Nineteenth-century ideas on the nature of the Sun can he divided into two distinct periods: the first characterized by the simple visual observations of sunspots, the second, by the application of photography and spectroscopy to the Sun.

In the early nineteenth century, sunspots were the major clue to the nature of the Sun. The common interpretation of sunspots as mountains or volcanoes emerging through an incandescent ocean was an intuitive one. In 1774, a British astronomer, Wilson, showed that this interpretation was wrong. From purely geometrical reasoning, he argued that the spots must be depressions rather than elevations. The idea was expanded upon by William Herschel in 1795 to produce a general picture of the Sun. one which was accepted until around the middle of the nineteenth century. According to it, the spots were interruptions in the cloud layers, exposing the solid body of the true Sun. There were two cloud layers, an upper luminous one (called the photosphere by Schroter ), and an inner one protecting the surface from much of the hot, outer layer. Beneath this was a cold and solid globe like a gigantic planet. At the time very little was known about the nature of heat, so this theory was acceptable.

Later, closer studies of sunspot activity began. From observations carried out at the Cape of Good Hope.
.John Herschel concluded that spots could be compared to tornadoes, an idea which has survived to the present day. At about the same time, a German amateur, H.S.Schwabe. showed that the number of sunspots varied over a 10-year period. A similar period was found by the astronomer J.Lamont in 1851 for the variation of the Earth’s magnetic field. In the same year, Sir Edward Sabine found that the number of magnetic storms varied over a 10-year period and that the variations were parallel to those of solar activity. This exciting correlation, the cause of which was a mystery at the time, stimulated interest in the study of solar activity. R.Wolf. a Swiss astronomer, undertook a statistical study of the maxima and minima of solar activity back to 1750 (observations before this were too irregular), and provided the framework for such statistical studies to be used as an index of solar activity. Another systematic study was undertaken by the English astronomer R.C.Carrington, who was able to announce a new result in 1859 – the Sun rotated differentially. He also noted that the mean latitude of spots varied systematically through a complete cycle; the result was obtained independently by a German amateur, G.Sporer, whose name is usually associated with it.

Attempts to determine the overall brightness and temperature of the Sun and the amount of solar radiation received on Earth were made by John Herschel and by Pouillet in France. The main problem lay in the allowance for atmospheric absorption. Pouillet found that the Sun’s rays falling on a square centimetre of the Earth’s surface could raise the temperature of 1.7633 grams of water 1°C per minute. He called this number the solar constant. Taking a mean between this result and his own, Herschel calculated that the ordinary expenditure of the Sun per minute would have enough power to melt a cylinder of ice 50m in diameter reaching from the Sun to a Centauri. A much more difficult problem was the calculation of the solar temperature. Pouillet estimated a temperature somewhere between 1461 and 1761°C, whereas Waterston obtained a potential temperature of 12 880 000°F.

Throughout the century, estimates of temperature ranged from several thousand to several million degrees. The exact relationship between the temperature of the surface arid the amount of radiation emitted was not solved until 1871), when the Austrian physicist J.Stefan announced what we now call Stefan’s law of radiation. Stefan showed that the total radiation increases as the fourth power of the temperature, a relationship subsequently derived theoretically by Ludwig Boltzmann. Application of this rule gave temperatures close to the present value, 6000°C; at the time, how¬ever, the relationship was mistrusted, partly because of its simplicity. More accurate temperatures were dependent on more precise values of the solar constant. In 1875, the French physicist Violle was the first to obtain an accurate result by observations of the Sun from Mont Blanc; this resulted in a solar constant of 2.5cal cm-2 min-1. Studies of solar radiation were aided by the invention in 1880 of a type of bolometer by the American physicist Langley. The instrument was used to study the variation of atmospheric absorption with wavelength, so as to produce a more accurate solar constant.

The source of energy remained a mystery until the first half of the twentieth century. In the 1840s the belief that energy must be conserved began to establish itself. Geological evidence required the Solar System to be much older than the biblical age of 6000 years; this meant that chemical burning (coal, oil, wood) was in¬adequate, so some other mechanism had to be introduced to account for the Sun’s heat. Mayer, in 1848, suggested that the heat was produced by the impact of meteorites on the solar surface. However, the density of meteorites required would cause the Sun’s mass to increase at an appreciable rate, resulting in an observable alteration in the Earth’s orbit. A theory which did become accepted was that expounded by Hermann von Helmholtz in a popular lecture in 1854, and it was to remain the accepted model for nearly 50 years. According to this new theory, the Sun’s heat was a direct result of shrinkage through cooling. By a decrease in diameter of 120m per year, Helmholtz calculated that the radiation could last 22 million years. Towards the end of the century even this lifetime became unacceptably small by comparison to geological estimates of the age of the Earth, but a better explanation had to await the discoveries of nuclear physics.

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