The extent, measured as an angle, to which the illuminated area of the full Moon differs from a complete circular disc because of the inclination of the Moon’s orbit to the ecliptic.

The ratio between the Bond albedo and the geometric albedo of an asteroid or other planetary body.

The third-brightest star in the constellation Ursa Major. It is an A star of magnitude 2.4.

The inner of the two small satellites of Mars, discovered by Asaph Hall in 1877. Images from the Viking spacecraft of 1977 show Phobos to be ellipsoidal in shape 28 × 20 km and covered with craters. The largest, Stickney, is 10 kilometres in diameter, one-third of the satellite’s largest dimension. A series of striae emanating from Stickney appear to be fractures caused by the impact that created the crater.

Two Soviet space missions to Mars launched in 1988. Contact with one was lost en route and the other operated only briefly, returning limited data.

A group of asteroids at a distance of 2.36 AU from the Sun, with orbits inclined at 24° to the plane of the solar system. The group is separated from the main belt by one of the Kirkwood gaps and is not a true family with a common origin. The group is named after 25 Phocaea, diameter about 70 km 45 miles.

The outermost satellite of Saturn, discovered by W. Pickering in 1898. Phoebe appears to be spherical, 220 kilometres in diameter, and of very dark material that reflects less than 5 per cent of incident light.

A southern constellation introduced in the 1603 star atlas of Johann Bayer. Though not particularly conspicuous, it does contain seven stars brighter than fourth magnitude.

Asteroid 5145, diameter 190 km, discovered in 1991. It follows a highly unusual, remote orbit, on which it ranges between 8.7 and 32 AU from the Sun. With Chiron, and five other asteroids in orbits with similar characteristics, it forms the group termed Centaurs. Pholus has a low albedo of 4.4 per cent, and is very much redder in colour than typical asteroids.

The ionization of an atom by the absorption of a photon of electromagnetic radiation. Ionization can take place only if the photon carries at least the energy corresponding to the ionization potential of the atom, i.e. the minimum energy required to overcome the force binding the electron within the atom.

Making use of the photoelectric effect, whereby electrons are liberated from the surface of a solid material when it is struck by photons of electromagnetic radiation. Since the number of electrons emitted is proportional to the intensity of radiation, photoelectric detectors provide an effective method of measuring light intensity.

The accurate measurement of the magnitudes of stars by means of a photomultiplier or CCD. See also: photometry.

magnitude.

A special telescope, mounted vertically, used for the very accurate determination of the positions of stars and for monitoring irregular changes in latitude and time, arising from polar motion and the Earth’s rotation.

The accurate determination of the magnitudes of stars, or other astronomical objects, within specified wavelength bands. Photometric measurements can be used to deduce broad physical characteristics of stars, without the need for detailed study of their spectra. Several photometric systems are used for this purpose, most commonly UBV photometry and uvby photometry. Photometric measurements are also important for determining the light curves of variable stars. Photometry is carried out using photoelectric measurements; it is very difficult to determine magnitudes accurately from photographs or visually. Photoelectric measurements also allow monitoring of light intensity changes over very short timescales.