The second-brightest star in the constellation Pegasus. It is a supergiant M star and varies in brightness between magnitudes 2.4 and 2.8. The name is derived from Arabic and probably means shoulder.

The brightest star in the constellation Cassiopeia. It is a supergiant K star with a magnitude near 2.2, though it is slightly variable. The Arabic name means breast.

A large lunar crater, 227 kilometres 141 miles in diameter, near the Moon’s south-west limb. The darkness of the crater floor indicates that it may be flooded by lava.

A type of wide-field astronomical telescope, designed purely for photographic use. It was invented by Bernhard Schmidt in 1930. The light collector is a spherical mirror. Correction for spherical aberration is achieved by means of a thin glass corrector plate with a complex profile, placed at the end of the telescope tube, beyond the focal point. The photographic plate is placed at the prime focus, where the focal plane is curved. A special plate holder is used to bend the photographic plate into the focal plane. By this means, sharp undistorted images can be obtained over very wide fields of view - up to tens of degrees across.

A design of optical telescope, incorporating features of both the Schmidt camera and the Cassegrain reflector. The Schmidt-Cassegrain employs a spherical primary mirror and a corrector plate to compensate for spherical aberration, as in the Schmidt camera. However, the prime-focus plate holder is replaced by a small convex secondary mirror, which reflects the light back down the tube and through a hole in the primary mirror. The image can thus be viewed visually, or a camera can be mounted at the back of the main tube. The resulting telescope is very compact, so the design is particularly suitable for portable telescopes and amateur and educational use.

A discrepancy between the observed and predicted phase of Venus around the time of dichotomy half-phase. Eastern dichotomy evening apparitions, when Venus is waning is usually a few days early, and western dichotomy morning apparitions, when Venus is waxing a few days late. The cause is not known for certain, but it may simply be that the region of the terminator the division between night and day is less bright than the rest of the illuminated hemisphere of Venus. Johann Schröter 1745-1816 was the first to draw attention to the phenomenon in 1793.

A winding valley in the Oceanus Procellarum on the Moon. It starts in a small crater just outside the northern wall of the crater Herodotus, and extends for about two hundred kilometres.

The metric of spacetime used in the presence of a spherical mass. This mathematical description of the gravitational field of a spherical mass, calculated by Karl Schwarzschild in 1915, did not find general application until the 1950s when it was revived by cosmologists. It leads directly to the concept of black holes.

The critical radius at which the spacetime surrounding a sphere becomes so curved that it wraps round to enclose the body. An object that has collapsed inside its Schwarzschild radius is a black hole, from which nothing can escape into the outside world. The Schwarzschild radius for an object the mass of the Sun is 3 kilometres; for an object the mass of the Earth it is 1 centimetre.

Comet Schwassmann-Wachmann 1.

Twinkling - the rapid variations in the brightness of a star caused by random refraction in turbulent layers of the Earth’s atmosphere. A similar phenomenon affecting radio signals from celestial sources occurs in the Earth’s ionosphere and also in the ionized gas in both the interplanetary and interstellar media.

A radiation detector used in astronomy to detect gamma rays. The device uses crystals that emit flashes of light when gamma-ray photons strike them. Each flash, or scintillation, is picked up by a photomultiplier, the signal from which is then a measure of the flux of gamma rays on the counter.

A term used for a lobate or irregular scarp on a planetary surface.

Alternative chiefly astrological name for the constellation Scorpius.

English name for the constellation Scorpius.