Among the attempts to shake Bolognese astronomy from the lethargy it had fallen into in the first half of the XIXth century was the purchase of this meridian circle, ordered in 1846 by the then director of the Specola, Ignazio Calandrelli. The latter left Bologna for Rome in 1848 but returned for a while three years later to discharge his duty of mounting the instrument, together with its designer Ertel. In Bottigari’s 1851 Chronicle (vol. II, p.224) we read that "The celebrated artist Mr. Ertel of Munich has been in Bologna for some time: he is at present busy setting up his great Meridian Circle in the observatory of our University in those rooms that were expressly built under the plans and direction of our architect Filippo Antolini."
Among the manuscripts present in the Archives of the Department of Astronomy, the instructions for setting up and using the instrument can be found in a dossier called Uso del Circolo meridiano, written in Calandrelli’s own hand.
A room was expressly built next to the great hall of the turret for housing the meridian circle. The detailed estimate of costs and the plans for building the room, signed by the architect engineer Carlo Parmeggiani on 5 May 1848, can be found in the Archives of the Department of Astronomy. Four granite pillars were built into the floor [Inv. MdS-60 a,b,c,d] to support the axis of the telescope and rails (today disappeared) needed for carrying the wrought iron trolley [Inv. MdS-57] were fixed to the floor for inversion of the instrument. This operation was necessary in order to estimate certain measuring errors, by carrying out observations before and after 180° inversion.
It had two micrometers and four celestial eyepieces, as well as a bubble level [Inv. Mds-59], with relative leaf-spring support [Inv. MdS-58], for checking the horizontalness of the axis.
Despite the fact that this type of instrument required maximum stability, it was placed about 37 metres above the ground, on one side of the tower, which meant it was difficult to use and not very accurate; Lorenzo Respighi ended up using it exclusively as a zenithal instrument, by reflection in a basin of mercury placed in the room below.
On one of the arms is written the trademark Ertel & Sohn, Münich. In 1814, T. Ertel had become partners with Reichenbach - who had just left Utzschneider, Liebherr and Fraunhofer - soon becoming his successor and faithful practitioner of his building technique. They put their names to many instruments under the trademark Reichenbach und Ertel, but, when Reichenbach withdrew in 1820, Ertel continued alone until, in 1834, he was joined by his son Georg with whom he constituted the trademark Ertel & Sohn, that was to last until 1890. Ertel also designed meridian circles for the observatories of Christiania, Glasgow and Warsaw. The meridian circle built for the Istituto Idrografico della Marina in Genoa, later transferred to the Brera Observatory in 1924 (see Miotto et al., op. cit.) was of about the same period, while those made for Rome (Campidoglio) and Catania (already at the Collegio Romano in Rome) were before. The first volume of the Astronomical observations made during the year 1845 at the National Observatory, published by the Washington Observatory (now US Naval Observatory), contains a detailed description of a very similar instrument; the volume affirms that on account of the suspected presence of instrumental imperfections due to the divisions, flexures or other reasons, the hundreds of observations made with Ertel’s instrument were not published.
The room that housed it, altered mid-way through this century by Guido Horn-d’Arturo to take the multi-mirror telescope [file 50], was restored in 1988. On the south-east wall is the wooden cupboard built to hold the accessories of the meridian circle.
Atti della Pontificia Accademia dei Lincei, Tomo V, anno V, 1851-52, pp.162 ff.
J.A. Bennett (1987), pp. 155, 160.
H.C. King (1955), p. 242.
E. Miotto, G. Tagliaferri, P. Tucci (1990), pp.81 and 100.
M. Rajna (1906).
J.A. Repsold (1914), p. 16.