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On 1 January 1801, the first evening of the new century, Piazzi observed an unfamiliar point of light in the sky. He thought that the object might be a new star. Over the next three evenings he observed it again and noticed that it had shifted its position at the same rate as on the preceding days. Piazzi was now sure that it was not a fixed star. Thinking it might be a comet, he continued to follow it until 11 February when an illness cut short his work. However, on 24 January he announced his discovery to Bode, the French astronomer Joseph Lalande, and his friend Barnaba Oriani, director of the Brera Observatory in Milan.

He confided only to Oriani that it might possibly be a new planet: 'I have announced this star as a comet, but since it is not accompanied by any nebulosity and, further, since its movement is so slow and rather uniform, it has occurred to me several times that it might be something better than a comet. But I have been careful not to advance this supposition to the public.'

Oriani replied: 'I congratulate you on your splendid discovery of this new star. I do not think that others have noticed it, and because of its smallness, it is unlikely that many astronomers will see it.' Bode thought that Piazzi's discovery marvellously fulfilled his prediction of a planet between Mars and Jupiter. Von Zach was elated and reported the news in Monatliche Correspondenz, under the heading 'On a long supposed, now probably discovered, new major planet of our solar system between Mars and Jupiter'.

But Bode and von Zach could not verify the discovery. Such was the state of the postal service in those days that Bode did not receive Piazzi's letter until 20 March. By now the new planet had ceased its retrograde motion and had begun to advance, and had moved near enough to the Sun that it could not be seen. Everyone eagerly awaited its emergence from the other side of the Sun in July. Herschel was the first to search for it in July, and he and many other astronomers continued their search for months. The planet was missing again.

It required the genius of the German mathematician Carl Friedrich Gauss to recover Piazzi's lost planet. Gauss, then 24 and at the beginning of a brilliant career that placed him in the company of Archimedes and

Newton, calculated the orbit of the planet from Piazzi's few observations. Gauss's calculation of the planet's position was so accurate that on 31 December 1801, within a few hours of one another, von Zach and the German amateur astronomer Heinrich Olbers independently recovered the lost planet.

Piazzi named it Ceres Ferdinandea, but it was soon shortened to Ceres. King Ferdinand wanted to strike a gold medal with Piazzi's effigy, but the astronomer requested the privilege of using the money to buy a much-needed equatorial telescope for his observatory.

The discovery of Ceres posed a problem for astronomers. Herschel's observations showed that it was a most unusual planet, being too small to show a planetary disc. The problem was compounded when, on 28 March 1802, Olbers discovered 'another Ceres', a body that was also orbiting in the 'gap' between Mars and Jupiter. Olbers, a medical doctor by profession, named it Pallas. The discovery was most perplexing.

Olbers wrote to Herschel: 'Could it be that Ceres and Pallas are just a pair of fragments, or portions of a once greater planet which at one time occupied its proper place between Mars and Jupiter, and was in size more analogous to the other planets, and perhaps millions of years ago, either through the impact of a comet, or from an internal explosion, burst into pieces.'

Within a month of the discovery of Pallas, Gauss calculated its mean distance from the Sun, almost the same as Ceres. He also noted that Ceres and Pallas had many characteristics that made them quite unlike planets. Bode was not yet convinced that his law (which he had unceremoniously pinched from Titius) was not true. He wrote to Herschel: 'I hold myself convinced that Ceres is the eighth primary planet of our solar system and that Pallas is a special exceptional planet - or comet - in her neighbourhood, circulating around the sun. So there would be two planets between Mars and Jupiter, wherever since 1772, I have expected only one; and the well-known progressive order of the distances of the planets from the sun, is by this fully proved.' Herschel was not influenced by Bode's plea for his law. He was now convinced that Ceres and Pallas represented a new and different class of celestial bodies.

He also believed that since Ceres and Pallas did not occupy the space between Mars and Jupiter with 'significant dignity', these new bodies were not worthy of the name 'planet'. He proposed that they should be given the name 'asteroid' (from the Greek asteroeides, 'starlike'), since they are intermingled with, and similar to, the small fixed stars. He went on to advocate three forms of celestial bodies - planets, asteroids and comets.

Most astronomers now accepted that Ceres and Pallas were not planets, but Piazzi was not happy with Herschel's celestial hierarchy of planets, asteroids and comets, and retorted: 'Soon we shall be seeing counts, dukes and marquesses in the sky.' He suggested the name 'planetoids', pointing out that 'asteroid' would be more appropriate for 'little stars'. The term 'asteroid' has persisted, but they are sometimes also referred to as 'planetoids' or 'minor planets'.

Von Zach's 'celestial police' did not give up their quest, and continued to scan the heavens through their telescopes. Their efforts were rewarded when German astronomer Karl Harding discovered the third asteroid, Juno, on 1 September 1804. Olbers added the fourth, Vesta, on 29 March 1807. Both were too tiny to be qualified as a planet. The talented Olbers also suggested an idea which has passed the test of time: noting that the brightness of Ceres and Pallas appears to vary from one observation to another, he said that asteroids have an irregular rather than round shape. To Olbers, asteroids appeared like rocks tumbling through space. And he was right. However, he is now best known for the Olbers paradox, the answer to the deceptively simple question: why is the sky dark at night?

The thousandth asteroid was discovered in 1923 and was named Piazzia in honour of Piazzi (who had died nearly a century earlier in 1826 at the age of 80). Since that time, no year has passed without the discovery of new asteroids. Now discoverers do not have to fight for their right to name their discovery. Once the precise orbit of an asteroid is determined, it is given a permanent catalogue identification consisting of a number that denotes its order of entry, which is usually followed by a name proposed by the discoverer, for example, 1 Ceres, 2 Pallas. Until the discovery is approved by the International Astronomical Union, the asteroid is provisionally known by its year of discovery followed by two letters, and numbers if necessary, indicating the date and sequence of discovery; for example, 1950 DA (the letter 'D' signifies that it was discovered in the period 16-28/29 February, and the letter 'A' that it was the first discovery during that period).

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