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.
