I went to the Reuben H. Fleet Science Center for my birthday, and I noticed something odd. Upstairs they have, in a glass case, what they call "English Planetarium, circa 1810." It's really nice, but the weird thing is that it has models of all nine planets. I'm not sure when the last few planets were discovered, but I know that Pluto was discovered this century. Did they just stick on new planets as they were discovered? It doesn't look like it. Incidentally, the newly redone science center is really cool.
-- Nephi, College Area
The Fleet folks appreciate your enthusiasm, but you sent them into orbit for a while trying to resolve the planetarium anomaly. Officially, the device is called an orrery; and this particular one consists of a circular brass box on feet; it has a post and a knob (the sun) sticking up through the middle with nine arms extended out around it. Little planet balls sit on pegs at the ends of the arms. And some of the little planet balls are surrounded by even tinier moon balls. There's a mechanism inside that propels the arms to make planets travel in their proper relative motions. The Fleet gang admits that in 1810 we had only confirmed seven planets. Neptune was 35 years away and Pluto more than 100.
According to Dennis Mammana, Fleet's resident astronomer, their orrery was made by a father-and-son team of London instrument makers who worked from 1794 to 1823. Around the turn of the 19th Century, some astronomers would include some of the newly discovered minor planets (asteroids) in a planetarium. They gave up the practice, sez Dennis, "when they realized how many asteroids there were and how futile it was to include them with the planets." Adding up all their clues, the Fleet force estimates that their orrery was actually made between 1802 and 1804, when there were seven known planets and two known asteroids.