One of the two axes about which an equatorially mounted telescope can rotate the other being the polar axis. Movement around the declination axis allows the telescope to point to different declinations while the right ascension remains constant. See also: equatorial mounting.

A graduated disc or circle fitted to an equatorial mounting in order to indicate the declination coordinate of the direction in which the telescope is pointing.

The era in the evolution of the universe, about 300,000 years after the Big Bang, when electrons and protons began to combine to form hydrogen atoms. At this point, the radiation in the universe ceased to scatter material particles and began to propagate freely. In other words, the radiation and the matter became decoupled. This phase is also called the recombination epoch.

An object of astronomical study that does not belong to the solar system. The expression is not precisely defined and is used primarily by amateur astronomers as an umbrella term for both galactic and extragalactic objects, such as nebulae and galaxies.

A ground-based network of radio dishes used by NASA to communicate with space missions to solar system destinations beyond the Moon. The three 70-metre 230-foot antennas are located at Goldstone in California, near Madrid in Spain and near Canberra in Australia.

The proportion of the disc of a planetary body, measured as an angle, that is not illuminated as viewed by an observer on Earth.

A basic circular orbit, which, in combination with the epicycle, was necessary to the theory of planetary motions developed by Ptolemy in the second century AD. The planets were supposed to travel uniformly around a small, circular epicycle, the centre of which in turn moved around a larger circle, called the deferent.

A term that covers both white dwarfs and neutron stars, which are made up of degenerate matter. These stars are in an advanced state of evolution and have suffered extreme gravitational collapse. Normal atoms cannot exist under the conditions of very high pressure. In white dwarfs, the electrons and atomic nuclei collapse from the normal open atomic structure into a dense, compressed mass. A quantum-mechanical effect called degeneracy pressure counters further gravitational collapse. However, if the total mass of the star exceeds 1.4 times the mass of the Sun, the degeneracy pressure is insufficient to balance the gravitational force and a neutron star results. Electrons and nuclei combine into a form of matter consisting of tightly packed neutrons. See also: black hole.

One of the two small satellites of Mars, discovered in 1877 by Asaph Hall. Images obtained by the Viking 2 Mars mission show a heavily cratered surface. The low albedo and properties of the surface suggest that both satellites have a composition similar to that of the carbonaceous chondrite meteorites, and it has been suggested that they are captured asteroids.

Comet Delavan.

A small faint but distinctive constellation lying in the Milky Way just north of the celestial equator. It is one of the constellations listed by Ptolemy c. AD 140.

Aquarids

A yellow giant star in the constellation Cepheus that varies in brightness between magnitudes 3.6 and 4.3 over a period of 5.37 days, and is the prototype of the Cepheid variable stars.

A member of a group of luminous stars of spectral type A and F, characterized by weak absorption lines of calcium in their spectra. The star Delta Delphini is the brightest of the group and regarded as the prototype. Some are low-amplitude, short-period variables similar to the Delta Scuti stars, and they may also be related to the Am stars.

A short-period pulsating variable star. Delta Scuti, found to be variable in 1935, is the prototype of the group. Typically, the periods are less than eight hours and the variation amounts to only a few hundredths of a magnitude, which is imperceptible to the naked eye. Stars of this type are also known as dwarf Cepheids or AI Velorum stars.