Abbreviation for light year.

The island in the Canary Islands group where the Observatorio del Roque de los Muchachos is located.

The observatory of the European Southern Observatory, located in the southern part of the Atacama desert, about 600 kilometres 370 miles north of Santiago de Chile, at an altitude of 2,400 metres 7,900 feet. The instruments include a 3.6-metre 142-inch telescope, the 3.5-metre 138-inch New Technology Telescope and the 15-metre 50-foot Swedish/ESO Submillimetre Telescope.

A landslide. A descriptive term used in naming planetary features.

A complex system of intersecting valleys on the surface of a planet.

A small inconspicuous constellation between Cygnus and Andromeda. It was introduced by Johannes Hevelius in the late seventeenth century and contains only one star brighter than fourth magnitude.

BL Lac object.

Literally lake, a term used in the names of certain dark, isolated features on the Moon. Its use dates from a time when it was believed that the darker features on the Moon were of liquid water; this is now known to be untrue. In recognition of its use over a long period, the term has been retained in official place names on the Moon. Lacus was also formerly used in the names of certain features on Mars. Though these names are still often used by amateur astronomers, they have been replaced officially by more precise descriptive terms. Thus, what was known as Solis Lacus is now properly called Solis Planum.

A luminous nebula in the constellation of Sagittarius. It is a complex of ionized hydrogen, gas and dust with hot, recently formed stars. A star cluster, NGC 6530, lies near the centre of the nebula. The light from two naked-eye stars in the cluster, 7 and 9 Sagittarii, is responsible for ionizing the gas. The nebula is estimated to lie at a distance of 4,500 light years.

Points in the orbital plane of two massive objects circling about their common centre of gravity where a particle of negligible mass can remain in equilibrium. There are five such points for two bodies in circular orbits around each other, but three are unstable to small perturbations. The other two, at points 60° either side of the less massive body and in the same orbit, are stable. The Trojan asteroids, which share the orbit of Jupiter, are examples of masses trapped at the two stable Lagrangian points in an orbit. See also: Roche lobe.

A large lunar crater, 132 kilometres 82 miles in diameter, on the eastern border of the Mare Fecunditatis. It is a bright crater with a central peak, terraced walls and a modest ray system.

A telescope consisting of two 8.4-metre 27.5-foot mirrors on a single mount, to be constructed at the Mount Graham International Observatory in Arizona. The project is a collaboration between the University of Arizona and the Arcetri Astrophysical Observatory in Florence, Italy. The binocular arrangement will give the telescope a light-gathering power equivalent to a single 11.8-metre 39-foot mirror, and a resolution corresponding to a 23-metre 75.5-foot telescope.

Magellanic Clouds.

A satellite of Neptune 1989 N2 discovered during the flyby of Voyager 2 in August 1989.

An observatory in the Sierra del Condor, Chile, at a height of 2,300 metres 7,500 feet, operated by the Carnegie Institution of Washington. The main instruments are 2.5-metre 100-inch and 1-metre 40-inch reflectors.