"Hence, we may justly conclude that the Returns of Comets are much more frequent than is vulgarly reckoned."
There is something truly thrilling about the first-hand accounts of scientists who have homed in on an idea or a fact that is now common knowledge. A few artifacts of such moments -- Darwin's early tree-of-life sketches or the first few minutes of this BBC documentary on Andrew Wiles' work on Fermat's Last Theorem -- convey the emotions and the creativity that surround those discoveries. A paper, published in Philosophical Transactions Vol. 30 (1717-1719) relaying the details of the discovery of a comet as "seen at London on the 10th of June 1717" and now available on JSTOR, does much the same.
Edmond Halley described the events as such:
On Monday, June 10, in the Evening, the Sky being very serene and calm, I was desirous to take a view of the disk of Mars (then very near the Earth, and appearing very glorious) to see if I could distinguish in my 24 Foot Telescope, the Spots said to be seen on him. Directing my Tube for the purpose, I accidentally fell upon a small whitish Appearance near the Planet, resembling in all respects such a Nebula ... The Reverend Mr. Miles Williams, Mr. Alban Thomas, and myself contemplated this Appearance for above an Hour ... and we could not be deceiv'd as to its Reality; but the slowness of its Motion made us at that time conclude that it had none, and that it was rather a Nebula than a Comet.
Of course we know now that it was indeed a comet, which the trio confirmed when they returned later in the week. Halley continued:
However, suspecting that it might have some Motion, I attended the next Night, June 11th, at the same Hours and in the same Company, when with some Difficulty by reason of the Thickness of the Air, we found the two little Stars, but the Nebula could not at that time be seen, which we then inputed to the want of a clearer Sky. But on Saturday, June 15, the Moon being absent, and the Air perfectly clear, we had again a distinct View of the two Stars, with an entire Evidence that there remained no Footstep or Sign of it, in the place where we had first seen this Phenonmenon, which we therefore now found to be a Comet.
As Halley writes in the introduction to his story, it was still quite unknown at the time whether comets were in abundance in the Solar System or were, in fact, quite rare. Halley theorized that, on account of several recent discoveries, there were perhaps many more than had been earlier supposed. "Hence," he wrote, "we may justly conclude that the Returns of Comets are much more frequent than is vulgarly reckoned."
But finding them would be tough, and require luck. "There may be still a much greater Number of these Bodies, which by reason of their Smallness and Distance are wholly invisible to the naked Eye," he observed, "So that unless Chance do direct the Telescope of a proper Observer, almost to the very Points where they are (against which there are immense Odds) it will not be possible for them to be discovered." And that's what it took -- a bit of luck, a bunch of curiosity, lots of telescope pointing -- to bring Halley, and with him, us, one step further in filling out the picture of what exists in the space around us.