Every so not-very often, the planet Mercury passes between the Earth and the Sun. It only does this about 13 times each century, and only for a few hours, and even then the weather might be poor enough to obscure the lot. If you're lucky, and have access to a telescope with the appropriate filter, what you see is a tiny black dot scuttling slowly across the face of the Sun. Yesterdaywe gotlucky.
The sky was blue with a few streaks of white at noon as the Flamsteed Astronomy Society set up their equipment. This was considerably better than the forecast had suggested, but the Society are a bunch of optimists so had made the decision to turn up anyway, and were rewarded with almost ideal conditions. A wide range of tubular instruments was lined up across the courtyard, some long and thick, others rather shorter and more compact, with a volunteer or two watching over each one. Most wore hats, generally of the broad-brimmed kind, hinting strongly that they'd done this kind of standing around for a long time in the Sun before. There was no fuss as twelve minutes past twelve ticked round, just a few fingers raised, as the shadow of Mercury nudged fractionally onto the Sun's disc. A few hundred years ago this would have been a key scientific moment, the timing of first contact essential data in attempting to calculate the scale of the solar system. These days a transit of Mercury is much less important, all the appropriate measurements have been taken, but none the less impressive in the awe and wonder stakes.
A few tourists wandered by towards the cafe, oblivious of the miniature spectacle above their heads. But for any member of the public who was interested, or could be cajoled, the courtyard area was freely accessible and a short queue awaited. I was surprised quite how short, I'd have expected more astronomically-minded attendees in a city of eight million, but that's a Monday lunchtime for you. When my turn came I stepped up to the mark and squinted down the eyepiece to see a blazing red disc with a spot on it. It was a very small spot - the diameter of Mercury is only 1/158th that of the Sun - but crisp and round and pinpoint sharp. And it was also very close to the edge, as befits the start of the transit, in this case right over near the very right hand side, around 3 o'clock. And that was Mercury, a planet generally invisible except around sunrise or sunset, plain as anything in the middle of the day.
Back in 2004 I came to Greenwich to watch the Transit of Venus, again courtesy of the Flamsteed Astronomy Society. Venus is rather bigger, and closer, so the black disc crossing the Sun was visible to the protected eye. Venus's orbit is also rather longer, which means it's even rarer that the planet's oblique orbit crosses our own. Transits of Venus occur in pairs, but a century apart, so the next isn't due until 2117 by which time we'll all be dead. Transits of Mercury are a little more accessible, but it's still awkward if you miss one - the next three visible from London will be in November 2019, November 2032 and November 2039, and the next one almost as good as today's will be in May 2049. And yes, transits of Mercury only ever happen in May or November, more often in the latter, but fractionally larger in the former.
Chasing the transit across town, for a subtly different view, I headed next to a courtyard off Piccadilly. You'll know it better as the entrance to the Royal Academy, but a variety of age-old scientific organisations also have their HQ in rooms around the quadrangle. The Royal Astronomical Society had set up in the sunniest corner, with three different instruments for public view and a small queue curling across the front of the art gallery. The volunteers here were rather more likely to be astrophysics graduates, and the attendees too had more of a scientific bent. The lady in front of me was a serial eclipse chaser, and had spent much of March attempting to reach a remote Micronesian island for totality. She'd even brought her special glasses, but only out of habit, because Mercury's much too small to be seen with the naked eye.
The first piece of transit-watching apparatus was nothing more than a pair of upturned binoculars with a sheet of cardboard across the top and another underneath to act as a screen. The second was essentially a lens shoved through a cardboard box, and the third a proper spotlight-sized refractor. All were susceptible to high level cloud, of which there was now rather more, with Monday's threatened frontal system increasingly evident in the southern sky. Indeed the thickest clump of greying altocumulus appeared to have settled in precisely the worst spot, as if it were sentient or something, and the wrong kind of shadow was in control. But waiting patiently intermittently paid off, at least for those hoping to use the main telescope, and there was the crisp sharp dot once more, but shifted further across the main disc. Tempus fugit.
As a planetary bonus, skygazers were also invited to cross the courtyard to view a Mercury-related exhibition inside the Royal Astronomical Society. This almost-200 years old organisation boasts a charming balconied library upstairs, its walls rammed with books and periodicals, and also a fine collection of vintage volumes. A few of these were laid out on the far table, the earliest recounting (in Latin) the first human observation of a Transit of Mercury in 1631. Alongside, in his actual handwriting, William Herschel's account of the transit of November 1802, a constant battle against thick cloud which made taking measurements almost impossible. Meanwhile downstairs in the lecture theatre a live broadcast of the 2016 event was being screened, oddly rather blurrier than could be seen outside, which might have explained the lack of audience.
Shifting location once more, I headed to almost the highest point in Hampstead to one of London's finest amateur observatories. This perches on top of a reservoir, its grass cover freshly mown, and is accessed via a staircase from a side road. Inside the dome the main telescope had been set up to point directly at the Sun (no simple task when it keeps moving), focused to project the shadow onto a small formica plate. A bit annoyingly this appeared to have a scratch on it, or some other thin mark which wasn't actually present on the surface of the Sun. But there beside it were two dots, one an irregular sunspot and the other the planet we were seeking. Mercury had moved even further across the solar disc by this point, one entire crossing (on this occasion) taking nearly seven and a half hours. And while we watched, swirly cloud permitting, a volunteer imparted knowledge much more fully than at the other venues.
It's a very friendly group of astronomers that meet here, I could tell. Anyone can come and visit when the observatory is open, which tends to be Friday and Saturday evenings, or Sunday lunchtimes, or for special occasions such as this. You never know who you'll meet... Jon Culshaw was there during my visit, not to do an impression of Sir Patrick Moore but to keep tabs on the solar spectacle. Meanwhile back outside the dome a couple of smaller telescopes had been set up, this time with almost no queue for viewing. I took a last opportunity to view Mercury against a yellow background on one, and against a red background on the other, now rapidly approaching its halfway point. But it wouldn't be long before the cloudbase extended across the entire sky, making any attempt to view the second half of the transit almost entirely impossible. Your next opportunity will be Armistice Day 2019, assuming your thoughts aren't elsewhere, and the November sky permits.
