On this night in 1787, William Herschel discovered the two largest moons of Uranus: Titania and Oberon. He had also discovered the planet six years earlier. Of course, “discovered” is a strange word, because Hipparchos recorded it as early as 128 BC. But he thought it was a star. Herschel himself originally thought it a comet. This is one of the reasons that I think everyone really owes it to themselves to go visit some amateur astronomers. Spend a night in the cold and see with our advanced technologies, just how hard it is to tell a star from a comet from a planet. And also note just how many stars are in the sky. And how vast it all is. It is a spiritual experience.
But the issue at hand today is the discovery of these first two moons. They’re tiny. Well, a bit more than half the radius of Pluto, and a quarter its mass. Larger, but comparable in size to Pluto’s biggest moon Charon. I still find it amazing. Our Moon is the fifth largest moon in the solar system; it’s very large compared to the Earth. And Charon is huge compared to Pluto. But Uranus has these tiny moons. Of course, that makes the detection of them all the more amazing.
Titania and Oberon are very similar: size, mass, inclination, eccentricity. They have the same composition: about half and half rock and ice. In fact, they are so much alike, it’s kind of creepy. The main difference between them is that Oberon has a whole lot more craters. It is assumed that Titania experienced some recent major event that covered over the old surface. If you look at the image above, you can see an enormous impact in the upper right-hand corner. Could it be that a huge impact temporarily liquefied the surface? I’ve never heard anyone argue that, and I do know that our Moon has bigger impacts. But it isn’t half made of ice!
It’s interesting that Herschel named the moons after the two most boring characters in A Midsummer Night’s Dream. Okay. Not the most boring, but I’ve always found that subplot very lame and kind of misogynistic. Anyway, I don’t know what to think of objects like these in space. The solar system is a bizarre place. And our solar system is really boring. Thank God! If I gave it more thought, I’d spend all my time hiding under the bed. But it’s great that people like William Herschel are around looking at the skies. And like I said: you really should go out. I’ve put in my time. Although I still gaze at the night sky every chance I get — just not huddled around a telescope for hours at a time.