A type of Soviet manned space station. Seven were placed in Earth orbit between 1971 and 1982. Supplies could be brought to the stations, enabling cosmonauts to remain on board for many months.

Abbreviation for either Smithsonian Astrophysical Observatory or Special Astrophysical Observatory of the Russian Academy of Sciences.

Abbreviation for Smithsonian Astrophysical Observatory Star Catalog. This general catalogue contains 259,000 stars down to an approximate magnitude limit of 9. It was published in 1966; the epoch of the positions given is 1950.0.

Abbreviation for synthetic aperture radar.

The period of time over which the sequence of lunar and solar eclipses repeats. Its length, 6,585.32 days about 18 years, has been known since antiquity. After this period, the Earth, Sun and Moon will have returned to the same relative positions. Succeeding eclipses in a particular saros series occur about 8 hours later and fall nearly 120 degrees of longitude further west. This was known to the ancient Babylonian and Mayan astronomers and to the builders of Stonehenge.

A sequence of lunar or solar eclipses occurring at intervals of one saros. Since there are up to seven eclipses every year, there are more than 80 saros series running concurrently. In any one series, an eclipse will recur at a particular longitude after an interval of three saroses 54 years. However, each succeeding eclipse in the series moves systematically in latitude either north or south from one pole to the other, until the series is completed.

Abbreviation for Small Astronomy Satellite.

Any body in orbit around a larger parent body. Most planets in the solar system have natural satellites, otherwise known as moons. Artificial satellites are man-made objects launched into orbit around the Earth, or another moon or planet.

A dwarf galaxy in orbit around a larger one. A number of satellite galaxies accompany our own Milky Way Galaxy and the Andromeda Galaxy in the Local Group.

A technique for measuring with a high degree of accuracy the rotation and gravitational field of the Earth. Trains of laser light pulses 4 centimetres long are beamed to special satellites from which they are reflected back. There are 30 satellite laser ranging stations around the world.

The sixth major planet of the solar system in order from the Sun. Saturn is one of the four gas giants, second in size only to Jupiter. Its equatorial diameter is 9.4 times the Earth’s and its mass 95 times greater. However, its average density is only 0.7 times that of water. Hydrogen and helium make up the bulk of the mass. There is a central core, ten or fifteen times the mass of the Earth, made of rock or a mixture of rock and ice. In the high-pressure region surrounding the core, the hydrogen takes on the form of a metal. The outer half of the planet consists of a deep atmosphere; the visible features of the planet are cloud bands at the top of this atmosphere. The cloud patterns on Saturn do not normally show much colour contrast. However, storm activity is occasionally observed. In late September 1990, a large white spot developed, expanding over a period of weeks to encircle much of the planet’s equatorial region. This apparent eruption of material from the lower atmosphere followed a pattern of such occurrences over a 30-year cycle, corresponding to the orbital period. Similar spots were seen in 1876, 1903, 1933 and 1960, close to saturnian mid-summer in the northern hemisphere. Less extensive eruptions occur from time to time. One was observed by the Hubble Space Telescope in 1994. Computer processing of images obtained by Voyagers 1 and 2 during their encounters in 1980 and 1981 reveal complex circulation currents, similar to those observed on Jupiter. Saturn rotates rapidly, spinning once every 10 hours 32 minutes on average, though the rate varies with latitude. The resulting flattening at the poles is significant; the polar and equatorial diameters differ by 11 per cent. The most striking feature is the spectacular ring system. The rings lie in the planet’s equatorial plane, which is tilted at an angle of 27° to its orbit round the Sun. They are easily visible in a small telescope. As the relative positions of the Earth and Saturn change, the rings are presented at differing angles, sometimes appearing open, at other times edge-on so that they disappear from view. The rings have the appearance of a series of zones of differing brightness, separated by dark divisions. The most marked divisions are Cassini’s and Encke’s. The Voyager images of the rings showed that they consist of many thousands of narrow concentric ringlets, resulting in a grooved appearance. They are only one kilometre thick and are made up of a huge number of separate rocks and particles, perhaps ranging in size from a hundred metres down to a micrometre. Before 1980, ten satellites of Saturn were known. More have been discovered since, some telescopically in 1980 when the ring system was edge-on thus removing the glare and some by the Voyager 1 and 2 spacecraft in 1980 and 1981. Eighteen are now known for certain, and there are probably three others and possibly more, subject to confirmatory observations. See also: Cassini mission, planetary rings

A planetary nebula in the constellation Aquarius. Its unusual shape, with a faint partial outer ring, resembles the planet Saturn. The double ring may be the remains of separate shells thrown off by the central star.

English name for the constellation Libra.

A process in which all or part of a beam of electromagnetic radiation or particles is deflected from its initial direction of travel, without any absorption or emission. Light is scattered by fine particles, such as dust, through the mechanism of reflection or diffraction or both. If the particles are smaller than the wavelength of the light, the effect is purely one of diffraction and the phenomenon is known as Rayleigh scattering. The intensity of scattered light, as viewed from any particular direction, varies with wavelength λ as 1/λ4. This means that blue light is scattered more effectively than red. The daytime sky is blue because the blue component of sunlight is scattered by air molecules, while the rising or setting Sun, viewed through a thick layer of atmosphere near the horizon, is red because blue light is removed by scattering from the rays being observed directly. The same phenomenon produces interstellar reddening. Scattering may also be caused by direct interaction between a beam of radiation and the nuclei or electrons in the material through which it is passing. See also: Compton effect.

The upper limit on the mass of hydrogen that can be converted to helium in the core of a main-sequence star before core burning ceases and hydrogen burning moves to a shell around the core. The limit is about 12 per cent of the star’s original mass.