The effect of random turbulent motion in the atmosphere on the quality of the image of an astronomical object. In conditions of good seeing, images are sharp and steady; in poor seeing, they are extended and blurred and appear to be in constant motion. Amateur astronomers sometimes use a scale of seeing quality indicated by Roman numerals. I is the best, II-III typical, IV poor and V extremely bad. The scale is due to Eugenios Antoniadi 1870-1944. Seeing may also be described quantitatively in arc seconds.

Centred on the Moon.

The study of the surface features and topography of the Moon.

The study of lunar rocks and of the surface and interior of the Moon: the lunar equivalent of geology.

A binary star system in which one star has expanded to the extent that its outer layers are pulled away by the gravitational attraction of the companion star. Material streams on to the companion from the larger star, which is described as having filled its Roche lobe.

Half the length of the maximum dimension of an ellipse. See also: orbital elements.

A member of the group of pulsating variable stars that show some periodicity in their variations but are nevertheless unpredictable. The magnitude range is typically only one or two magnitudes, and the periodicity can range from a few days to several years. A number of subgroups have been distinguished. Types A and B are red giants, type C red supergiants and type D giants and supergiants of spectral types F, G and K. An example is Mu μ Cephei, known as the Garnet Star.

The angular distance between two stars in a visual binary star system, measured in arc seconds. It is one of two standard measurements used to record the relative positions of the members of a binary system, the other being the position angle.

One of the 48 constellations listed by Ptolemy c. AD 140 and unique in that it is split into two parts, one either side of Ophiuchus The Serpent Bearer. The two parts are known as Serpens Caput the head and Serpens Cauda the tail. In all, Serpens contains nine stars brighter than fourth magnitude.

English name for the constellation Serpens.

English name for the constellation Ophiuchus.

A structure for the open tube of a large reflecting telescope, designed to allow equal degrees of flexure when the orientation of the telescope is changed. It is impossible to make a completely rigid tube for the largest telescopes. The solution proposed by Mark Serrurier for the design of the 200-inch Hale Telescope tube was to allow flexure in such a way that the optical alignment is not disturbed.

Abbreviation for Swedish-ESO Submillimetre Telescope. See also: submillimetre wave astronomy.

Abbreviation for Search for Extraterrestrial Intelligence. It is used as a general term rather than for any particular experiment.

Graduated scales attached to the rotation axes of a telescope mounting that indicate the celestial coordinates of the point in the sky at which the telescope is pointed.