I had taken the plates of the telescope the previous month, in January 1930, and I did not know that I had recorded the image of Pluto on those plates, not until I scanned them later, in February. And so you passed your gaze over all these stars — you have to be conscious of seeing every star image because you don’t know which one’s going to shift, if they shift. So it’s very tedious work, and you go through tens of thousands of star images. Well, I came to one place where it actually was, turned to the next field, there it was. Instantly, I knew I had a planet beyond the orbit of Neptune because I knew the amount of shift was what fit the situation, and that was the most instantaneous thrill you can imagine. It just electrified me!

It was an intense thrill. It made my day! That was the 18th of February 1930, about four o’clock in the afternoon. I realized, in a few seconds’ flash, that I’d made a great discovery, that I’d become famous, and I didn’t know what would happen after that. So it was a very intense thrill. You don’t have that kind of a thrill very often!

At first, I had a little sense of caution. I thought I’d better check this with a third plate, which is another date, to see if there’s an image there in the right place that would be consistent with the images on the other plates. That was the final proof. Sure enough, it was there. That was when I was 100 percent sure.