A book of tables giving the future positions of the Moon, planets and other celestial objects, often compiled with additional information of practical value to users. An almanac normally covers the period of one calendar year.

A circle on the celestial sphere parallel to the horizon.

An instrument for measuring altitude and azimuth.

A variant spelling of the star name Elnath.

One of the three bright stars forming Orion’s belt. The Arabic name means string of pearls. Alnilam is a supergiant B star of magnitude 1.7.

One of the three bright stars forming Orion’s belt. The Arabic name means the girdle. Alnitak is a supergiant O star of magnitude 1.8.

The third brightest star in the sky, part of a multiple system of three stars. In even the smallest telescopes, it appears as a double star. The brighter of these is almost identical in size and temperature to the Sun. The other, which orbits it every 80 years, is smaller and more yellow. Alpha Centauri lies 4.4 light-years away. The third star in this multiple system, Proxima Centauri, an 11th magnitude red dwarf, is the closest star to the Sun, 4.2 light-years away.

The brightest star in the constellation Centaurus and the nearest bright star to the Sun, at a distance of 4.34 light years. It is a visual binary star with an orbital period of 80 years. The two components are of spectral types G and K and have a combined magnitude of -0.27. The eleventh magnitude star Proxima Centauri, though two degrees away on the sky, is thought to be associated with this star system because it has a similar motion in space. Proxima, a dim M star, is the nearest star to the Sun at a distance of 4.24 light years. Alpha Centauri is also called by the Arabic name Rigil Kentaurus sometimes Rigel, or shortened to Rigil Kent, which means the foot of the Centaur. An alternative name is Toliman.

The nucleus of a helium atom, consisting of two protons and two neutrons. Alpha particles are emitted by many radioactive isotopes and also play an important role in nuclear fusion processes within stars.

The brightest star in the constellation Hydra. Its Arabic name means the solitary one of the serpent. It is a K star of magnitude 2.0.

The brightest star in the constellation Corona Borealis. It is an A star of magnitude 2.2. The Arabic name, also spelt Alphecca, means bright one. This star is sometimes called by the Latin name Gemma, the jewel in the crown.

The brightest star in the constellation Andromeda, marking one corner of the Square of Pegasus. It was formerly considered to belong to that constellation, being designated Delta Pegasi. Alpheratz is an A star of magnitude 2.1.

A set of tables giving the positions of the Sun, Moon and planets, published in 1252 under the patronage of King Alphonso X of Castile. They were computed by a team of astronomers using the principles set out by Ptolemy in the Almagest but incorporating more recent observations. They were in use in Europe for nearly 400 years, until superseded by the work of Johannes Kepler, during which time they were the best available.

A lunar crater, 118 kilometres 73 miles in diameter. A prominent ridge runs across the centre, almost along a north-south line, through a central peak about one kilometre high. Temporary reddish clouds were observed there in 1958 and 1959, possibly due to the release of gas from the rocks. See also: transient lunar phenomenon.

A flat-bottomed valley, 150 kilometres 95 miles long, crossing the lunar Alps and connecting the Mare Frigoris with the Mare Imbrium.