Observations of Mercury transit were reported as early as the ninth century, but after Galileo introduced the telescope in 1610, it became clear that earlier observers probably saw more sunspots than Mercury.
For the 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, suggesting that he could not quite believe what he saw at the time.
"I was far from anticipating that Mercury would project such a small shadow," Gassendi wrote.
The priest suspected that the small spot he saw was only a sunspot because he expected the Mercury disk to cover about one-tenth of the sun, although in reality it was more like a hundredth of the size of our star appears.
"It is indeed strange that early observers thought they were looking at Mercury on the Sun when they saw a sunspot, and that now Gassendi, as he watched Mercury on the Sun, thought he was looking at a sunspot," Wrote Albert Van Helden in 1976 for the Journal of the History of Astronomy.
At Gassendi's time, there was still a great deal of disagreement over the arrangement and size of the cosmos. During this time, the scientists argued about whether the earth revolves around the sun or vice versa. Interestingly, both camps more or less agreed on the approximate size of the planets. And when it came to Mercury, both were wrong.
"Fortunately, he continued to watch for several hours, noting that the tiny dark spot moved across the face of the sun much faster than a sunspot," writes Todd Timberlake, author of Finding Our Place in the solar system.
This perseverance would pay off in the long run and lead to some important corrections in our understanding of Mercury orbit and the size of the planets. The unexpected small size of the mercury would eventually lead to a more accurate measurement of the distance between earth and sun. This in turn would give us a better idea of how big the universe is.
All this understanding, however, was almost delayed when Mercury was written off for a moment as just another point on the Sun.