Asteroid 349, diameter 164 km, discovered by A. Charlois in 1892. It belongs to the rare class of R-type asteroids and is a member of the Budrosa family.

Algol.

The brightest star in the constellation Cygnus. It is a supergiant A star of magnitude 1.3. The Arabic name means tail.

An A star of magnitude 2.1, the third-brightest in the constellation Leo. The name, of Arabic origin, means the lion’s tail.

In cosmological theory, the ratio of the total density of the universe to the critical density.

The passage of a compressed region through a material medium while the elements of the material itself move by only a relatively small amount around their average positions. Sound waves are a common example of density waves. A density-wave theory has been proposed to explain the structure of spiral galaxies. According to the theory, spiral arms are not permanent structures but regions where stars and interstellar matter concentrate as the density wave passes through.

ascending node.

One of the small satellites of Uranus discovered during the Voyager 2 encounter with the planet in 1986.

A large lunar crater, diameter 234 kilometres 145 miles, on the southern border of the Mare Nubium. Its walls are overlapped by several other craters, including Regiomontanus, Walter and Lexell. The crater Hell lies within it.

A satellite of Neptune 1989 N3 discovered during the flyby of Voyager 2 in August 1989.

A binary star system in which neither star fills its Roche lobe. See also: semi-detached system, contact binary.

The element of an instrument system that is sensitive to the incoming radiation or particles to be observed.

An extension fixed to a telescope tube to inhibit the condensation of water droplets on the optical elements. A simple tube extension works by lowering the cooling effect of direct air currents. A more sophisticated dew cap may incorporate a small electric heating element.

An attachment for a small telescope, containing a small plane mirror or prism, used to turn through a right angle the beam of light in the telescope and the direction of the draw-tube into which the eyepiece is inserted. Use of a diagonal may be helpful when the visual user of a small amateur telescope would have difficulty accessing the eyepiece in its normal tube. However, there is some loss of light when a further optical element is introduced, and there may also be loss of image quality. The image in a diagonal is reversed right-to-left.

In a Newtonian telescope, the flat secondary mirror, which is positioned diagonally i.e. at an angle of 45° to the optical axis of the telescope.