An observation shortly after sunset of a star that is just rising or just setting. Such observations of bright stars e.g. Sirius were used in ancient times for keeping track of the seasons. See also: heliacal rising.

The brightest star in the constellation Crux. Appearing to the unaided eye as a white star of magnitude 0.9, it is a visual double, the two components being B stars of magnitude 1.4 and 1.9. The separation is 4.4 arc seconds.

An instrument for measuring the intensity of solar radiation. Such instruments are now more usually called pyrheliometers.

The region at the center of certain galaxies from which an enormous amount of energy is released. It is believed that this energy comes from matter accreting via an accretion disk on to a supermassive black hole of more than 100 million solar masses. All the various forms of AGN may come from a single type of source whose appearance depends on its orientation relative to the observer. The AGN may consist of a compact source surrounded by a torus doughnut-shaped ring of gas and dust. Narrow jets of gas are emitted at velocities approaching the speed of light along the axis of rotation, which is at right angles to the ring. If the line of sight were through the torus, the central source would be obscured, and the AGN would appear to be a radio galaxy. If the torus were tilted toward the line of sight, the central source would be visible, and the AGN would be seen as a quasar. Looking directly along, or close to, the jet, would result in the appearance of a blazar.

A galaxy emitting a large amount of energy from a central compact source. Such a galaxy shows activity similar to that of a quasar. It is not known whether all galaxies undergo this type of behavior at some time, or if active galaxies are different from the majority of galaxies. There are a few main types of active galaxies. Seyfert galaxies, named for US astronomer Carl Seyfert, who identified the first ones in 1943, have optically bright centers, but are much less energetic than quasars and much more numerous. Most Seyfert galaxies are spirals or barred spirals. Radio galaxies are unusually strong sources of radio emission. At radio lengths, they show structure similar to that of quasars, with symmetric lobes and often a jet, but their centers are not as bright optically. A blazar is another kind of active galaxy. Blazars are more luminous and more variable than standard quasars.

The Sun during periods of solar activity.

Any one of a number of indicators of the level of solar activity at a given time. Indices used to measure solar activity include the Wolf sunspot number, the total area covered by sunspots on the visible hemisphere, and a K-plage index derived from the areas and brightness of plages. In addition, the total radio and X-radiation from the Sun are also regarded as indices of solar activity.

An English mathematician.  From his analysis of the motion of Uranus, Adams had worked out an approximate position for an unseen more distant planet by 1845.  Urbain LeVerrier 1811-77, a French astronomer, arrived at a similar conclusion in 1846.  Using LeVerrier's calculations, German astronomers Johann Galle 1812-1910 and Heinrich D'Arrest 1822-75 found Neptune on September 23, 1846.  Both Adams and LeVerrier are now credited with the discovery of Neptune.

A technique for improving the quality of the image produced by an astronomical telescope in which the optical system compensates for changes in the quality of seeing - constantly varying distortions produced by refraction in the Earth’s atmosphere. The distortion is corrected by the rapid bending of a small, very thin mirror placed a short distance before the focus. To be effective, the system needs an image sensor, a microprocessor and actuators to apply forces to the thin mirror, all of which must have response times shorter than a hundredth of a second.

he second-brightest star, after Sirius, in the constellation Canis Major. It is a giant B star of magnitude 1.5 and has an eighth magnitude companion. The Arabic name means the virgins.

Asteroid 2101, diameter 2 km, discovered by E. Delporte in 1936. It is a member of the Apollo asteroid group and came within 2 million kilometres of the Earth in 1937, but was then lost until recovered in 1977 following recomputation of its orbit.

A small satellite of Jupiter number XV, discovered by David Jewitt in 1979.

Abbreviation for Aitken Double Star Catalogue.

A US spacecraft, launched in August 1997, carrying nine scientific instruments for determining the isotopic and elemental composition of the solar corona, the interplanetary medium, the local interstellar medium and galactic material. It was placed in solar orbit, at one of the Earth’s Lagrangian points, 1.5 million km nearer the Sun than the Earth, where it will maintain an almost constant position relative to the Earth.

An American satellite, scheduled for launch via the Space Shuttle in late 1998, which will provide a national X-ray astronomy facility.