Abbreviation for Giotto Extended Mission. See also: Giotto.

A powerful gamma-ray source in the constellation Gemini discovered in 1972 by the orbiting observatory SAS Small Astronomy Satellite 2. Weak X-rays from Geminga were detected by the Einstein Observatory and its optical counterpart appears as a twenty-fifth magnitude star. Geminga is thus a very unusual object in that it emits almost all its energy as gamma rays; the X-ray emission is a thousand times weaker and its luminosity in visible light a thousand times weaker again. It is thought to be relatively close, probably within 700 light years. Observations made by ROSAT confirmed the X-ray emission and showed it to be pulsating with a period of about a quarter of a second. Gamma-ray pulsations have also been detected by the Compton Gamma Ray Observatory. Geminga thus appears to be a gamma-ray and X-ray pulsar. Why the bulk of its radiant energy is emitted in such a high-energy form is not known.

One of the twelve constellations of the zodiac in the list drawn up by Ptolemy c. AD 140. The two brightest stars in Gemini, both first magnitude, bear the names of the twins Castor and Pollux of classical mythology. Pollux, though the brighter of the pair, was given the designation β Beta by Johann Bayer.

A series of manned, orbiting spacecraft, launched by the USA in the 1960s. They were an important part of the development of manned spaceflight technology in preparation for the Apollo programme of Moon landings. Gemini 3 in 1965 was the first American flight with a crew of more than one astronaut, and Gemini 8 in March 1966 achieved the first successful docking in space. The last of the series was Gemini 12 in November 1966. Many of the astronauts who later took part in the Apollo Moon landings also flew in the Gemini programme.

Two 8-metre telescopes for optical and infrared astronomy, resulting from an international collaboration between the USA, the UK, Canada, Chile, Brazil and Argentina. One is sited in the northern hemisphere, at the Mauna Kea Observatories in Hawaii, the other in the southern hemisphere on Cerro Pachón in Chile, near the Cerro Tololo Inter-American Observatory. The siting of the two telescopes ensures complete sky coverage between them. The Hawaii telescope is being completed during 1998, and its more southerly twin in 2000.

A major annual meteor shower, the radiant of which lies near the star Castor in the constellation Gemini. The shower peaks around 13 December, and the normal limits are 7-16 December. The meteor stream has an unusual orbit with a perihelion distance of only 0.14 AU. IRAS, the Infrared Astronomical Satellite, discovered a cometary nucleus in 1983, designated as asteroid 3200 Phaethon, which appears to be the parent body for this stream.

An alternative name for the star Alphekka.

A theory of gravitation, published in its final form in 1916. It was developed by Albert Einstein 1878-1955 from his earlier 1905 Special Relativity theory. One of the fundamental postulates of the general theory is that, over a limited region of spacetime, it is impossible for observers to tell whether they are undergoing uniformly accelerated motion or are in a gravitational field. This is known as the principle of equivalence. Einstein showed that it was not necessary to think of gravity as a force acting at a distance. Instead, he described gravity in terms of its local effects on space and time, i.e. as the curved geometry of spacetime, which is determined by the distribution of matter and energy. A good three-dimensional analogy to help understand the meaning of curved, four-dimensional spacetime is geometry on the surface of a sphere. For example, two travellers who set out from different places on the equator and travel due north will eventually find that their paths cross, even though they started out travelling parallel to each other. This contrasts with what happens on a flat surface, where parallel lines never cross. The two travellers might say they had been pulled together by some force gravity, for example, but their experience is more effectively explained in terms of geometry. In regions where the gravitational field is weak, General Relativity approximates to the theory set out by Isaac Newton. For a strong gravitational field, General Relativity gives the best description yet devised. There are other theories of gravity but none has met with the total success enjoyed by General Relativity. Several areas of astronomy have proved to be testing grounds for the theory. The perihelion of Mercury’s elliptical orbit around the Sun advances by 43 arc seconds per century more than is predicted by Newton’s gravitational theory, but General Relativity explains it exactly. The light from stars deviates from a straight line if it passes very close to the Sun, and this has been observed at solar eclipses. The motion of pulsars in binary systems is readily accounted for by General Relativity. The most frequent application of General Relativity is in cosmology, since gravity is the dominant factor in all attempts to make mathematical models of the universe.

A model of the solar system that places a stationary Earth at the centre of the motion of the Sun, Moon and planets. This was the universally accepted view of the cosmos until Copernicus 1473-1543 suggested that a solar system with the Sun in the middle would provide a more elegant explanation of observed planetary motions. Predictions of the movements of the planets in the geocentric system were made on the basis of a complex theory of epicycles set out by the Greek astronomer and mathematician Ptolemy c. AD 100-170. See also: heliocentric model.

The outermost part of the Earth’s atmosphere, consisting of a halo of hydrogen gas that extends over a region extending to about 15 Earth radii.

The shortest path between two points in spacetime and therefore the path followed by photons.

The measurement of the precise figure of the surface of the Earth and the Earth’s gravitational field.

Asteroid 1620, diameter 2 km, first discovered in 1951 by R. Minkowski and A. Wilson, and recovered in 1969 when it made a close approach to the Earth. It is a member of the Apollo group.

A surface defined by mean sea level in the open ocean and, on land, the surface that would be taken up by water in an imaginary network of frictionless channels connected to the sea.

The magnetic field in the vicinity of the Earth. To a first approximation, the Earth’s magnetic field is like that of a bar magnet dipole currently displaced 451 kilometres from the centre of the Earth towards the Pacific Ocean and tilted at 11° to the rotation axis. The strength and shape of the geomagnetic field varies gradually over a timescale of years. The intensity of the geomagnetic field is denoted by a vector quantity F or B, and is measured in gauss G, tesla T or gamma γ. 1 tesla = 10,000 gauss; 1 gamma = 1 nanotesla = 10-5 gauss. The direction of the field at any point can be described by two angles: I, the dip angle or inclination, the angle between the horizontal and the field, taken as positive when downwards; D, the declination, the azimuth from the northward horizontal direction, measured towards either east or west.