In 1817, when the place of Second Pupil was freed, Mossotti was advanced to Second Pupil and started to receive a salary. In 1817 and 1818 Mossotti was working at the ephemerides, and published some supplements to his Nuova analisi del problema di determinare le orbite dei corpi celesti. His fame was by now truly international: in 1817 Gauss published in Göttingen a review of the aforementioned paper, and in 1818 Encke published in Tübingen an abridged German translation of the paper and the supplements.
On 22 August 1818 (M17) de Cesaris wrote to the Government proposing the promotion of the Second and Third Pupils to (respectively) First and Second, and the appointment of Gabrio Piola as Third Pupil. It is clear from the letter that Mossotti was kept in exceptionally high consideration by Brera Observatoire. About half of the letter is devoted to the praise of Mossotti's fame and ability, in which the director of the Observatory writes in fact a short eulogy of his pupil, saying that he is a man of ``wise conduct and continuous application", that he tackles theory as well as practical observations, and both to a very high level indeed. There is a short account of his two main publications, the paper Sopra il movimento di un fluido elastico che sorte da un vaso, which in de Cesaris' opinion shows ``how much he [Mossotti] is advanced in the art of sublime mathematics", and the appendices to the ephemerides and their supplements (Nuova analisi del problema di determinare le orbite dei corpi celesti), ``full of elegant formulae and profound calculations, and whose abstracts were published in the German journals of Göttingen and of the Baron of Lindenau". De Cesaris goes on pleading for a high salary to be assigned to Mossotti. In the rest of the letter, de Cesaris devotes to the advancement of the Third Pupil (Enrico Brambilla) to Second Pupil only a very short paragraph, saying that Brambilla is very good in calculations and necessary to the Observatoire because he speaks German fairly well. For the occupation of the place of Third Pupil de Cesaris recommends Gabrio Piola, a young Milanese nobleman who had previously studied in Pavia with Brunacci, saying that Piola had been helping in the Observatoire for the previous two years.
The reply of the Government dates 13 April 1819 and nominates Mossotti as First Pupil and Brambilla as Second Pupil. For the place of Third Pupil the Government organized an examination to be taken in the universities of Padua and Pavia. The recommendation of de Cesaris probably bore considerable importance as there were only two candidates to the examination, one of which was obviously Piola, who finally got the job (not to mention the fact that the examination marker was de Cesaris himself!).
With Mossotti's nomination to First Pupil a long quarrel between Brera and the Government began over Mossotti's salary. In the nomination letter, Mossotti's salary had been fixed to 1200 florins per annum, but when he went to collect his pay they only gave him 1200 lire (one lira was worth about a third of a florin). Mossotti informed de Cesaris who wrote to the Government, which replied that the copist had made a mistake. After many pleas from Brera and negative replies from the Government, there came a letter from the Government saying with threatening tones that Mossotti's salary had been fixed at 1200 lire, i.e. 400 florins per annum, and there could be no further appeal: that decreed the end of the disagreement.
In 1819 Mossotti published another interesting paper as an appendix to the Effemeridi astronomiche di Milano per l'anno 1820, in which, in order to explain the difference in the results of the calculations made by Littrow and Piazzi to determine the diameter of the sun, Mossotti advances the supposition that the sun is in fact an ellipsoid and not a sphere. According to Mossotti, that is the ``simplest and most probable cause" to explain the fact that people in different places observe different diameters.