Often known popularly as the ‘evening star’ or ‘morning star’, Venus is the brightest of all the planets in the night sky. The early Greeks once thought Venus was really two separate objects, being the evening star Hesperus, and the morning star, Phosphorus, while the early Romans thought it was both Vesper and Lucifer. The Babylonians who adorned the planet as the goddess of beauty and love, with the name Ishtah meaning the “light-bringer”. These same middle-easterners were also first to realise that both objects were the same, and the Greeks soon adopted this too, which they named the now solitary Greek god, Aphrodite, who even later, the Romans named Venus. Terms like “Venusian atmosphere” derive from the genitive, Veneris, but fortunately we have not use the traditional terms for planets. Else Venus would have an “Aphrodisiac atmosphere” certainly giving the wrong connotation!
Of a similar size to Earth, Venus is some 12 104 km across. Closer to the Sun than us, Venus’s almost circular orbit takes 224.7 days to complete at the mean solar distance of 0.72 A.U. (108 million km). Like Mercury, Venus is a so-called inferior planet that experiences both inferior and superior conjunctions. Never drifting very far from the Sun, the maximum angle reaches only 47o. Each full “cycle” of the phases, called the synodic period, takes 1.60 years or 583.9 days, with its apparent size changing anywhere between 9.6" and 66" arc seconds. Venus’s distance from Earth may change anywhere between 38 million kilometres and 261 million kilometres, with its visual brightness varying between -4.7 and -3.5 magnitude. Transits across the solar disk do occur, but because of the orbital inclination of 3.4o, these are quite rare. Coming in pairs once every 127 years, we saw the last events in 1874 and 1882, recently on the 8th June 2004. The next awaited transit is on 6th June 2012.
Discoverer : Prehistoric
Satellites : None
Equatorial : 12 103.6 km
Polar : 12 102.4 km
Period (P) : 224.701 days
Synodic Period : 583.92 days
Orbital Velocity : 35.03±0.26kms-1
Eccentricity (e) : 0.0067
Inclination (i) : 3.39o
Mean Density : 5.243 g.cm-3
Mean Distance : 108.21±0.73 x106 km
Sidereal Rotation : 243.02 days
Day Length : 116.75 days
Maximum Diameter : 66.0"arcsec
Minimum Diameter : 9.7"arcsec
Maximum Magnitude : -4.7
Venus is a total cloud-shrouded world that was mysteriously hidden until the 1960’s. Once considered as Earth’s twin, some quite fancifully imagined a prehistoric version of the Earth, including dinosaurs and other primitive life-forms. First explored by the Mariner 2 spacecraft in 1962, and dispelled this theory. Venus is a hellish world. The atmospheric pressure is ninety-two times of Earth’s, whose thick corrosive atmosphere contains 90% carbon dioxide (CO2). This is mixed with smaller quantities of nitrogen, sulphur dioxide, sulphuric acid, argon and only minute traces of water. Fast winds move once around Venus every 4.3 days, so this spreads evenly the hot 460oC surface temperature on both the dark and sunlit sides of the planet. This internal heat is partly due to the proximity of the Sun, but it has become much hotter because the CO2 has trapped the sunlight via a runaway Greenhouse effect. This is even more remarkable because Venus only spins once every 243.7 days in a retrograde direction and opposite to all the other planets.
Many spacecraft have visited Venus and each gathered a wealth of information. Radar observations by the Pioneer Venus Orbiter and Magellan spacecraft found evidence of cratering, recently active volcanoes, plate tectonics, and several unknown geological features not seen on Earth. Venera 9 also obtained surface pictures in October 1975, showing that the ambient light is similar to the sky brightness of a full moon. No suggestion of any magnetic field has been found, so that Venus’s core must be quite different in composition than that of the Earth’s.