He investigated Herschel's large telescopes (indeed, he fell and broke his arm while examining one of them that was mounted outdoors) and, with Maskelyne, observed at Greenwich the solar eclipse of 3 June 1788. His first astronomical work, a study of the difference in longitude between various observatories, based on that of Greenwich, was published in the 'Philosophical Transactions' of the Royal Society in 1789. The most important result of Piazzi's English visit, however, was the great five-foot vertical circle, a masterpiece of eighteenth-century technology, that he commissioned from Ramsden.

When in 1792 I started to study the stars that are in Mr Wollaston's catalogue, I decided to study the stars that are in the area of my telescope and note them.

Most people are aware that the celebrated astronomer Piazzi discovered the small planet Ceres at Palermo in this very observatory, with an instrument of Ramsden's which we had the satisfaction of seeing. It was made on the 1st of January, 1801, at which period the present astronomer, Cacciatore, was Piazzi's assistant in the observatory of which he is now the chief. As Piazzi was at that time engaged in making the noble catalogue of the stars, which has since become so well known, he placed himself at the telescope, and observed the stars as they passed the meridian, while Cacciatore wrote down the times, and the polar distances, as they were read off by his chief. Certain stars passed the wires, and were recorded as usual on the 1st of January, 1801. On the next night, when the same part of the heavens came under review, several of the stars observed the evening before were again looked at, and their places recorded. Of these, however, there was one which did not fit the position assigned to it on the previous night, either in right ascension, or in declination. "I think," said Piazzi to his companion, "you must, accidentally, have written down the time of that star's passage, and its distance from the pole, incorrectly." "To this," said Cacciatore, who told me the story, "I made no reply, but took especial pains to set down the next evening's observations with great care. On the third night there again occurred a discordance, and again a remark from Piazzi that an erroneous entry had probably been made by me of the place of the star. I was rather piqued at this," said Cacciatore, "and respectfully suggested that possibly the error lay in the observation, not in the record. Under these circumstances, and both parties being fully awakened as to the importance of the result, we watched for the transit of the disputed star with great anxiety on the fourth night. When lo, and behold! it was again wide of the place it had occupied in the heavens on the preceding and all the other nights on which it had been observed. 'Oh, oh!' cried the delighted Piazzi, 'we have found a planet while we thought we were observing a fixed star; let us watch it more attentively?'" The result soon confirmed this conjecture, and thus was made one of the most interesting, and I may say useful, astronomical discoveries of modern times.

Sicily - Palermo, January 15. On the 1st of this month, a new comet in the shoulder of Taurus, near the 19th star of Mayer, was discovered from our observatory. It was observed on 1st, 2nd, 3rd and 4th, as it passed the meridian. Although it is not covered with any kind of nebulous spot, it still cannot be seen with the naked eye. Its movement is retrograde; it goes forward towards the north.

I have announced this star as a comet, but since it shows no nebulosity, and moreover, since it had a slow and rather uniform motion, I surmise that it could be something better than a comet. However, I would not by any means advance publicly this conjecture. As soon as I shall have a larger number of observations, I will try to compute its elements.

We further remark that these four planets [Uranus, Ceres, Pallas, and Juno] were found while searching for something else, and conclude that the real way to deserve and to encounter such accidents is to be occupied in some grand undertaking, which in itself is of real use, and keeps us constantly on the route to such discoveries; it is, for example, to work, as M Piazzi, to perfect and augment the stellar catalogue, observing each star repeatedly for several days: this method has the double advantage to register in the catalogue only the reliable positions, and to evidence in the long run the planets that could still be confused among the innumerable quantity of very faint stars scattered in the sky.

... are going to think that I am a fool because I have deduced the proper motions of a few stars on the observations of only ten years, and in some cases less than that.

Edmond Halley, towards the beginning of the last century, having compared the longitudes of the stars reported in the 'Almagest' with those observed in his day, was the first to recognise in some of them motions which were fully distinguished from the annual precession.

Jacques Eugène d'Allonville de Louville and Jacques Cassini confirmed this interesting discovery, and Tobias Mayer extended it and gave further confirmation, comparing his observations with those of Ole Romer. These first steps, however, did not advance beyond a very few stars, and even these were not able to be given with precision. Quite exact and remote observations were lacking, so nothing could be affirmed with certainty. Our age was somewhat less difficult; and through the catalogues of John Flamsteed, and principally of Nicolas-Louis Lacaille and Tobias Mayer, it was possible to assign particular motions to several stars. We owe this to the work of Maskelyne, Lalande, and others; but above all to that of the Vienna astronomer Franz de Paula Triesneker. Very little has been done up to now, and very much remains to be done. If motions are found, to be well established they need to be verified and confirmed; and I want to examine no less the many other stars registered in the catalogues which are not yet subjected to discussion. For this reason, after the publication of my catalogue, having in 1803 re-observed many stars contained therein, I entered into the hope of being able to somewhat promote this still nascent very important part of astronomy. For now I have chosen about three hundred stars among those re-observed in 1803; and I have tried to investigate the variations to which they seem subjected. If this first essay will be so fortunate to obtain the approval of the National Institute, to which I dedicate it, I hope to be able to subsequently give it a continuation regarding all the stars of Flamsteed, Lacaille and Mayer.

Upon the whole, this work reflects great credit on the distinguished astronomer who now conducts the observatory at Palermo. The plan, although nearly the same as was pursued by M Piazzi, and therefore not altogether new, is different from the printed observations of other observatories: and is in many respects worthy of imitation. For, although we would not wish to be considered as discouraging the printing of observations in the order, and in the manner in which they are actually made, yet we are certainly desirous, in common with many others that are fond or astronomy, of seeing the reductions of those observations a little more frequently than we do ...

When the direction of this Observatory was entrusted to me, and the task of guiding the young students to the knowledge of astronomical things, I devised and arranged those courses of lessons which seemed to me the most suitable for this purpose. In view of the progress which has been made in this science since that time, I have not neglected to insert appropriate additions from time to time, particularly after the publication of the distinguished work 'Astronomie Theorique et Pratique' by Delambre. In doing this I have never aimed at anything other than private instructions, which I regularly give within the domestic walls of the Observatory itself; nor had it occurred to me to write for the public, writing only for my students. But this book happened thanks to the authority of the Supreme Magistrate of Studies, who graciously welcoming and seconding the attentions of the young scholars, wanted these writings to be printed and brought to light. Here they are therefore as they are, not as I would like them to be; it is my belief that they will be of some benefit to the young, for whom they are only intended.

When Piazzi discovered Ceres, exploring it was beyond imagination. More than two centuries later, NASA dispatched a machine on a cosmic journey of more than 3 billion miles to reach the distant, mysterious world he glimpsed. Our knowledge, our capabilities, our reach and even our ambition all are far beyond what Piazzi could have imagined, and yet it is because of his discovery that we can apply them to learn more, not only about Ceres itself but also about the dawn of the solar system.