Last year, the solar system was officially reduced to eight planets when Pluto was demoted by the International Astronomical Union. Here a member of the IAU explains why and how the decision was madeby Stephen Eales / May 26, 2007 / Leave a comment
My Very Easy Method Just Speeds Up Naming Planets. Since 24th August 2006, children have had to find a new mnemonic for the names of the planets in the solar system, because on that day the International Astronomical Union voted to remove Pluto from the list of planets. The peculiar story of the fall of Pluto shows how human nature and politics can interfere even in astronomy, the most ethereal and beautiful of sciences.
The story starts over 200 years ago, in the back garden of a house in Bath, when William Herschel discovered a new planet—the first in modern times. In an act of spectacular grovelling, Herschel tried to call the planet Georgium sidus (George’s star). The name didn’t stick—the planet was named Uranus—but George III took the point; he granted Herschel a pension that allowed him to give up teaching music to young ladies and take up astronomy full time.
Herschel hadn’t stumbled on Uranus by chance. He was one of the first astronomers to realise the importance of systematic surveys, and over several thousand cold nights, he surveyed the whole sky five times. During the second of these surveys, he discovered Uranus, which he knew must be a planet because it slowly moved across the sky from night to night, unlike stars, which stay fixed in position.
The discovery of Uranus soon led astronomers to another planet. They realised that Uranus was not following the path predicted by Newton’s law of gravity, and in 1846 the French mathematician Urbain Leverrier showed that the discrepancy could be explained if Uranus’s orbit was being perturbed by the gravitational field of a planet further from the sun. A German astronomer, JG Galle, discovered a planet—later named Neptune—at the predicted position, which led to another outbreak of human nature. It turned out that a young British mathematician, John Couch Adams, had done the same calculation before Leverrier, but none of the senior British astronomers had taken him seriously. This produced a predictable Anglo-French spat over which scientist should share the credit for the discovery.
The final planet in the traditional set, Pluto, was discovered in 1930 by Clyde Tombaugh, an American astronomer. Tombaugh went back to Herschel’s method of looking for moving objects. The tool he used was a blink comparator, a device for displaying in rapid alternation two photographic plates…