On this day in 1906, the astronomer Clyde Tombaugh was born. He is known for having discovered Pluto. I saw him lecture when he was in his 80s. He was a very charming guy. And the work he did was absolutely the most difficult and tedious of observational astronomy. Basically, he would take a picture of a certain part of the sky on two separate days — usually roughly a week apart. Then, he would display them overlapped — fading from one to the other. And he would note any tiny differences. That’s how Pluto was discovered.

How hard was it? Here are two photographic plates, separated by six days. The arrows indicate where Pluto is. All I can say for sure is that there is no way that I would have been able to notice this difference. It is ridiculously small:

What’s most interesting about Tombaugh’s discovery is that it was a fluke. In the early part of the 20th century, Percival Lowell proposed that problems with Uranus’ orbit could be resolved with a ninth planet: Planet X. According to Lowell, the planet would have to be out at about 45 AU with a mass seven times that of Earth. Well, Pluto does have more or less that distance — but it is quite a lot closer these days. The bigger problem is that Pluto is really small. The initial estimate was that it was about the size of earth. But that estimate has only gone done over time. We now know it is about two-tenths of one percent the mass of the earth. It is less than 20% the mass of the moon. Eventually, it was found that the problems with Uranus’ orbit were due to an error in the mass of Neptune.

Still, Pluto is a cool bit of space debris. And until quite recently, it was the largest bit of space debris that we knew about. But in 2005, Eris was discovered. It is roughly a third more massive. And it is bizarre. But they are all the sun’s children and we love them all.

Happy birthday Clyde Tombaugh!