Asteroid 704, diameter 338 km, discovered in 1910 by V. Cerulli. It is the sixth-largest asteroid known.

A filter that uses the phenomenon of interference in a thin film to transmit light selectively over a narrow wavelength band. See also: interferometer.

In astronomy, an instrument in which the electromagnetic radiation from a celestial object is collected along two or more different path lengths and then brought together to form an interference pattern. When any electromagnetic wave is sampled it has amplitude how strong the wave is and phase the point the wave pattern has reached between peaks and troughs. If two light beams from the same point source have travelled slightly different paths and are then brought together, an interference pattern is formed. Where the two sets of waves are in phase, the pattern is bright, and where they cancel it is dark. Astronomical interferometers exploit this basic physical principle as a way of increasing resolving power. A star image, for example, is a disc and not a point source. With a pair of mirrors there will be a critical separation at which the interference pattern disappears because the bright fringes from one side of the disc coincide with dark fringes from the other side. The first successful astronomical application of this principle was the Michelson stellar interferometer. In basic radio interferometry, telescopes are used in pairs and the resulting interference patterns analysed by computer. By using more than two elements e.g. mirrors or antennas in an interferometer, it is possible to produce high-resolution maps or images, a technique often known as aperture synthesis. Interferometry has been an essential observational tool in radio astronomy for decades. More recently it has become feasible to extend the technique fully into the infrared and optical wavebands. A number of dedicated instruments for optical interferometry have been constructed, such as the Cambridge Optical Aperture Synthesis Telescope in the UK and the Navy Prototype Optical Interferometer in the USA. In addition, several projects for the construction of very large telescopes have been specifically designed so that optical interferometry can be carried out. These include the Keck Observatory, the Very Large Telescope and the Large Binocular Telescope. See also: radio interferometer, very-long-baseline interferometry.

intracluster medium.

polar.

An organization formed in 1919 for fostering international cooperation in astronomy. It is composed of member countries represented by national academies or similar institutions, not governments and around 8,000 individual members. Along with similar organizations for other branches of science, it belongs to the International Council of Scientific Unions ICSU, which has its headquarters in Paris. The history of the IAU may be traced back to the international cooperation established for the Carte du Ciel project. From 1887 the Permanent Commission on the Photographic Carte du Ciel extended its interest into other areas of astronomy and may be regarded as the parent organization of the IAU. The IAU is recognized as the international authority on astronomical matters requiring cooperation and standardization, such as the official naming of astronomical bodies and features on them. The Central Bureau for Astronomical Telegrams and the Minor Planets Center located at the Smithsonian Astrophysical Observatory operate under its auspices. The IAU is also concerned with the promotion of astronomy in developing countries. Its General Assembly meets every three years, and specialist symposia and colloquia are sponsored regularly. Work is organized through Commissions and Working Groups.

A continuous timescale resulting from analyses by the Bureau International des Poids et Mesures of atomic time standards in many countries. See also: atomic clock.

A spacecraft, originally known as ISEE-3 International Sun-Earth Explorer when it was launched in 1978, which was diverted and reactivated in order to pass through the tail of Comet Giacobini-Zinner in September 1985, and to observe Halley’s Comet from 28 million kilometres 17 million miles in March 1986.

An organization based in the USA, established to combat light pollution.

A European/Russian orbiting observatory for spectroscopy and accurate imaging of gamma-ray sources. Launch by a Russian Proton launcher is planned for 2001.

The period from 1 July 1957 to 31 December 1958 during which an internationally coordinated programme of geophysical research was organized to coincide with a period of maximum solar activity.

Name of three spacecraft built by NASA and ESA to study the influence of the Sun on the Earth’s space environment and magnetosphere. ISEE-1 and 2 were launched in 1977. ISEE-3, launched in 1978, later became the International Cometary Explorer ICE.

An astronomical telescope with a 45-centimetre 18-inch primary mirror, designed to work in the ultraviolet region of the spectrum, which was launched into Earth orbit in 1978. A joint NASA-ESA-UK project, it continued to observe successfully for 18 years, finally ceasing operations in September 1996.

The period 1964-5 when, following the success of the earlier International Geophysical Year, an international programme of geophysical research was coordinated to coincide with a period of minimum solar activity.

Micrometeoroids present in the space between the planets. The particles are thought to originate in collisions between asteroids in the asteroid belt and from the gradual break-up of comets. Fragments from comets initially form meteor streams, but will disperse over long periods of time. The zodiacal dust cloud extends out from the centre of the solar system for at least 600 million kilometres 373 million miles. Its presence is observed as the zodiacal light: sunlight scattered by dust particles a few tens of micrometres in size. Its very low density is equivalent to one grain in a cube hundreds of metres across at the Earth’s distance from the Sun. Near the Sun, the particles are eroded by collisions, evaporation and the action of the solar wind. The Infrared Astronomical Satellite IRAS located three dust bands between Mars and Jupiter in the asteroid belt. The brightest is centred on the ecliptic, but there are fainter bands 10° to the north and south of the ecliptic. These are thought to consist of dust particles in orbits tilted at about 10° to the ecliptic. See also: Poynting-Robertson effect.