Uranus at opposition tonight
William Herschel's planet, Uranus, which sounds remotely repulsive whether you pronounce it the right way (URINE-us) or the wrong way (yer-ANUS), is at opposition today. Wouldn't it have been nicer if they'd transcribed the Greek in a more appropriate phonetic style as Ouranos? Then we wouldn't have so many schoolboy jokes about this planet. There. Now, having established my anti-schoolboy street cred, I can go on to explain that I am talking about astronomical opposition, not some sort of Jungian midlife crisis thing. (Were I copyediting my own copy, I would strike the extraneous "thing" in that last sentence. But I'm not going to kill my own joy that way. Wait a minute. Does the fact that I'm pursuing my own pleasures in self-expression by not censoring myself mean that I am entering a second childhood. Already? Maybe this is a midlife crisis thing.) statistics: Uranus is the penultimate (i.e., seventh) planet from our Sun. At 2.8 x 109 km distance, its orbit around the Sun takes 84 Earth years. Like its larger, closer cousins, Jupiter (orbital period 12 years) and Saturn (orbital period ~29.5 years), Uranus has rings. Unlike them and all the other planets in our solar system, though, its axis of rotation is nearly perpendicular to the ecliptic plane, presumably as a result of a collision with an Earth-sized body in the past. This peculiar orientation makes seasons on Uranus rather interesting, although slow to evolve. Each season takes about 20 of our Earth years, and they progress from pole to equator to pole, unlike the situation on Earth, where we have a tropic zone that has no astronomically-related seasons. Uranus also has rings, as seen here through the Hubble Space Telescope in 1995: rings, like Jupiter's, are pretty much invisible from Earth, although they are detectable at particularly favorable occultations of stars, as happened in 1977. Uranus was discovered, if that's the word for it, in 1781 by one of the greatest observational astronomers of his, and any, time: William Herschel. The discovery is actually a sequence of events, as many astronomical discoveries are: observation, misunderstanding, insight, correct understanding. In the case of Uranus, the person who first observed it was by no means its discoverer; it had been seen and (mis)catalogued dozens of times before 13 March 1781, when Herschel trained his telescope on the stars near H Geminorum--and Herschel didn't know what it was when he found it, although he was sure what it was not: a star! (So it makes sense that the name he would propose for it, Georgium Sidus, means George's star!) The Greeks probably had been able to see Uranus; it's a 6th magnitude object, so it is visible, if nowhere near as bright as the other planets, in our night sky. John Flamsteed, the royal astronomer, observed it on at least 6 separate occasions in 1690,1712, and 1715 (Price, Planet Observer's Handbook, 321) but because of its slow movement and his smallish telescope which didn't enable him to see, or observe, its disk, he cataloged it in the Historia coelestis Britannica, as a star, 34 Tauri. The star didn't make it into the 1753 edition of the Atlas Coelestis, though. At least, I can't find it on the Taurus plate; maybe you can? Another near-discoverer of the planet was Pierre Charles Le Monnier, who, according to Price, "had recorded the position of the planet on four consecutive nights and two more times within nine days. These six 'stars' were actually the one moving planet. Unfortunately, Le Monnier did not compare his observations; if he had done so, Uranus would have been discovered 12 years earlier in 1769." It took Herschel's homemade 6.2-inch reflector with a seven foot focal length (typical for the time) to see that the object had an extended disc, and was therefore not a star. Here is how he described the observation in a letter read to the Royal Society on April 26, 1781:
On Tuesday the 13th of March, between ten and eleven in the evening, while I was examining the small stars in the neighbourhood of H Geminorum, I perceived one that appeared visibly larger than the rest: being struck with its uncommon magnitude, I compared it to H Geminorum and the small star in the quartile between Auriga and Gemini, and finding it so much larger than either of them, suspected it to be a comet. (Bakich, Cambridge Planetary Handbook, 264)There are at least two reasons why Herschel, an amateur astronomer--excellent, but amateur-- would have assumed he had discovered a comet rather than a planet. (As you can imagine, I have nothing against amateurs, being one myself!)
- Everyone knows (or they did at the time, anyway) that there are only 6 planets*, and since we're on one of them, there were only 5 to see in the sky: Mercury, Venus, (Earth, which doesn't count), Mars, Jupiter, and Saturn. No one, in the entire history of the world, had ever discovered a planet. Comets, on the other hand, were quite a bit more common.
- As Herschel increased the magnification of his 7-foot (focal length, not aperture) telescope, the disc of the object appeared to increase in diameter. Further "proof" of its nature as a comet came 4 days later, when he checked its position, and it had moved.