A 37-metre 120-foot radio dish located at the Haystack Observatory in Massachusetts, north-west of Boston. It is a facility of the Massachusetts Institute of Technology, and the astronomy research is conducted under the auspices of a consortium of thirteen educational establishments. It is equipped for use in the wavelength range 2.6 mm-13 cm; the surface is accurate to half a millimetre. The antenna is fully steerable and of a Cassegrain configuration. Among its discoveries are a number of interstellar molecules. It is also used for very-long-baseline interferometry. When first built in the 1960s, it was used mainly for radar studies of the Moon and nearer planets.

A prefix used for star catalogue numbers in the Henry Draper Catalogue.

The nucleus and coma of a comet, excluding the tail.

A radio galaxy with radio emission streaming away to one side of the corresponding optical galaxy, giving a shape resembling a tadpole.

Abbreviation for High Energy Astrophysical Observatory.

English name for the constellation Gemini.

Asteroid 6, diameter 204 km, discovered in 1847 by K. L. Hencke.

A 10-metre 33-foot telescope at the Mount Graham International Observatory operating in the submillimetre waveband between 0.3 and 1 mm. First observations were made in 1994. It is operated jointly by the University of Arizona and the Max-Planck-Institut für Radioastronomie in Bonn, Germany.

Asteroid 624, the largest of the Trojan asteroids, discovered by A. Kopff in 1907. Its brightness varies by a factor of three as it rotates in a period of just under 7 hours. Measurements of the variation indicate that Hektor is cylindrical in shape, 150 km wide by 300 km long. It has been suggested that Hektor may in fact be two asteroids in contact or a close binary.

A small satellite of Saturn discovered in 1980. It is an irregular object, measuring 36 × 30 kilometres.

The rising of a bright star just before the Sun. In practice, the date of heliacal rising would be when the star was first observable in the eastern dawn sky. The heliacal rising of Sirius was used by the ancient Egyptians as a herald of the flooding of the River Nile.

A model of the solar system that places the Sun at the centre with the planets in orbit around it. Although such a system had been suggested as early as c. 200 BC by Aristarchus of Samos, it offered no particular advantage at the time for predicting the positions of the planets and the idea of a moving Earth was philosophically unacceptable. The geocentric model, refined by Ptolemy c. AD 100-170, was in general use until the work of Copernicus 1473-1543. By this time, the idea that the Earth was the centre of the created universe was strongly rooted in religious dogma. In his book De revolutionibus, Copernicus argued the advantages of considering the solar system as Sun-centred. However, the idea did not gain general acceptance until the observational work of Galileo 1564-1642 and Kepler 1571-1630, whose results made better sense in the context of a heliocentric system. In the Copernican system, the planetary orbits were assumed to be circular. This meant that the theory was no more successful on a practical level than the Ptolemaic theory in predicting planetary positions, though it was more elegant and provided a natural explanation for the retrograde motion of the planets. Kepler’s discovery that the planetary orbits are elliptical resolved this problem and the first telescopic observations by Galileo revealed phenomena, such as the phases of Venus, that could be explained only on the basis of a heliocentric model.

annual parallax.

Angular distance on the Sun's surface north (positive) or south (negative) of the Sun's equator. The solar equator, which intersects the ecliptic at an angle of 7° 15', changes its apparent location on the solar disc as the Earth orbits the Sun. Tables are published in handbooks for observers giving the heliographic latitude of the centre of the Sun's disc, from which other latitudes may be calculated.