Herschel’s discovery of stars embedded in nebulosity, the so-called planetary nebulae, reinforced a possible interpretation of the nebulae as planetary systems in the making. Such an interpretation correlated very well with the accepted theory of the origin of the Solar System developed by Pierre Simon de Laplace towards the end of the eighteenth century. According […]
The Nature of Nebulae Certain nebulous patches, such as the Magellanic Clouds in the south and the Andromeda Nebula in the north, are clearly visible to the naked eye and have undoubtedly been observed from early times. Comments on the Andromeda Nebula date back to the Arabs in the middle of the tenth century. Similarly, […]
The Draper catalogues provided astrophysicists with the massive numbers of spectra needed for statistical studies of the distribution of stellar types, which in turn might provide clues on stellar evolution. Two independent studies of particular interest, one by E.Hertzsprung, a Danish astronomer, the other by the American astronomer H.N.Russell, resulted in one of the corner-stones […]
In the mid-nineteenth century, very little was known about stars other than positions, apparent magnitudes, and, in a few cases distances. The method of determining distance, by trigonometrical parallax, was limited to those stars within a hundred parsecs of the Sun ; thus, even by 1890 fewer than 100 stars had known distances. Spectral analysis […]
The dark lines in the solar spectrum, noted by Wollaston in 1802, were first mapped systematically by the physicist Joseph Fraunhofer. By 1817 Fraunhofer had described hundreds of lines and bands, the principal ones to which he assigned letters A, B, C and so on, which are used even today. Furthermore, he believed that the […]
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 […]
The discovery at the turn of the nineteenth century that the light from the Sun dispersed through a spectroscope produced not only a continuous spectrum, as Newton had shown in 1666. But one crossed by dark lines – the Fraunhofer lines as they came to be known – provoked much concern among astronomers, physicists and […]
Vital step were taken towards the detection of stellar parallax during the eighteenth century through the collection of extensive and precise stellar catalogues. In addition, more precise instruments, impossible for the crude technologies of earlier times, were being constructed. Both these factors contributed to many false alarms until the actual minuteness of stellar parallaxes was […]
The desire to detect both solar and stellar parallaxes provided great incentive for observational astronomers during the eighteenth century. Lunar distances also were important because observations of the Moon were felt to be a viable approach to the problem of determining longitude at sea. One of the most elaborate scientific expeditions of the eighteenth century […]
Meanwhile, investigations into the physical mechanism of the planets culminated in 1687 with the publication of Isaac Newton’s Principia’. Although others had made important contributions to the question of the mutual attraction of bodies, Newton was the first to assimilate it all and to derive the exact equations for the fall of a body on […]