Theory The Discovery of Asteroids

New Year's Day, 1801, by the dawn of the 19th century, Sicilian astronomer Father Giuseppe Piazzi pointed his telescope at the sky and observed a distant object that we now know as minor planet Ceres. Piazzi wasn't sure what he was seeing when he noticed a small, faint light through his telescope. Piazzi was the director of the Palermo Observatory in Sicily, Italy. According to the observatory, Piazzi had been working on a catalog of star positions on January 1, 1801, when he noticed something whose 'light was a little faint and colored as Jupiter.' He looked for it again on subsequent nights and saw that its position changed slightly. He wrote to fellow astronomers Johann Elert Bode and Barnaba Oriani to tell them he had discovered a comet, albeit he stressed that it lacked of nebulosity, its motion being so slow and rather uniform and that it could be 'better than a comet.' Piazzi told the press too that this object was a comet, but did not provide data from his observations, which generated criticism from other astronomers. Piazzi then became sick for a time, and said he could not observe the object any more. As newspapers spread the word that a comet had been found, astronomer Jerome de Lalande, based in Paris, wrote to Piazzi requesting relevant data in February. The Italian astronomer obliged in April, after recovering from his illness. One of Lalande's students, Johann Karl Burckhardt, performed calculations that revealed Piazzi's discovery did not have an orbit consistent with a comet's orbit. Instead, the data appeared to better fit a circular orbit. Letters that Piazzi wrote to his friends Bode and Oriani, on a other hand, about the so-called comet had been delayed due to the Napoleonic Wars. They finally reached the astronomers in March

The news was especially interesting to Bode because he had championed the Titius-Bode hypothesis: that the positions of planets in our solar system follow a specific pattern, which predicts each planet's distance from the Sun. Uranus, discovered in 1781, fit the prediction, too. But the pattern also demanded that there be a planet, yet undiscovered, between Mars and Jupiter. To find this missing planet, a group of German astronomers had established a society called the 'Celestial Police' ('Himmelspolizei' in German), with Franz Xaver von Zach as its secretary, in 1800. There were 24 astronomers who each scoured a 15-degree piece of zodiacal sky for the missing object. However, Piazzi did not receive his invitation to join this group until after he had spotted Ceres. Bode calculated a orbit based on Piazzi's data, and he believed that the object seen was the missing planet that fit his formula. Oriani, meanwhile, also calculated a orbit, and on April 7 asked von Zach to publish the news in his well-known astronomy journal, 'Monatliche Correspondenz,' that such a planet may have been discovered. As of spring 1801, besides Piazzi, no one had been able to observe the new celestial object because of cloudy skies and the object's position in its orbit -it was no longer visible at night, and the Sun blocked astronomers' views. Meanwhile, Piazzi still did not publish anything on the object, while he continued to refine his data. Several of his colleagues grew upset with Piazzi for holding back information. Without the data from his observations that concluded on Feb. 11, 1801, confirming his discovery would be more difficult. Since February, Ceres indeed had been lost. Why did Piazzi hesitate to make his data public? One reason might be that, though Piazzi was a skilled observer, he didn't have a solid theoretical knowledge of astronomy, so he couldn't calculate orbits quickly. Secondly, he risked the credibility and reputation of both himself and the Palermo observatory. But while he wavered, colleagues in Germany such as Bode firmly believed that there needed to be a planet between Mars and Jupiter. It was their conviction that helped keep the work going on this object. At last, in July 1801, Piazzi worked on calculating the object's orbit and made public his data about his observations from earlier in the year. And while other astronomers had already come up with their own names -such as Juno, Hera and Piazzi (to honor the astronomer)- Piazzi himself announced that the 'new star' was called Ceres Ferdinandea. Ceres was the Roman goddess of agriculture, and also the patron deity of Sicily as 'Ferdinandea" stood for honoring King Ferdinand of Sicily. Bode, who had wanted to call the object Juno, agreed on Ceres, telling: 'You have discovered it in Taurus, and it was re-observed in Virgo, Ceres of the old times. These two constellations are the symbol of agriculture. This occurrence is quite unique.' By the end of July 1801, many astronomers believed Ceres was a planet, but they needed additional confirmation and observations. Piazzi published his complete data set in von Zach's journal in September and, by doing so, got the attention of a young mathematician who would become instrumental in the fate of Ceres. Twenty-four-year-old Carl Friedrich Gauss had been experimenting with mathematical methods for which he would later become famous. When he applied those methods to Ceres, he came up with different predictions for its position than what others had calculated. Though some were skeptical about Gauss's results, his calculations enabled von Zach to be the first to see Ceres again, on Dec. 7, 1801, followed by other prominent astronomers of the time, and by Piazzi himself on February 23, 1802

As Gauss had calculated the orbit of Ceres, he had not resolved the fundamental question of defining what that new object was. In March 1802, Heinrich Wilhelm Matthias Olbers discovered a second, similar object, which later became known as Pallas. William Herschel, one of the most famous astronomers in history, then wrote an essay proposing that both Ceres and Pallas represented an entirely new class of objects he called 'asteroides.' Herschel wrote: 'If we called it planet, it would not fill the space between Mars and Jupiter with the dignity required by that position.' Though Herschel considered it an achievement that Piazzi had encountered the first example of a asteroid, Piazzi was disappointed. He thought that Herschel, who had discovered Uranus, just wanted to downplay Ceres. Piazzi wrote to Oriani: 'Be they called planetoides or cometoides then, but never asteroides [...] If a Asteroid Ceres must be called, so must also be called Uranus." Nonetheless, the door had opened for many more asteroids to be observed. The discoveries of Juno in 1804 and Vesta in 1807 reinforced Herschel's notion that asteroids were a class of their own. Herschel coined the term 'asteroide' because of their star-like appearance in telescopes