A large decrease, typically of a few hours’ duration, in the horizontal component of the Earth’s magnetic field. The cause is the arrival in the vicinity of the Earth of electrically charged particles plasma from the Sun, generated usually by a solar flare. Auroral activity and disruption of radio communications are common during such storms.

The ratio between the brightness of a planetary body, as viewed from the direction of the Sun, and the brightness of a hypothetical white, diffusely reflecting sphere of the same size and at the same distance. See also: albedo.

The region of space around the Earth incorporating Earth’s magnetosphere and ionosphere.

geosynchronous orbit.

An orbit around the Earth in which a satellite has a period exactly equal to the Earth’s sidereal rotation period of 23 hours 56 minutes 4.1 seconds. If such an orbit is circular and in the plane of the Earth’s equator, the satellite appears to be practically stationary in the sky and the orbit is described as geostationary. A geostationary orbit is at an altitude of 35,900 kilometres. A satellite in a geosynchronous orbit inclined to the Earth’s equatorial plane appears to trace out a figure-of-eight shape over the course of a day.

A particular form of equatorial mounting.

Medieval French Jewish astronomer, mathematician and philospher, considered one of the greatest of all medieval astronomers. Also known as Rabbi Levi ben Gershon, or Ralbag. He rediscovered the law of sines and published a sine table, correct to the 5th decimal. In his study of eclipses, he became the first astromer to rely on his own observations, rather than on Ptolemy’s. In his work,Sefer Tekunah, completed in 1328, Gersonides exposed the defects of Ptolemy’s system. Gersonides is the first person kown to have used a Camera Oscura for astronomical observations. Gersonides invented Jacob’s staff, a navigational instrument used for measuring angular separations, that was widely used for three centuries.

Popular name for NGC 3242, a planetary nebula in Hydra.

Comet Giacobini-Zinner.

Alternative name for the meteor shower also known as the Draconids. See also: Comet Giacobini-Zinner.

The region on a Hertzsprung-Russell diagram occupied by points corresponding to stars that have evolved into red giants. See also: stellar evolution.

A radio telescope located near Poona in India. It consists of 30 45-metre 146-foot dishes arranged in an array extending for 25 kilometres 16 miles and is the most powerful telescope for radio astronomy at metre wavelengths.

molecular cloud.

A term used to describe the planets Jupiter, Saturn, Uranus and Neptune to contrast them with the smaller, rocky terrestrial planets.

A member of a broad category of stars that have luminosities between 10 and 1,000 times that of the Sun, and radii typically between 10 and 100 times the Sun’s. A star becomes a giant when the hydrogen fuel available for nuclear fusion reactions in its core is depleted and the adjustment to the new energy balance causes the outer layers to expand greatly. The surface temperature drops, but the total luminosity rises because of the great increase in surface area. Examples are Capella, Aldebaran and Arcturus. Massive hot stars, which are very large in comparison with the Sun even before they reach a late stage of evolution, are sometimes also referred to as giants. See also: Hertzsprung-Russell diagram, red giant, stellar evolution.