Most of us above a certain age have been, at some point, moved to tears by Longfellow's maudlin epic of maritime meteorological catastrophe: It was the schooner Hesperus, Who sailed the wintry sea. . . But the original Hesperus was not a ship at all: it was a planet.

Hesperus, meaning "western", was the name given by the ancient Greeks to the planet Venus when it appeared in the western sky as the evening star. To Venus as the morning star, which presumably they took to be a different body altogether, they gave the name of Lucifer.

Now this planet, the cynosure of every eye last Tuesday morning, displays regular "phases" like our Moon when viewed from Earth. These, though not visible to the naked eye, occur for the same reasons as do lunar phases. And it sometimes happens, albeit rarely, that when Venus is at the "crescent" stage, the dark sector of its disc is faintly visible, illuminated by an anaemic, dull, red glow called "Ashen Light".

As it happens, the Moon is often seen with "the old Moon in the new Moon's arms", but the reasons are well known; "earthshine" is caused by light reflected from the illuminated Earth.

But this cannot explain the Ashen Light of Venus, so many alternative explanations have been put forward over the centuries since it was first noticed in 1643.

On one end of the spectrum of opinion are those who say that the Ashen Light does not exist at all; it is an optical illusion.

At the other extreme there has been at least one eccentric who assigns the illumination to some kind of celebratory son et lumiere arranged from time to time by joyful Venus citizens, en fete.

More realistic explanations range from sunlight being scattered through Venusian clouds to suggestions of electrical activity similar in origin to our own aurora borealis.

More recently, a theory has emerged which seems more plausible than most.

It has been known for decades that the surface of Venus is very hot indeed; an atmosphere over-rich in carbon-dioxide has produced a runaway greenhouse effect, so the dry, rocky surface of the planet sizzles at well over 400C. Venus is hot enough to melt tin and lead, to boil mercury, and, indeed, hot enough to have a dull, red glow.

Now, we know also that Venus is enveloped in an unbroken layer of dull, yellowish cloud. If this were like the clouds on Earth, it would obscure the glowing surface of the planet underneath, but the Venusian cloud, unpleasantly, consists almost entirely of tiny droplets of concentrated sulphuric acid.

Some time ago it was realised that such a cloud, unlike our water clouds, is semi-translucent to light of certain colours. What we see as Ashen Light may be the glowing surface of the planet faintly visible, its elusive, variable and patchy nature being caused by changes in the structure and thickness of the sulphuric acid cloud.