BBC News Online science editor, Dr David Whitehouse, images the planet Mercury as it walks across the face of the Sun

Mercury appears as a passing dot

Like thousands across the globe I watched the transit of Mercury with interest, and not a little wonder.

I was in an ideal location at an observatory in Hampshire, England, where in the pale light of the pre-dawn, I gathered with a few other enthusiasts to watch the event.

A transit of Mercury is a rare thing. These days it is of no great scientific importance, but it is fascinating to watch nonetheless as an example of the "clockwork" of the Solar System.

Tiny Mercury races around the Sun every 88 days but because its orbit and the Earth's are tilted, transits occur infrequently.

They could not be seen before the invention of the telescope. The first person to see one was French astronomer Pierre Gassendi in 1631.

Small dot

Right on cue, Mercury appeared, a tiny dot encroaching on the disc of the Sun.

Yet this small dot is a real world, somewhat larger than our own Moon to which it bears close resemblance.

Mercury is seen (circled) with a sunspot (top left)

Its smallness impresses two things upon you: the enormous size of the Sun and the vastness of the distances between the planets.

Mercury was 91 million kilometres (57 million miles) away, the Sun 150 million km (93 million miles) - scales that dwarf human imagination yet are utterly insignificant in cosmic terms.

It was the second time I had seen Mercury in recent weeks. I saw it when it was in the evening sky; you had to know just where to look, when it was at its furthest point from the Sun.

Mercury never strays far from the Sun. The ancient Greeks had two names for it: Apollo in the morning sky and Hermes in the evening.

The scale of space

It was realised very early on that observations of transits could help establish a fixed distance scale for the Solar System from which the true measurements to the other planets could be gauged, calibrating the scale of space if you like.

Edmund Halley saw their significance in 1677 and then realised the greater importance of the Venus transits.

About 12 Mercury transits occur each century

These are very rare. There have been only seven since the invention of the telescope and the last one was in 1882 - so it has passed out of living memory.

But our generation is lucky as Venus transits the Sun next year, an event to look forward to.

Back to Mercury, slowly making its way across the Sun's face. Telescopes from all over the world turned to it and a few in space as well. Everyone was curious.

But it was soon gone and those observing it surprisingly mourned its passing, and remembered the enchantment of its shadow moving to the clockwork of the planets.