Asteroid 253, which was imaged by the Near-Earth Asteroid Rendezvous NEAR mission in a close flyby on 27 June 1997. Mathilde is a uniformly dark C-type asteroid with an albedo of only 3 per cent. NEAR found its mean diameter to be 52 km 33 miles. Five craters with diameters in excess of 20 km 12 miles were identified on the side of the asteroid in sunlight at the time of the encounter. Its rotation period was also measured and found to be unexpectedly long at 17.4 days.

Observatories on a mountain-top site on the island of Hawaii at a height of 4,200 metres 13,800 feet. It is one of the best sites in the world for optical, infrared and sub-millimetre-wave astronomy and has been developed for this purpose since 1970. The first large telescope to be installed was a 2.24-metre 88-inch reflector of the University of Hawaii. In 1979 three more major telescopes began operation at the observatory: the United Kingdom Infrared Telescope UKIRT, NASA’s Infrared Telescope Facility IRTF and the Canada-France-Hawaii Telescope. The James Clerk Maxwell Telescope working in the millimetre wave region, and the Caltech 10.4-metre 34-foot Submillimeter Array were opened in 1987. The first of the two Keck Observatories was completed in 1992, and the second in 1996. Two 8-metre 26-foot instruments, the Japanese Subaru Telescope and one of the international Gemini Telescopes, are due for completion in 1999.

butterfly diagram.

An interval of about 70 years, starting around 1645, during which solar activity was consistently at a low level and sunspots were rare. For 37 years no aurora was recorded.

A German astronomy research institute with headquarters at Heidelberg and an observatory at Calar Alto in Spain.

A German institute for research in radio astronomy which has its headquarters in Bonn and an observing station at Effelsberg, 40 kilometres 25 miles to the south-west of Bonn. The observatory operates a 100-metre 328-foot, fully steerable radio dish on a computer-controlled altazimuth mounting, which was opened in 1971.

The highest mountain peaks on Venus, located on Ishtar Terra. They rise to 11.5 kilometres 38,000 feet above the venusian datum level.

James Clerk Maxwell Telescope.

A 4-metre 158-inch optical reflecting telescope at Kitt Peak National Observatory, belonging to the US National Optical Astronomy Observatories. It has been in operation since 1973.

An observatory belonging to the University of Texas located on Mount Locke in the Davis Mountains near Fort Davis. It was established in 1932, financed by a bequest from a wealthy Texan banker and amateur astronomer, William J. McDonald. The original instrument, completed in 1938 and still in use, is a 2.08-metre 82-inch reflector, known as the Otto Struve Telescope after the observatory’s first director. In 1969, a 2.72-metre 107-inch reflector came into use. The latest addition is the Hobby*Eberly Telescope, completed in 1996. There are also 91-centimetre 36-inch and 76-centimetre 30-inch reflectors dating from 1956 and 1970 respectively. The 76-centimetre mirror is the central portion of the 2.08-metre, cut away to create the hole for the Cassegrain system. Additionally, there is a 76-centimetre telescope dedicated to satellite laser ranging work and a 5-metre 16-foot millimetre-wave dish.

A large solar observatory located at Kitt Peak and belonging to the US National Solar Observatory. The main telescope, completed in 1962, consists of a 1.6-metre 60-inch mirror, mounted on a tower, which directs the sunlight along a long light shaft inclined at 32° to the horizontal, much of which is below ground. A high-resolution image of the Sun 75 centimetres 30 inches in diameter is produced by this system. The entire building is encased in copper, and coolants are piped through the outer skin to maintain a uniform temperature inside.

For a body travelling in an undisturbed elliptical orbit, the product of its mean motion and the interval of time since it passed pericentre. The mean anomaly is thus the angular distance from pericentre of a hypothetical body travelling with a constant angular speed equal to the mean motion. See also: anomaly.

The equinox determined by considering only precession and ignoring small variations of short period. Star positions are normally referred to the mean equinox of a standard epoch.

The constant angular speed that would be required for a body travelling in an undisturbed elliptical orbit of specified semimajor axis to complete one revolution in the actual orbital period. It is thus the angular speed a body would have if it were following a circular orbit with a radius equal to the semimajor axis of its actual elliptical orbit.

The same as statistical parallax.