The largest and youngest of the very large circular impact features on the Moon. It was subsequently flooded by lava to form the dark area known as the Mare Imbrium, which is 1,300 kilometres 800 miles in diameter. The Imbrium Basin is surrounded by three concentric rings of mountains, though only the outer one is defined with any clarity, by the Carpathian, Apennine and Caucasus mountains. The Alps form part of the second ring.

Abbreviation for initial mass function.

The disappearance of a star, planet, moon or other body at the beginning of an occultation or eclipse.

One of the principal orbital elements used to define an orbit, giving the angle between the orbital plane and a reference plane. For the orbits of planets and comets around the Sun, for example, the reference plane is the ecliptic. For satellite orbits, it is the plane of the equator of the parent planet. The term inclination is also used for the angle between a body’s rotation axis and a reference plane, normally the body’s orbital plane.

A supplement to the New General Catalogue NGC of nebulae and star clusters, compiled and published by J. L. E. Dreyer in two parts, in 1895 and 1908.

English name for the constellation Indus.

An inconspicuous southern constellation, representing an American native Indian. It was introduced in the 1603 star atlas of Johann Bayer and contains no stars brighter than the third magnitude.

An irregularity in orbital motion.

The position of either of the planets Mercury or Venus when it lies directly between the Earth and the Sun. Because of the relative tilts of the planetary orbits, only rarely does Mercury or Venus actually transit the face of the Sun. Normally, at inferior conjunctions, the planets pass in the sky to the north or south of the Sun.

The crossing of the meridian by a circumpolar star when at its lowest point in the sky.

Either of the two planets Mercury or Venus, whose orbits around the Sun lie inside that of the Earth.

A class of Big Bang models of the universe that include a finite period of accelerated expansion in their early histories. Such an event would have released enormous energy, stored until then in the vacuum of spacetime. The horizon of the universe expanded, temporarily, much faster than the speed of light. This theory is able to account satisfactorily for the present vast extent of the universe and its uniformity.

A two-dimensional infrared imaging device, consisting of an array of small, individual electronic detectors, each of which records a pixel in the image.

A highly successful orbiting infrared telescope which operated from launch on the night of 25 January 1983 until the supply of coolant ran out on 23 November 1983. It was a collaborative mission between NASA, which designed and built the telescope, the Netherlands Aerospace Agency, which was responsible for the basic satellite, and the UK, which was responsible for the day-to-day tracking and data reception. IRAS was a Ritchey-Chrétien telescope with mirrors made of beryllium rather than glass in order to withstand the low operating temperature. The diameter of the primary was 57 centimetres 22.5 inches. The telescope was cooled to 2 K by means of liquid helium. The detector consisted of an array of 62 elements, and filters were used for operation in wavebands centred on 12, 25, 60 and 100 micrometres. The different wavebands were used to distinguish between sources at different temperatures. The satellite orbit was oriented in the north-south direction and devised to rotate by about 1° per day so that it was always along the terminator with the telescope pointing away from the Sun. During the ten-month mission, 96 per cent of the sky was scanned twice so that an overall map of the infrared sky could be plotted. In addition, there was a spectrometer and a mapping facility for individual sources. A quarter of a million individual sources were detected, including stars, galaxies, dense interstellar dust clouds and some unidentified objects. Five comets were discovered. The first and brightest, Comet IRAS-Araki-Alcock, discovered in May 1983, passed within five million kilometres 3 million miles of the Earth - the closest approach of any comet for 200 years.

The study of infrared radiation from astronomical sources. Infrared is electromagnetic radiation with wavelengths in a range between the visible red spectrum and radio waves. The definition is not precise but the wavelength range from about 0.1 to about 100 micrometres is normally considered to be the infrared. It is invisible to the human eye and is absorbed almost completely in the lower layers of the Earth’s atmosphere, primarily by water vapour. For this reason, infrared astronomy observations have to be conducted from the highest mountain sites, or from aircraft or satellites. The first infrared observation was made accidentally by William Herschel in 1800 when a thermometer he placed just to one side of the red end of a visible solar spectrum recorded a rise in temperature. Infrared images predominantly show the distribution of heat. All warm objects radiate infrared, so infrared telescopes must be cooled to a few degrees above absolute zero to prevent their being blinded by their own emission. The development of systematic infrared astronomy began in the 1960s, when suitable detectors became available. The first infrared survey of the sky was carried out by Gerry Neugebauer and Robert Leighton of the California Institute of Astronomy Caltech. They published a list of 5,612 sources in 1969. In 1968, Eric Becklin and Neugebauer announced that the infrared emission from the galactic centre at a wavelength of 2.2 micrometres is more than a thousand times stronger than might have been anticipated from radio observations. Infrared astronomy has made important advances with the development since the 1980s of two-dimensional arrays of infrared detectors, capable of making a complete image in a single exposure. Infrared astronomy received an enormous boost from the successful operation of IRAS, the Infrared Astronomical Satellite, in 1983. Its successor, the Infrared Space Observatory ISO was launched in November 1995. The best ground-based site for infrared astronomy is the Mauna Kea Observatories in Hawaii. Three infrared telescopes started operation there in 1979: the United Kingdom Infrared Telescope UKIRT, NASA’s Infrared Telescope Facility IRTF and the Canada-France-Hawaii Telescope CFHT, which also functions as an optical telescope. The telescopes of the Keck Observatory also have infrared capability. Infrared radiation is detected from stars and galaxies, and from dust clouds within the solar system and in the interstellar medium. Strong infrared emission is particularly characteristic of dust that has been heated by shorter-wavelength visible and ultraviolet radiation from stars. Protostars in process of formation and evolved red giant stars are surrounded by shells of dust, giving rise to infrared emission. Unlike visible light, infrared radiation passes relatively unimpeded through dust clouds. So, for example, the galactic centre, which is largely obscured by dust in the visible spectrum, can be explored by means of infrared and radio astronomy. The way in which infrared radiation is scattered from the surfaces of objects in the solar system provides important clues to their composition. Infrared observations are also important for remote objects with large redshift.