The smallest image a particular telescope can make of a point source of light, such as a star. Diffraction as the light passes through the telescope aperture causes the image of even a point source to have a finite size. The diameter of the Airy disc is smaller for larger apertures. In practice, it is rarely possible to achieve images as small as the Airy disc because of turbulence in the Earth’s atmosphere, which distorts and enlarges the perceived image.

A catalogue of over 17,000 binary stars compiled by Robert G. Aitken and published in 1932. Its formal title is New General Catalogue of Double Stars.

A common abbreviation for the Astronomical Journal.

Asteroid 702, diameter 202 km, discovered by J. Helffrich in 1910.

The proportion of incident radiation that is reflected by a non-self-illuminating body. Albedo is measured in a scale from 0 to 1, 0 referring to a perfectly absorbing black object, and 1 referring to a perfectly reflecting white object. Spherical albedo assumes that the object is a perfect sphere, with a diffuse surface that reflects incoming parallel light in all directions. Bond albedo is the fraction of the total incident energy reflected in all directions and calculated over all wavelengths. Geometrical albedo is the ratio of the reflectance of the object to the reflectance that would be produced by a flat white surface of the same surface area at the same distance.

Asteroid 719, diameter 2.6 km. It was discovered by J. Palisa in 1911, when it made a close approach to the Earth, but is now lost.

The second-brightest star in the constellation Cygnus. Visual observers regard it as one of the most beautiful double stars. The primary star is a yellow-orange K star, a giant of magnitude 3.2, and its companion a bluish B star of magnitude 5.4. The two components are separated by 35 arc seconds.

Alternative form of Alkaid.

The forerunner of scientific chemistry. In Arabic science, astronomy and alchemy were closely related. Jabir ibn Hayyan, who died c. 803 CE, devised distillation techniques that led to the discovery of new substances such as nitric acid and alcohol. Medieval European alchemy, as well as metallurgical chemistry, were built upon Arabic foundations. It was believed that the Sun, Moon and planets formed gold, silver and other metals in the Earth. Alchemists believed that they could speed up this process in their laboratories.

The fourth magnitude A star that forms a naked-eye double with Mizar in the bear’s tail. The two stars are separated by 11.5 arc minutes on the sky and 10 light years in space.

The brightest member of the Pleiades star cluster in the constellation Taurus. The name is that of the daughter of Atlas and Pleione in Greek mythology. Alcyone is a B star of magnitude 2.9.

The brightest star in the constellation Taurus. Its Arabic name means the follower. Aldebaran is a giant K star of magnitude 0.9. Although it appears in the sky to be part of the Hyades star cluster, it is not in fact a cluster member, lying only half as far away.

The brightest star in the constellation Cepheus. It is an A star of magnitude 2.7. The name, which is of Arabic origin, means the right arm.

An American astronaut. On July 20, 1969, Buzz Aldrin became the second person on the moon, after Neil Armstrong, on Apollo 11. Aldrin arrived on the moon 15 minutes after Armstrong. Once on the moon, Armstrong and Aldrin set up seismic, solar wind and laser-ranging experiments, collected samples and took photographs. They conducted a short telephone conversation with President Nixon, and they unveiled a plaque on one of the legs of the lunar module celebrating Apollo’s achievement for all mankind.

Magnetic waves that can propagate through an electrically conducting fluid, such as an ionized gas, in a magnetic field.