The ratio of the focal length of a lens, mirror or compound optical system to its aperture. It is normally written using the prefix f: for example, f/10 means a focal ratio of 10.

A term used to describe the position of a planetary feature or an object on the celestial sphere relative to another. Following features or objects come into view later than the reference feature or object; those coming into view earlier are described as preceding p.

The brightest star in the constellation Piscis Austrinus, an A star of apparent visual magnitude 1.2. The name, of Arabic origin, means the fish’s mouth.

Minkowski’s Footprint.

spectral lines, not normally observed under laboratory conditions because they have an intrinsically low probability of occurrence, or because they result from a transition between a metastable excited state and the ground state. Under typical conditions, an atom in a metastable state will lose energy through a collision before it is able to decay to the ground state. Under astrophysical conditions, however, where there are huge numbers of atoms and densities may be very low, it is possible for such forbidden transitions to occur and to produce strong spectral lines.

A particular form of equatorial mounting.

A small, inconspicuous constellation of the southern sky. It was introduced in the mid-eighteenth century by Nicolas L. de Lacaille with the longer name, Fornax Chemica - the chemical furnace. None of its stars are brighter than fourth magnitude.

A strong radio source in the constellation Fornax associated with the spiral galaxy NGC 1316.

The second-brightest globular cluster in the sky after Omega Centauri. To the naked eye it appears as a fifth magnitude star; this explains why it was originally designated 47 Tucanae, a form of name appropriate to a single star. It lies at a distance of 13,000 light years and is a relatively young globular cluster.

A long, narrow, shallow depression on a planetary surface.

A technique for testing the accuracy with which the surface of a mirror or lens is figured. It was introduced by J. B. L. Foucault 1819-68 in 1851. The method utilizes the fact that a knife-edge moved slowly across the focal plane will cause the image of a beam of light to darken uniformly only if the imaging surface is perfectly spherical: any depressed or raised areas on the lens or mirror cause light and dark zones to appear in the image as the test is carried out.

A long, free-swinging pendulum devised in 1851 by Foucault to demonstrate the rotation of the Earth. If undisturbed, the plane in which the pendulum swings slowly changes at a rate of 15 × sinangle of latitude degrees per hour of sidereal time.

In an eclipse of the Sun, the point when the Moon’s disc finally moves clear of the Sun’s photosphere. In a lunar eclipse, fourth contact occurs when the Moon leaves the full shadow umbra of the Earth. The term may also be used to describe the similar stage in the progress of a transit or occultation.

English name for the constellation Vulpecula.

A lunar crater, 95 kilometres 60 miles in diameter, close to the Apollo 14 landing site. It gives its name to the geological formation sampled by the Apollo 14 astronauts, this being part of the ejecta or debris created by the formation of the Imbrium Basin. See also: Apollo programme.