A reflecting telescope in which the image is brought to a focus just behind a central hole in the primary mirror. The design was proposed in about 1672 by Jacques Cassegrain 1652-1712, professor of physics at Chartres in France, some four years after Isaac Newton constructed the first reflecting telescope. It employs a convex secondary mirror, rather than the flat one in Newton’s own design. Cassegrain did not build a telescope himself and it was some years before his idea was put into practice. Today, the Cassegrain focus is popular and widely used in both modest amateur instruments and large professional telescopes.

The conspicuous gap, 2,600 1,600 miles kilometres wide, between the two main components A and B of the rings of Saturn. See also: planetary rings

A joint NASA/ESA mission to explore the Saturnian system, including the planet, its rings, its magnetosphere and several of its moons. Launch was October 1997. Cassini has been designed to use gravity assist flybys of Venus April 1998 and June 1999, Earth August 1999 and Jupiter December 2000, resulting in arrival at the Saturnian system in 2004. It is intended to operate in orbit around Saturn for four years. One of the major objectives of the mission is a study of Saturn’s moon Titan. Cassini carries the Huygens probe, an instrument package which will parachute down through Titan’s atmosphere and land on the surface. The Huygens probe is the ESA contribution to the mission.

Three empirical laws describing the rotation of the Moon about its centre of mass, stated by Jacques Cassini 1677-1756 in 1721: 1. The Moon rotates eastwards about an axis fixed within it, with constant angular velocity in a period of rotation equal to the mean sidereal period of revolution of the Moon about the Earth. 2. The inclination of the mean plane of the lunar equator to the plane of the ecliptic is constant. 3. The poles of the lunar equator, the ecliptic and the Moon’s orbital plane all lie on one great circle, in that order.

A conspicuous W-shaped constellation near the north celestial pole. It is represented by the seated figure of Queen Cassiopeia of classical mythology, and was among the constellations listed by Ptolemy c. AD 140. It is the site of a supernova observed by Tycho Brahe in 1572 and of the strongest radio source in the sky, a supernova remnant known as Cassiopeia A.

The brightest radio source in the sky other than the Sun, identified as the remnant of a supernova that must have occurred around AD 1667. No records exist of any observation of a supernova at this time. It is assumed that it was obscured by large quantities of dust lying in the line of sight to the object, which is 10,000 light years away. The radio emission is concentrated in a ring-like shape 4 arc minutes in diameter, suggesting a shell of material ejected in an explosion, and some faint nebulosity can be detected in optical photographs of that region of the sky. X-ray emission is also detected in the same ring shape.

The second-brightest star in the constellation Gemini, after Pollux. Its magnitude as seen by the naked eye is 1.6, but this is the combined brightness of a multiple system with at least six components. There are two A stars of magnitudes 2.0 and 2.9 forming a close visual pair, each of which is a spectroscopic binary, and a more distant ninth magnitude red star, which is an eclipsing binary.

A popular name for the planetary nebula NGC 6543. It is about 3,000 light years away and lies in the constellation Draco. Estimated to be about 1,000 years old, its intricate structure suggests that there is a binary star at its centre.

A star whose brightness increases dramatically and suddenly in response to an explosive event. The term is applied particularly to novae and supernovae. See also: flare star, dwarf nova.

Describing optical systems that employ a combination of reflecting and refracting elements. The Schmidt camera is an example of such a system. See also: catoptric, dioptric.

The intersection of the hour circle of zero right ascension of a particular catalogue with the celestial equator. See also: dynamical equinox, equinox.

A chain of craters on a planetary surface.

Describing optical systems that employ only reflecting elements i.e. mirrors. See also: catadioptric, dioptric.

A range of mountains on the Moon forming part of the border of the Mare Imbrium.

A hollow or irregular depression on the surface of Mars.