Abbreviated form of the title, Dialogue Concerning the Two Chief World Systems, the Ptolemaic and the Copernican, a book by Galileo Galilei 1564-1642 published in 1632. Aware of potential opposition from the Church authorities, Galileo had delayed writing such a book at all. Encouraged by a change in the papacy in 1624, Galileo eventually started his work but cast it in the form of a conversation between three men in order to appear as if he were not supporting one argument against another. However, he presented the arguments in favour of a Copernican, Sun-centred solar system so persuasively that the few disclaimers were ineffectual, and Galileo, then aged 68 and infirm, was tried before the Inquisition in Rome. He was forced to abjure his heresies and spent the rest of his life under house arrest. See also: Copernican system, Ptolemaic system.

A phenomenon observed at the very beginning and end of totality in a total solar eclipse when the last or first glimpse of the brilliant photosphere of the Sun shines through a valley on the limb of the Moon. The visual effect is very much that of a flashing solitaire diamond ring. See also: Baily’s beads.

The time when the Moon, Mercury or Venus is exactly at half phase.

A technique used in radio astronomy, particularly in microwave work, in which the strength of a cosmic radio signal is compared with the strength of a known terrestrial standard by rapidly switching between the two.

A branch of mathematics that deals with the properties of curved spaces, and is applied in cosmology to the analysis of the geometry of the universe.

The rotation of a gaseous body, such as the Sun or the planet Jupiter, at a rate that varies with latitude, or the rotation of a non-solid, disc-shaped structure, such as a galaxy, at a rate that varies with distance from the centre. A solid planet like the Earth must rotate so that the angular velocity is the same everywhere. However, the equatorial regions of a gaseous planet or star rotate more quickly than regions at higher latitudes, so two features at differing latitudes will move relative to each other. In a galaxy, the component parts stars and clouds of interstellar material are in individual orbits around the centre of the galaxy. The angular velocity varies with radial distance from the centre, so the galaxy does not rotate like a solid disc.

The process in which a body such as a terrestrial planet, which is initially homogeneous in composition, becomes stratified into different regions, usually of different densities. In the case of a planet, these would be the core, mantle and crust.

The spreading of a beam of light as it passes by the edge of an obstacle, into what geometrically would be the shadow. When diffraction occurs, interference between different parts of the light beam results in a pattern of light and dark areas called a diffraction pattern.

An optical device used to disperse light into a spectrum. It consists of a large number of narrow, closely spaced lines ruled either on glass to form a transmission grating or on polished metal to form a reflection grating. Typically there are several thousand rules per centimetre. Interference between the beams of light created by diffraction at each slit result in the dispersion of the light with wavelength. Diffraction gratings can produce very high dispersion spectra of good quality, and are used for this purpose in astronomical spectrographs.

A cold, dark, relatively small cloud of interstellar matter, several light years in diameter. Such clouds are of relatively low density, and contain gas mainly in the form of atoms and atomic ions, with a sparse population of interstellar molecules.

Features of unknown origin between 440 and 685 nanometres in the absorption spectrum of the interstellar medium.

The general interstellar medium, which is not in discrete nebulae.

A gaseous nebula. The use of the adjective diffuse dates from a time when all objects of fuzzy appearance were classed as nebulae, and it was necessary to distinguish between them. In current terminology, star clusters and galaxies are no longer called nebulae, the word being reserved for diffuse clouds of gas and dust.

A type of stony meteorite composed of the silicate minerals pyroxene and plagioclase.

A medium-sized satellite of Saturn, discovered by G. D. Cassini in 1684. Images from the Voyager 1 mission show several different types of terrain on Dione: heavily cratered areas, cratered plains with a lower density of craters, and smooth plains with few craters or other features. The largest craters are over 200 kilometres 125 miles across, and craters over 100 kilometres are common in the heavily cratered areas. Another noticeable feature is an irregular network of light wispy streaks on a dark background, which, it has been suggested, may be frosty deposits.