English name for the constellation Grus.

A circular feature on the surface of a planetary body. The word literally means cup, and craters are typically shaped like a shallow cup, lower in the centre than the surrounding terrain, with raised walls. The vast majority of craters observed on planets and their satellites are now known to have resulted from the impact of meteorites and the word is often treated as synonymous with impact crater. However, volcanic craters calderas also occur, on Mars, for example. Craters range in size from the smallest discernible to hundreds of kilometres across. The largest impact features are often termed basins. The detailed forms of craters depend on many factors, including the composition of the planetary surface, the speed, mass and direction of the impacting body and subsequent weathering or geological activity. They may contain a central peak, or a depression.

A small, faint constellation on the southern border of Virgo. It is one of the ancient constellations listed by Ptolemy c. AD 140 and is said to represent the goblet of Apollo. None of its stars are brighter than fourth magnitude.

One of the rings of Saturn the C ring, fainter than the prominent A and B rings. It lies within ring B, extending about halfway between the B ringÕs inner edge and the planet. The name comes from a description of its appearance given by the observer W. Lassell after the announcement of its discovery by W. C. Bond in 1850.

A phase of the Moon, Venus or Mercury when less than half disc is illuminated.

Popular name for NGC 6888, a diffuse shell of gas surrounding the Wolf-Rayet star HD 192163. One crescent-shaped segment of the spherical shell is relatively bright.

One of the small satellite of Uranus discovered during the Voyager 2 encounter with the planet in 1986.

An observatory of the Russian Academy of Sciences, located near Simeiz in the Crimea. It is an important solar observatory, and there is a 2.6-metre 102-inch optical telescope and a millimetre-wave telescope 22 metres 72 feet in diameter.

In cosmology, the minimum density of matter that would ensure that the universe could not expand for ever. The observed expansion can be reversed by gravity only if the density is sufficiently high. The critical density is defined as the density that would ensure that both the deceleration and the velocity of expansion would become zero at the same time. Its value is between 10-29 and 2 × 10-29 g/cm3, about ten times larger than the density inferred from visible matter, such as stars and galaxies. For philosophical and aesthetic reasons many cosmologists like to imagine that the universe is closed. If it is, the amount of so-called missing mass is considerable. Dark matter in the form of particles other than baryons - such as neutrinos - would be required in order to match the actual density to the critical density. See also: closed universe, expanding universe, Big Bang.

English name for the constellation Crux.

The ratio between the diameter of a rich cluster of galaxies and the average random velocity for galaxies within the cluster, which is thus a measure of the time for a galaxy to drift through the cluster. Typical crossing times are about one-tenth the age of the universe. This result implies that members of clusters are gravitationally bound together: otherwise they would have dispersed long ago.

A 90-centimetre 36-inch reflecting telescope at the Lick Observatory, presented in 1895 by an Englishman, E. Crossley. The mirror was figured more accurately than had been possible previously and the instrument demonstrated the potential for larger reflectors. Its success stimulated work on the construction of instruments of greater size.

English name for the constellation Corvus.

The outermost solid layer of a planet or satellite, usually consisting of rock, ice, or a mixture of the two.

The best-known of all southern constellations, formerly included in Centaurus which surrounds it on three sides before A. Royer introduced it as a separate constellation in about 1679. It is the smallest constellation by area, yet one of the most distinctive and recognizable. It lies in the Milky Way and contains a fine star cluster known as the Jewel Box.