A transit is the movement of a body directly between the Earth and the Sun. The planets Mercury and Venus do so on occasions, when they appear as a black spot crossing the Sun's disk. Accurate measurements of such transits helped to establish the scale of the Solar System. (Adapted from the Philip's Astronomy Dictionary)

Viewing Location: The Royal Observatory, Greenwich
Weather: Perfect (if a little hazy at times - but brilliant for Britain!)

The Courtyard at the Royal Observatory. Who's that in the nice red shirt?

All things astronomical in Britain normally have one big problem: the weather. Those who went to Cornwall in the hope of seeing the Total Eclipse of the Sun back in August of 1999 know all about that. It is therefore more with fingers crossed that keen astronomers wait for any rare(ish) events that show up on the calendar. Having watched the weather reports for this particular event over the past few days it, for once, looked very promising - I write this after the transit, so I can laugh at 'tempting fate'.

The transit itself was the first since, wait for it... November 1999. "That's not long ago, I don't call a transit of Mercury very rare!" I hear you cry. No, but the Sun had already set in Britain when this one took place, and the same thing happened back in November 1993. Previous to that was November 1986, when only half of the crossing took place with the Sun in the sky. So we travel back to November 1973 before a nicely decent transit happened. The problem also with November events (and the next one is November 2006) is that the Sun doesn't get very high in the sky. Today's spectacle sees the Sun rise to a very respectable height, which is better for viewing. You see, the higher something is the less atmosphere you have to look through, leading to a less blurry view of the thing.

And so it was to be: at 6.13am a small perfectly round black dot appeared on the edge of the Sun. The low haze near the horizon made for difficult viewing early on, but as the Sun rose into only a whispy cloudy sky the majestic event slowly unfolded. Mercury moved gracefully across the solar disc until 11.32 when the scene was given back to the Sun and its associated sunspots.

11.23, a few minutes before Mercury left the scene.

So, even though thirteen or fourteen of these transits happen each century, once everything is taken into account (weather, Earth rotation, low Sun, broken telescopes, bad days, very bad days, etc.) these do turn out to be rare events - and this was just one of those equally rare times when everything worked out perfectly. Well done Mercury, you did a splendid job.