A B star of magnitude 2.8 that marks one corner of the Square of Pegasus. The Arabic name means the side. The name is also sometimes applied to the star Alpha Persei, more usually known as Mirfak.

A visual binary star consisting of two yellow giants, separated by 4 arc seconds. The magnitudes are 2.3 and 3.5, and they orbit around each other in a period of 620 years.

A star in the constellation Perseus, also known as Beta Persei. Algol is a prototype of eclipsing binaries, close double stars that periodically pass in front of each other as they move along their orbits, temporarily blocking the light from the other star. Every 2 days and 21 hours, Algol, which is normally a 2nd magnitude star, dips to one third of its usual brightness and remains there for ten hours as its dimmer companion partially eclipses it, blocking some of its light.

Perhaps the best-known variable star, its magnitude varying between 2.2 and 3.5 in a period of 2.87 days. The variation occurs because Algol is an eclipsing binary system in which the two stars regularly cross in front of each other as viewed from Earth. Algol is regarded as the prototype eclipsing binary. The brighter of the components of Algol is a B star and the fainter a G star. The decline in brightness as the G star cuts off the light from its brighter companion takes four hours, and minimum lasts only twenty minutes. The secondary eclipse, when the fainter star causes a dip in brightness of only 0.06 magnitude, is not detectable by eye. Variations in the spectrum of Algol with a period of 1.862 years reveal the presence of a third star in the system. There is also evidence in the spectrum for mass transfer between the two close companions, supported by observations that Algol is a radio star, erratically flaring up to twenty times its normal radio brightness. The radio emission is attributed to gas streaming on to the primary star.

A Canadian radio astronomy observatory in Ontario. The main instrument is a 46-metre 150-foot fully steerable dish.

An Arabic scientist. Alhazen did fundamental work in optics, building on Aristotle’s Meteorologia. Alhazen dissected the eyes of cattle to reveal their optical structures, made simple lenses, projected images with a camera oscura, and studied the way white light broke down into different colors when refracted.

A movable sighting arm on an astrolabe that is used in making observations of the altitudes and azimuths of celestial objects.

Asteroid 887, diameter 4 km. It was discovered in 1918 by Max Wolf when it made a close approach to the Earth. It is a member of the Amor group.

The brightest star in the constellation Ursa Major, the Greek letters in this case being allotted in order of position rather than of brightness. Alioth is an A star of magnitude 1.8.

A star in Ursa Major, at the end of the bear’s tail. It is a B star of magnitude 1.9. The Arabic name means chief of the mourners, for the Arabs saw the constellation as a bier rather than a bear.

A camera with a very wide-angle, fish-eye lens, capable of photographing all or most of the visible hemisphere of the sky on one exposure. Cameras of this type have applications in routine surveys for meteors and artificial satellites.

A region in Antarctica from where large numbers of meteorites have been recovered. The meteorites become concentrated in the area by natural movements in the ice sheet, and are relatively easy to identify against the ice.

A research observatory of the University of Pittsburgh, Pennsylvania. The present building dates from 1912 but its predecessor was started in 1858 by several Pittsburgh businessmen. Inspired by Donati’s Comet of that year, they formed the Allegheny Telescope Association and acquired a 33-centimetre 13-inch refractor. In 1867, the telescope and observatory were given to Western University in Pennsylvania, the forerunner of the University of Pittsburgh. The first salaried director was Samuel Pierpont Langley. He was succeeded by James E. Keeler, co-founder of the Astrophysical Journal and subsequently director of the Lick Observatory. The 1912 building is equipped with three telescopes. The original 33-centimetre refractor is used primarily for educational and testing purposes. The others are the 76-centimetre 30-inch Thaw refractor and the 79-centimetre 31-inch Keeler Memorial Reflector. These continue to be used for research.

A meteorite of the carbonaceous chondrite type, which fell in Mexico in 1969. More than two tonnes of material fell, scattering over an area 48 by 7 kilometres, making it one of the most massive carbonaceous chondrites known.

A large astronomical treatise by the Greek astronomer Ptolemy Claudius Ptolemaeus, who worked in Alexandria between about AD 127 and 151. The name is an Arabic corruption of a Greek title, The Greatest, though Ptolemy’s original title was The Mathematical Collection. It ranks among the most important works on astronomy ever written. It included a star catalogue and dealt with the motion of the Moon and planets. The rules set out for calculating the future positions of the planets, on the basis of an Earth-centred universe, were used for centuries.