Clyde Tombaugh: It was daytime when I did the scanning. When I took the photographs, I had no idea that Pluto’s image was on those plates. Not until I began to scan them carefully sometime later — in fact, it was several weeks later when I got to that pair — and I encountered, as going through all these alternating views of starfields, and here’s one that shifted position by the right amount.  Now I had figured out beforehand that if there was Planet X, how I would recognize it if I encountered it, you see. So I thought all this out beforehand. And so when it came, there it was, and instantly, I realized I had found a planet beyond the orbit of Neptune by the amount of shift. I knew the scale of the plates, I knew the interval, and it was just a matter of geometry.