One post in every calendar year, that's my motto.

And so far it hasn't steered me wrong, as Theophile Escargot said in a different context.

Paris. November 7, 1631. Pierre Gassendi has been watching the sun for two days, now, hoping to see something that no one in the history of the world has ever before seen: the passage of the planet Mercury across the face of the sun, a transit of Mercury.
Not that this has never happened before. In fact, it happens about 13 times every century, at irregular intervals. But no one before Gassendi has had both the tools and foreknowledge to observe the transit.
The foreknowledge is courtesy of the recently deceased German astronomer Johannes Kepler, who four years earlier in 1627 had published his Rudolphine Tables, the most accurate astronomical tables computed up to that time (all calculations done by hand, of course) and a year before, in 1630, had published an "Admonition" pointing out to Europe's astronomers that a transit of Mercury was predicted for November 7th, and advising that they watch for it, cautiously suggesting that they watch from November 6th to the 8th in case of errors in his observations or computations. Gassendi, even more cautious, decided to start watching on the 5th.


The tool is the telescope, which had been invented two or three decades earlier by... well, we don't really know for sure who invented it. A Dutch spectacle-maker named Hans Lippershey or Lipperhey had applied for a patent for it in 1608, but at least two other men claimed priority, and in all likelihood, none of them had actually invented it.
At any rate, Gassendi had a telescope, and had set it up in a darkened upper room of a building in Paris to project an image of the Sun, about 8 inches across, onto a sheet of ruled paper. When the transit occurred, Mercury should show up as a black spot about half an inch across.
But Paris in November is far from an ideal location for making astronomical observations. It rained all day on the 5th, and the 6th was foggy. At first the 7th seemed hardly any better, but finally at about 9 in the morning, Gassendi got a clear image of the sun on his ruled paper. There was no sign of Mercury, but he did see a small sunspot, perhaps 1/10th inch across. He noted its location as a reference point, and stamped his foot to notify an assistant, stationed in the room below, to take a sighting of the exact angle of the Sun, allowing Gassendi to determine the time of the observation. This rather cumbersome method was necessary because Gassendi had no accurate clocks, which hadn't been invented yet.

continued in next post