Observations of the transit of Mercury have been reported as far back as the ninth century, but after Galileo introduced the telescope in 1610, it became clear that earlier observers probably saw sunspots rather than Mercury.
For transit on November 7, 1631, a number of astronomers were set up to capture Mercury's motion in front of our star. Only one, a Catholic priest in Paris named Pierre Gassendi, published his observations, indicating that he could not quite believe what he saw then.
"I never suspected that Mercury would project such a small shadow," Gassendi wrote.
The priest assumed that the little spot he saw was only a sunspot, because he expected Mercury's record to cover about a tenth of the sun, when in reality it seems more like a hundredth of the size of our star.
"It is really curious that early observers thought they were watching Mercury on the sun when they saw a sunspot and that now Gassendi, when he actually observed Mercury on the sun, thought he was looking at a sunspot," Albert Van Helden wrote 1976 for the Journal for the History of Astronomy.
Back in Gassendi's time, there was still a lot of disagreement about the arrangement and scope of the cosmos. This was the time period when scientists fought for the earth orbiting the sun or vice versa. Interestingly, though, both camps more or less agreed on the approximate sizes of the plans. And when it came to Mercury, they were both wrong.
"Thankfully, he continued to observe for several hours, noting that the small dark spot moved much faster across the sun's face and along a different path than a sunspot would do," writes Todd Timberlake, author of Finding Our Place solar system.
This endurance would pay off in the long run and lead to some important corrections in our understanding of Mercury's orbit and the size of the planets. Mercury's unexpectedly small size would also eventually lead to a more accurate measurement of the distance between the earth and the sun. This, in turn, would give us a better idea of how large the universe is.
But all this understanding was almost delayed, however, when Mercury was temporarily written off as just another spot on the sun.