association.

A star of spectral type O. O stars have surface temperatures in the range 28,000-50,000 K and are bluish white in colour. Their spectra are characterized by lines of both neutral and ionized helium; emission lines are also commonly present. The four brightest O stars in the sky are Delta δ and Zeta ζ Orionis, the easternmost stars in Orion’s belt, and the southern stars Zeta ζ Puppis and Gamma2 γ2 Velorum.

Abbreviation for Orbiting Astronomical Observatory.

One of the larger satellites of Uranus, discovered by William Herschel in 1787. Its surface is covered by numerous impact craters, many surrounded by bright rays and blankets of ejecta. Several have very dark material within them.

An old term for the objective lens of a refracting telescope. Its abbreviation, OG, is still often used when observations are recorded, to signify that the telescope used was a refractor.

The main light-collecting lens in a refracting telescope.

A diffraction grating, of the transmission type, placed over the aperture of a telescope in order to produce spectra of all the stars in the field of view.

A thin prism placed over the aperture of a telescope in order to produce spectra of all the stars in the field of view.

ellipticity.

The angle between the planes of the Earth's equator and the ecliptic. Its present value is approximately 23° 26'. The effects of precession and nutation cause it to change between extreme values of 21° 55' and 24° 18'.

The French national astronomical research institute, based at the original site in Paris where it was founded in 1667. This is the oldest astronomical observatory still in use for research. There is an astrophysics section, located at the Observatoire de Meudon, just outside Paris, and a radio astronomy station at Nançay. Research is carried out in many branches of astronomy. At the Paris site, there are three nineteenth-century instruments, including the telescope built for the Carte du Ciel project and a 38-centimetre 15-inch refractor, which is occasionally used for positional work. Systematic astrometric measurements are made with a prismatic astrolabe. The Observatoire de Meudon was founded in 1876. It became the Astrophysics Section of the Observatoire de Paris in 1926 when the two institutions were merged. The instruments there include an 83-centimetre 33-inch refractor dating from 1893, a 1-metre 40-inch reflector, also from 1893 but modernized in 1969, a solar tower telescope used for spectroscopic studies of the Sun, a spectroheliograph and a large siderostat used in conjunction with a solar magnetograph and instruments to monitor the solar chromosphere. The Nançay radio astronomy station, established in 1953, is a large site with many instruments. Observations of solar radio emission are made with a radioheliograph, a multi-channel instrument for spectral observations of the Sun in the radio band and telescopes for monitoring solar activity. A special array is used for observations of the Sun and the planet Jupiter at wavelengths between 3 and 300 metres. The largest radio telescope at the site is of a unique design, consisting of two immense reflecting surfaces, one flat and one concave, which face each other. The flat reflector consists of ten panels, each 20 × 40 metres 65 × 130 feet, which can be rotated about a horizontal axis. The concave reflector is 300 metres 980 feet long and 35 metres 115 feet high. Radio signals are reflected from the plane reflector on to the concave reflector, from which they are brought to a focus and collected by receivers mounted in a movable cabin. This telescope is used for studies of the 21-centimetre emission from neutral hydrogen, emission from the hydroxyl OH molecule, and other work.

An observatory on the island of La Palma in the Canary Islands Group. The observatory site, regarded as one of the best in the world, is operated by the Instituto de Astrofisica de Canarias, which is host to a number of different organizations that have telescopes there, including the Royal Greenwich Observatory RGO. The RGO’s Isaac Newton Group consists of the William Herschel Telescope, the Jacobus Kapteyn Telescope and the Isaac Newton Telescope. Other instruments at the observatory include a Swedish solar telescope, a 2.56-metre 101-inch telescope belonging jointly to Sweden, Norway, Finland and Denmark the Nordic Telescope, or NOT and the Carlsberg meridian circle operated by Denmark, the UK and Spain. The observatory occupies an area of nearly two square kilometres at an altitude of 2,400 metres 7,900 feet.

A place or building at which astronomical observations are or were formerly carried out, or the administrative centre for such work.

The passage of one astronomical object directly in front of another so as to obscure it from view as seen by a particular observer. See also: grazing occultation, eclipse.

mare.