ABSTRACT
The innermost planet, Mercury, is also the smallest of the principal planets–apart from Pluto, which may be in a completely different category. Plato commented upon the yellowish colour of Mercury, although most naked-eye observers will describe it as white. Mercury can actually become brighter than any star, but can never be seen against a really dark sky. Mercury, like Venus, can pass in transit across the face of the Sun and does so more frequently than with Venus, although during a transit it is not visible with the naked eye. Mercury has proved to be a world of craters, mountains, low plains, scarps, dorsa and valleys. The overall density of Mercury is greater than that of any other planet apart from the Earth, and the core is likely to be about 3600 km in diameter–larger than the whole globe of the Moon.
