A symbol used for visual magnitude, particularly that defined in the UBV photometry system devised by Johnson and Morgan, which is measured in a band centred around 550 nanometres and corresponds closely to traditional visual magnitudes.

A class of asteroid of which the only known example is Vesta.

A design of telescope for observations of the Sun. The example at the Sacramento Peak Observatory is typical. In that instrument, sunlight enters the tower 41 metres 135 feet above the ground. A further 67 metres 220 feet of the telescope lie below ground. The entire optical path is virtually air-free, to avoid distortion of the solar image that would be caused by the presence of hot air. In the observing room an image of the Sun 51 centimetres 20 inches in diameter is produced with a resolution better than a quarter of an arc second. Sunlight can be directed into spectrographs or other instruments by tilting the main mirror at the bottom of the central tube. The entire optical system is suspended near the top of the tower on mercury float bearings.

A large circular feature on Callisto, surrounded by fifteen concentric rings. The radius of the outermost ring is 1,500 kilometres 930 miles. The feature was caused by an impact, but no longer shows any significant relief because of the plastic nature of the crust when the impact occurred. The rings are effectively ripples.

A canyon system on Mars, extending more than 5,000 kilometres 3,100 miles in the east-west direction in the equatorial region. The western extremity is marked by the Noctis Labyrinthus, a complex area of fault valleys graben that cut the surface into polygonal shapes. The central section consists of several parallel canyons whose average depth is 6 kilometres. At the centre they join up with Melas Chasma, a depression 160 kilometres 100 miles across. The eastern end of the system is marked by Capri Chasma. Erosion of the canyon sides has created debris in the flat valley bottom and has revealed a layered structure in the surrounding plateau. Flow channels leading into the canyon suggest that erosion by water took place in the remote past. Valles Marineris is primarily a fault structure, thought to have been created by the uplift of the volcanic Tharsis ridge to the west.

A sinuous valley on the surface of a planet.

Two ring-shaped regions around the Earth where there are concentrations of high-energy electrons and protons that have been trapped by the Earth’s magnetic field. They were discovered by the USA’s first successful artificial Earth satellite, Explorer 1, which was launched on 31 January 1958. They are named after James Van Allen, the physicist who led the experiment on Explorer 1. The inner Van Allen belt lies about 0.8 Earth radii above the equator. The main concentration of the outer belt lies between about 2 and 3 Earth radii above the equator, but a broader region, extending from the inner belt out to as far as 10 Earth radii, contains protons and electrons of lower energy, believed to have come primarily from the solar wind. Because the Earth’s magnetic field is offset from the planet’s rotation axis, the inner belt dips down towards the surface in the region of the South Atlantic Ocean, off the coast of Brazil. This South Atlantic Anomaly presents a potential hazard to the operation of artificial satellites. In 1993, a region within the inner Van Allen belt was found to contain particles that have penetrated from interstellar space.

The companion to the star BD+4°4048, which, at the time of its discovery by George van Biesbroeck 1880-1974, was the least luminous known star. Its absolute magnitude is +18.

A nearby twelfth magnitude star, discovered by Adriaan van Maanen 1884-1946, identified as a white dwarf.

Any star whose light output varies, whether regularly or irregularly. The graph showing brightness as a function of time for a variable is known as the light curve. There are a number of physical reasons why stars vary, and these are used as the basis for broad groupings according to type. Within each group, many more specialized types may be distinguished, often named after a prototype star. Eruptive and cataclysmic variables are characterized by their unpredictability and include a wide variety from T Tauri stars, in the process of formation, to supernovae, which have reached an explosive end. This group also includes flare stars, novae and dwarf novae. Pulsating variables are physically oscillating because of internal instabilities. These include Cepheid variables, RR Lyrae stars and Mira stars. The other major group consists of the eclipsing binaries, in which the light variation is caused by one star periodically passing in front of another. Such binaries are often also interacting. The prime example is Algol. Some stars, BY Draconis stars for example, show variability as they rotate because their surface brightness is not uniform. The method of naming variable stars is somewhat strange. It is mainly due to F. W. A. Argelander 1799-1875, who used the letters R to Z in conjunction with the genitive case of the constellation name for the nine brightest variables in each constellation. After that, pairs of letters, RR to RZ, SS to SZ and so on to ZZ were used J being omitted. For further variables, the pairs of letters AA to AZ, BB to BZ and so on were introduced, bringing the number of available designations to 334. Since many more than 334 variables are now known in many constellations, they are designated as V335, V336, and so on.

A term in the mathematical expression for the ecliptic longitude of the Moon which varies with a period of half a synodic month and has a maximum value of 40 arc minutes. It arises because the Sun’s gravitational attraction on the Moon varies during the course of a synodic month, being greatest at new Moon and least at full Moon.

A widespread lowland area on the surface of a planet.

The brightest star in the constellation Lyra and the fifth-brightest in the sky. It is an A star of magnitude 0.03.

The name of two Soviet spacecraft launched in December 1984 with the dual function of dropping probes into the atmosphere of Venus in 1985 and encountering Halley’s Comet in 1986.

Part of the Cygnus Loop, an old supernova remnant.