Mercury, the innermost planet, is also the smallest. It always stays in the same part of the sky as the Sun, and can therefore never be seen against a really dark background, and is not a conspicuous naked-eye object, though at its best it is actually brighter than any star. Its quick movements led to its being named after Hermes (Mercury), the fleet-footed Messenger of the Gods.

Data for Mercury are given in Table 4.1.

VULCAN

It was once thought that a planet existed closer to the Sun than the orbit of Mercury. It was even given a name – Vulcan, after the blacksmith of the gods. Only in the twentieth century was it finally found to be non-existent and relegated to the status of a ghost.

The story of Vulcan really goes back to 1781, when William Herschel discovered a new planet, Uranus, moving far beyond the orbit of Saturn. Over the years it was found that Uranus was not moving quite as it was expected to do; something was perturbing it, and mathematicians began to suspect that there might be yet another planet still further from the Sun. From these tiny perturbations a leading French astronomer, U. J. J. Le Verrier, worked out the position of the unknown world, and in 1846 J. Galle and H. D'Arrest, at the Berlin Observatory, discovered Neptune, very close to the position given by Le Verrier.