Messier's Missed Discovery of Pallas in April 1779

Abstract

1 . IntroductionBy the beginning of the year 1779, Charles Messier (1730-1817) had listed about half the number of the objects in his famous catalogue of the Nebuleuses et amas d'etoiles. The aim of this catalogue, which he had begun to compile in 1758, was to avoid any confusion between the comets he searched for diligently and deep-sky objects. At that time, even the very existence of the asteroids was unknown. However, a comet came very close to changing this state of affairs.This comet, which barely became visible to the naked eye, has the modern designation C/1779 Al (Bode). It was discovered at Berlin by the German astronomer Johann Elert Bode on the evening of 6 January 1779. A calculation, by numerical integration, of the positions of the comet and of the brightest asteroids shows that the comet passed Io. 36 from minor planet (2) Pallas on 6 April 1779. The evidence that Messier definitely observed that celestial approach appears on the chart of the comet's path drawn by him on the occasion of its passage and in the list of the positions that he measured from 19 January to 17 May 1779.We find evidence ofthat historical observation in Histoire de l'Academie Royale des Sciences, Annee M. DCCLXXIX. l Messier's observations of comet Bode are accurately recorded in the Memoire contenant les observations de la XVIIe comete, on pp. 31 8-72. 2 We will examine the celestial map drawn by Messier, and then Tables II and I which will reveal to us the date and the hour at which the observation of Pallas was made. Once we have this information, we will be able to calculate the true position of the asteroid and to compare it to the position given by Messier.2. Messier's Celestial ChartLet us first analyse the celestial chart (see Figure 1) on which is depicted the path of comet Bode, the seventeenth comet observed by Messier. The size of the map is 49.5 ? 16.5 cm and the projection used is of the type Sanson-Flamsteed concave.Figure 1 shows the lower right corner of that map and represents the starry sky near the limits of the constellations Coma Berenices (above), Virgo (below) and Leo (to the right), a Comae is the star situated in the upper left corner of the image close to the globular cluster M53 marked Nebul. 1777. The bright stars labelled e and s are e Virginis (Vindemiatrix) and s Leonis (Denebola), respectively. The path of comet Bode, with the positions measured by Messier from 28 March to 17 May, crosses the picture diagonally towards the southwest. Notice the comet's short tail; it is directed to the northwest and is well apparent at the beginning but is imperceptible after 15 April.The arrow in the upper left part of the figure points to the position of asteroid Pallas. The slightly fainter star northwest (upper right) of Pallas is the double star STF 1686 (or SAO 100314-15), of magnitude 8.5, whose components are separated by only 5 arcseconds. As this star was one magnitude fainter than Pallas, Messier correctly used a different symbol to represent it. Notice the larger number of small stars along the comet's track. These stars, reaching magnitude 9, acted as reference points for Messier's measures of the positions of the comet.As a keen searcher for nebulae, Messier also depicted on his map the principal galaxies of the Virgo cluster. These are the dark spots numbered from 1 to 1 1 .Spaced 5° apart, the meridians of right ascensions go from 175° (at right) to 195° (at left), while the circles of declination are from the celestial equator (at the foot) to declination +20° (at the top). These coordinates are referred to the mean equinox of 1779.0. They allow us to obtain, simply by interpolation, a first approximation to the coordinates of Pallas as Messier measured them. Expressed in the same format as that used by Messier in his lists, we find:α = 191°05'6 and δ = +15°28'36 (1779.0).3. Table IILet us now search for the written record of these coordinates. …