that the pair of lines in the solar spectrum, which he marked D, coincided with the pair of bright lines emitted by incandescent sodium. Although he doubtless suspected that the lines conveyed intelligence regarding the elements in the Sun, he never was able properly to decipher their meaning. Had he lived, he would probably have made the great discovery; but these investigations were cut short by his sudden and untimely death on June 7, 1826.
After the death of Fraunhofer, very little was done to forward the study of spectrum analysis. Investigations in this branch of research were made, however, by Sir John Herschel (1792-1871), William Allen Miller (1817-1870), Sir David Brewster (1781-1868), and others. Two famous men of science had partly discovered the secret. These were Sir George Stokes (1819-1903), of Cambridge, and Anders John Angström (1812-1872) of Upsala. Of Angström’s work, published in 1853, it has been said that it would “have obtained a high celebrity if it had appeared in French, English, or German, instead of Swedish.”
It was not until 1859 that the principles of spectrum analysis were fully enunciated by Gustav Robert Kirchhoff (1824-1887), and his colleague in the University of Heidelberg, Robert Wilhelm Bunsen (1811-1899). Kirchhoff demonstrated that a luminous solid or liquid gives a continuous spectrum, and a gaseous substance a spectrum of bright lines. In the words of Miss Clerke, “Substances of every kind are opaque to the precise rays which they emit at the same temperature. That is to say, they stop the kinds of light or heat which they are then actually in a condition to radiate.... This principle is fundamental to solar chemistry. It gives the key to the hieroglyphics of the Fraunhofer lines. The identical characters which are written bright in terrestrial spectra are written dark in the unrolled sheaf of sun-rays.” Kirchhoff made several determinations of the substances in the Sun, proving the existence of sodium, iron, calcium, magnesium, nickel, barium, copper, and zinc. His great map of the solar spectrum was published by the Berlin Academy in 1860, and represented an enormous amount of labour. It was succeeded by another map by Angström, published in 1868. But both of these maps have been recently superseded by the investigations of Sir Joseph Norman Lockyer (born 1836), and of the American physicist, Henry Augustus Rowland (1848-1901). Rowland largely increased our knowledge of the elements in the solar atmosphere.
The spectroscope had become, by 1868, a recognised instrument of astronomical research, and in that year it was applied during the famous total eclipse, visible in India. There were many eclipse problems, arising from the observations made by the eclipse expeditions of 1842, 1851, and 1860. The eclipse of 1851 had finally proved that the red flames seen surrounding the Sun during total eclipses belonged to the Sun, and not to the Moon, as many astronomers had believed. At the eclipse of 1860, visible in Spain, the Italian astronomer, Angelo Secchi (1818-1878), and the Englishman, Warren De la Rue (1815-1889), secured photographs of the solar prominences. The problem of 1868 was the constitution of these prominences.
Pierre Jules César Janssen, born in Paris in 1824, was stationed at Guntoor, in India, to observe the eclipse. He succeeded in observing the spectrum of the prominences during the progress of totality, and found it to be one of bright lines, proving the gaseous nature of the sun-flames. During the progress of the eclipse, Janssen was specially struck by the brilliancy of the bright lines, and it occurred to him that the prominence-spectrum could be observed in full daylight, if sufficient dispersive power was used to enfeeble the ordinary continuous spectrum. At ten o’clock on the following morning, August 19, 1868, Janssen applied his spectroscope to the sun, and observed the prominence-spectrum. After a month’s observation in India, he sent to the French Academy an account of his success. A short time, however, before his report arrived, the Academy had received a similar one from Lockyer, who had independently made the same discovery. Two years previously, in 1866, the new method had occurred to him, but his spectroscope was not powerful enough; and although he ordered a more powerful one at once, it was not until October 16, 1868, that he had the instrument in his hands. Four days later he observed the prominence-spectrum in full daylight.