Being the innermost planet, Mercury never appears far from the Sun in the sky. The difficulty in observing Mercury led to a long-standing misconception about the time it takes to rotate on its axis. Towards the end of the 19th century, the Italian astronomer Giovanni Schiaparelli proposed, after a long series of observations, that the planet spins on its axis in 88 days, the same time as it takes to orbit the Sun. It would therefore keep one face turned permanently towards the Sun, as the Moon does to the Earth. In the 1920s, the Greek-born astronomer Eugene Antoniadi compiled a map showing smudgy markings on the surface of Mercury based on an assumed 88-day rotation period. This map seemed to settle the matter once and for all. Then, in 1965, came a surprise. At Arecibo Radio Observatory, astronomers Rolf Dyce and Gordon Pettengill bounced radio waves off the surface of Mercury. From the change in frequency of the reflected radio waves, they deduced that Mercury spins once every 59 days, two-thirds of the time it takes to orbit the Sun. Therefore the Sun does rise and set on Mercury, but very slowly. For the Sun to go once around the sky as seen from the surface of the planet - say, from one noon to the next - takes 176 Earth days, during which time Mercury orbits the Sun twice, spinning three times on its axis. In the skies of Mercury the Sun appears two and a half times as large as it does from Earth. The daytime side of Mercury is continually blasted by lethal doses of high-energy solar radiation. The Sun's intense heat roasts the surface rocks to over 400°C at noon on the equator, hot enough to melt tin and lead. Without an atmosphere to hold in the heat, the planet's surface cools to a frigid -180 °C during the long night. With a diameter of 4879 km, Mercury is only 50 per cent larger than our Moon, and smaller than every other planet in the Solar System except Pluto. Astronomers had long assumed that Mercury resembled our Moon in appearance, but it took the space probe Mariner 10 in 1974 to show how remarkably similar-looking Mercury and the Moon really are. As Mariner 10 flew past Mercury, its cameras photographed a surface heavily pockmarked with craters of all sizes, similar to the lunar highlands. Mariner 10 photographed less than half Mercury's surface, yet, if that portion is typical of the rest of the planet, it is enough to tell us much of the previously unknown story of Mercury. Craters on Mercury look almost identical to their lunar counterparts. There are deep, young craters, eroded ancient craters, craters with terraced walls, central peaks and bright rays. Many of the features on Mercury have been named after artists, composers and writers, thereby breaking the near-monopoly previously held by astronomers' names on Solar System bodies. For instance, we now find on Mercury memorials to Bach, Mozart, Van Gogh and Chekhov. Sometimes it is difficult to distinguish at a glance between a picture of Mercury and one of the Moon. Almost certainly, the craters on both bodies have been formed in the same way, by the impact of large meteorites early in the history of the Solar System. One noticeable difference on Mercury is that material ejected from the craters has not travelled as far as on the Moon, because Mercury's surface gravity is stronger - over twice that of the Moon, but still only 38 per cent of the Earth's. Another result of the higher gravity of Mercury is that craters tend to be shallower for a given diameter than on the Moon. Meandering cliffs called lobate scarps, several hundred kilometres long and a kilometre or so high, are a feature of Mercury's surface unlike anything on the Moon. These are believed to have been caused by a shrinking of the planet as its core cooled early in its history, leading to compression and faulting in the crustal rocks. The surface rocks of Mercury are actually slightly darker in tone than those of the Moon, reflecting a mere 11 per cent of the sunlight hitting them, compared with an average 12 per cent for the Moon; Mercury in fact has the darkest surface of any planet in the Solar System overall, although the Moon's maria are darker than Mercury. Between many of the large craters in the highlands of Mercury are areas peppered only with small craters. These areas, termed the intercrater plains, have no real counterpart on the Moon. They clearly pre-date the large craters, but whether they were formed by volcanic action or are deposits of ejecta from large impacts is uncertain. Other areas of particular interest for first-hand study are deep craters near the poles whose interiors are permanently shaded from the Sun, thereby preserving in deep freeze any gases that have seeped from the planet over its history as well as ice from cometary impacts. Of all the features seen by Mariner 10, the most prominent is an enormous bull's-eye structure, partly hidden by shadow, named the Caloris Basin. It is 13000 km across, similar to the Mare Imbrium on the Moon and fully one-quarter the diameter of Mercury. It was presumably formed by the impact of an asteroid after most of the surface had already been cratered. The Caloris Basin contains several concentric rings of mountains, and is surrounded by a number of radial ridges and grooves. Most importantly from a geological point of view, its interior, and much of the low-lying land around it, has been flooded by lava. Geological activity died out on Mercury over 3000 million years ago, as it did on the Moon. Since then, little has changed except for the random arrival of a stray meteorite. Despite its outward resemblance to the Moon, inwardly Mercury is believed to be more like the Earth. Mercury has a relatively large mass for its small diameter, which implies that it has a large iron core three-quarters of the planet's diameter. A core that size would be as big as the Moon. The existence of an iron core was directly confirmed when Mariner 10 measured a magnetic field around the planet, albeit only 1 per cent the strength of the Earth's magnetic field - but that is still far stronger than the magnetic fields of Venus and Mars. One explanation for such a disproportionately large core is that Mercury was originally much bigger, but had most of its rocky outer layers blasted off in a collision with a stray body of similar size to our Moon. This collision could also have knocked Mercury's orbit into its elliptical shape. In its own way, Mercury turns out to be a fascinating world containing many clues about the origin and development of the Solar System.