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 […]
Extinction and reddening The interstellar medium in nowhere completely transparent and nowhere completely opaque; a dusty region of space dims rather than blocks the- light passing through it. Thin dimming process is called INTERSTELLAR EXTINCTION. Interstellar extinction in front of a star can be expressed quantitatively, and is usually given the symbol Av whore Av […]
The origin of the planets constitutes one of the central problems of astronomy, perhaps the more so as it includes the origin of the Earth itself. The problem has remained a permanent challenge down the years, and has stimulated many attempts which even if not yet entirely successful have led to important new lines of […]
Comets have excited interest in men’s minds from time immemorial. There are two classes of comets. First, but less important, are the SHORT-PERIOD COMETS moving in eccentric orbits at planetary distances from the Sun, and of these about a hundred are known. Second, are the far more numerous LONG-PERIOD COMETS, of which almost 500 have […]
The dust which reaches the Earth , or is collected by spacecraft as micrometeorites makes its presence known in another way. The particles that make up this dust travel in elliptical orbits around the Sun and are concentrated in the plane of the ecliptic. They reflect the light of the Sun to give a luminous […]
If you view the sky on a clear moonless night you may see streaks of light several times per hour. These luminous streaks are called shooting stars or METEORS . The Solar System contains a large number of small particles moving in orbits around the Sun. In the vicinity of the Earth these particles typically […]
The Titius-Bode lair predicted that there should be a planet between Mars and Jupiter about 2.8 astronomical units from the Sun. In 1800 six astronomers, with Johann Schroter as president and Baron von Zach as secretary, worked out a scheme jointly to search for this missing planet. However, before their plans could be put fully […]
Details of the SATELLITES OF URANUS and NEPTUNE . The radii of all five satellites of Uranus are too small to be measured directly, but values can be calculated from their observed brightness and an assumed albedo. If they are ice-covered, the radii could be as small as 100km (Miranda) to 350km (Titania) whereas extremely […]
In addition to the ring particles, there are ten SATELLITES of SATURN . The diameters of the satellites are difficult to measure from the Earth. Turbulence in the Earth’s atmosphere blurs the image by at least 0.1 to 0.2 arc sec. This causes an appreciable error for an object whose apparent size may be only […]