Solar System

Mercury,the innermost planet of the solar system. It is visible as a magnitude 0 star at approximately two-monthly intervals alternating between the evening and morning twilight. The planet follows a remarkably elliptical orbit moving from 46 to 70 million kilometres from the Sun and completes one orbit every 88 days. In a small telescope it shows phases like those of the Moon, due to its varying position relative to the Earth and Sun. Very occasionally, the Earth, Mercury, and the Sun line up precisely and the planet is then seen to transit the Sun as a small dark spot crossing the solar disc. This last occurred on 12 November 1986 and will occur again on 14 November 1999. Observations of faint dusky markings seen on Mercury through large telescopes around 1900 led astronomers to conclude that the planet rotated on its axis and revolved round the Sun at the same 88-day rate, keeping the same face pointing sunwards. After being accepted for eighty years, the 88-day rotation period was finally replaced by the correct 58.5-day period deduced from radar observations.

 Mercury is 4,878 km in diameter, midway in size between Mars and the Moon and its daytime surface is baked by the proximity of the Sun. The midday equatorial temperature varies, due to the elliptical orbit, from 415 oC at the closest point to the Sun to 285 oC at the farthest. In contrast, the lack of an atmosphere allows the temperature to plummet to -175 oC at night. The spacecraft Mariner 10, which passed close to Mercury on three occasions in 1974 and 1975, revealed the planet as a barren airless world covered with impact craters similar to those on the Moon. The largest structural feature is the Caloris Basin, a ring basin 1,300 km in diameter, whose floor has been intensely disrupted by fractures and ridges. Caloris was probably caused by the impact of a body some tens of kilometres in diameter. Some craters show ray systems.

 The perihelion advance of Mercury has been used to confirm predictions made by the general theory of relativity.

Venus, the second major planet in order of distance from the Sun. Also known as Hesperus, the evening star, or Phosphorus, the morning star, Venus can be the brightest object in the sky after the Sun and Moon and follows a near-circular path some 106 million kilometres from the Sun, taking 225 days to complete one orbit. It shows phases in the same way as Mercury and can also pass in front of the Sun during a transit. The last transits were in 1874 and 1882 and the next are not until 7 June 2004 and 5 June 2012. In visible light the planet shows an almost featureless cloud-shrouded globe. However, in photographs taken with ultra-violet light, it shows dark patches and streaks, which are temporary breaks in the dense lower cloud levels. These race round the planet in only four days, carried by a high-altitude 350 km/h gale which never abates. The clouds closely resemble natural terrestrial fogs or industrial smogs and are composed of sulphuric, hydrochloric, and hydrofluoric acid droplets. The atmosphere is extremely dense and hot, made up of over 90 per cent carbon dioxide. The surface pressure is 91 times that of the Earth and the global surface temperature is kept permanently at about 475 oC by the greenhouse effect. The solid globe, with a mean density of 5,240 kg/m3, is 12,104 km in diameter and rotates slowly from east to west, that is, in a retrograde manner, with a period of 243 days so that its day is longer than its year. The Soviet spacecraft Venera 9 and 10 gave the first glimpses of the surface in 1975. Both spacecraft soft-landed and returned panoramic photographs showing hot stony desert landscapes scattered with rocks of all sizes. Recent radar studies suggest that this is typical of the whole planet and show that Venus has a cratered surface. Radar maps reveal two high continent-sized areas, Ishtar Terra and Aphrodite Terra, and a very high mountain feature, Maxwell Montes. There is also evidence for the presence of long canyons and volcanoes, though these seem to be inactive at present. Venus has no natural satellites.