Monday, November 30, 2009

Understanding Uranus and Neptune

Since ancient times, the inventory of the solar system was clear and seemingly complete: a sun and, in addition to Earth, five planets, Mercury, Venus, Mars, Jupiter, and Saturn. Then came along one of those scientific busybodies that the eighteenth century produced in abundance. Johann Daniel Titius, or Tietz (1729–1796), a Prussian born in what is now Poland, poked his curious nose into everything.
He was a physicist, biologist, and astronomer who taught at the University of Wittenberg.
It occurred to him, in 1766, that the spacing of the planetary orbits from the sun followed a fairly regular mathematical sequence. He doubled a sequence of numbers beginning with 0 and 3, like this: 0, 3, 6, 12, 24, 48, and so on.
He added 4 to each number in the sequence, then divided each result by 10. Of the first seven answers Titius derived—0.4, 0.7, 1.0, 1.6, 2.8, 5.2, 10.0—six very closely approximated the relative distances from the sun, expressed in astronomical units (remember, an A.U. is the mean distance between the earth and the sun), of the six known planets.
No one paid much attention to Titius’s mathematical curiosity until another Prussian astronomer, Johann Bode (1747–1826), popularized the sequence in 1772. Neat as it was, the sequence, which became known as the Titius-Bode Law or simply Bode’s Law, is now generally thought to be nothing more than numerology. For one thing, there is no planet at 2.8 A.U. This gap would be filled later by the discovery of the asteroid belt at this location. While the rule gives a number that is close to Uranus, it breaks down for the positions of Neptune and Pluto. Since those planets had yet to be discovered, no one saw it as a problem. But what about the numbers beyond 10.0 A.U.? Did the Titius-Bode law predict other, as yet unknown, planets?
The people of our planet did not have to wait long for an answer. On March 13, 1781, the great British astronomer William Herschel, tirelessly mapping the skies with his sister Caroline, took note of what he believed to be a comet in the region of a star called H Geminorum. On August 31 of the same year, a mathematician named Lexell pegged the orbit of this “comet” at 16 A.U.: precisely the next vacant slot the Titius-Bode Law had predicted.
Herschel, with the aid of a telescope, had discovered the first new planet since ancient times.
Once the planet had been found, a number of astronomers began plotting its orbit. But something was wrong. Repeatedly, over the next half century, the planet’s observed positions did not totally coincide with its mathematically predicted positions. By the early nineteenth century, a number of astronomers began speculating that the new planet’s apparent violation of Newton’s laws of motion had to be caused by the influence of some as yet undiscovered celestial body—that is, yet another planet. For the first time, Isaac Newton’s work was used to identify the irregularity in a planet’s orbit and to predict where another planet should be. All good scientific theories are able to make testable predictions, and here was a golden opportunity for Newton’s theory of gravity.
On July 3, 1841, John Couch Adams (1819–1892), a Cambridge University student, wrote in his diary:
“Formed a design in the beginning of this week of investigating, as soon as possible after taking my degree, the irregularities in the motion of Uranus … in order to find out whether they may be attributed to the action of an undiscovered planet beyond it ….”
True to his word, in 1845, he sent to James Challis, director of the Cambridge Observatory, his calculations on where the new planet, as yet undiscovered, could be found. Challis passed the information to another astronomer, George Airy, who didn’t get around to doing anything with the figures for a year. By that time, working with calculations supplied by another astronomer (a Frenchman named Jean Joseph Leverrier), Johann Galle, of the Berlin Observatory, found the planet that would be called Neptune. The date was September 23, 1846.

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