A large, bright constellation of the southern part of the zodiac, among those listed by Ptolemy c. AD 140. The brightest star is the first magnitude Antares. There are 16 other stars brighter than fourth magnitude. Scorpius is also known as Scorpio, particularly in astrological rather than astronomical contexts.

The brightest X-ray source in the sky and the first to be discovered. It is a low-mass X-ray binary star. The X-rays are thought to originate from a neutron star and an associated accretion disc.

A faint, inconspicuous constellation of the southern hemisphere introduced by Nicolas L. de Lacaille in the mid-eighteenth century. Its four brightest stars are fourth magnitude.

English name for the constellation Sculptor.

A small constellation near the celestial equator, introduced by Johannes Hevelius in the late seventeenth century with the name Scutum Sobieskii in honour of his patron, King John Sobieski III. There are no stars brighter than fourth magnitude though the constellation lies in rich star fields of the Milky Way. Its most notable feature is the star cluster M11, known as the Wild Duck Cluster.

English name for the constellation Capricornus.

English name for the constellation Hydra.

A small galaxy interacting gravitationally with a larger galaxy, NGC 5291. The interaction has distorted the shape of the Seashell to resemble a whelk - hence the name.

Part of a natural cyclical change in the prevailing environmental conditions on the surface of a planet, over the course of a complete orbit round the Sun. A planet experiences seasons if its rotation axis is not at 90° to the plane of the ecliptic. Seasonal effects, particularly on the polar ice caps, are quite marked on the Earth and Mars. Conventionally, four seasons are identified - spring, summer, autumn and winter - but there are no strict divisions between them and seasonal conditions may vary considerably from year to year.

A unit of time measurement defined in the International System of Units Système International, or SI as the duration of 9,192,631,770 cycles of radiation corresponding to the transition between two hyperfine levels of the ground state of the caesium 133 atom. See also: arc second.

In a total or annular eclipse of the Sun, the point when the edges of the Moon’s disc and the Sun’s photosphere are in contact at the start of totality or the annular phase. In a lunar eclipse, second contact occurs when the Moon just enters completely 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.

Continuing, or changing in a non-periodic way, over a long period of time.

The systematic increase in the velocity of the Moon in its orbit around the Earth, a result of tidal interaction with the Earth and the gravitational attraction of other planets.

The angular displacement over time of a star’s position caused by the Sun’s motion through space relative to the local standard of rest. Measurement of the secular parallax provides a method of determining the distance to nearby star groups, on the assumption that the individual motions of the stars in such a group are random with an average of zero. The Sun’s relative velocity, also needed in the calculation, is 19.5 km/s, which amounts to 4.11 AU per year.

A rock made of fragments of pre-existing rock or, on Earth, the hard parts of dead organisms that have been deposited as a sheet, for example on a sea-bed.