Meridian Circle

Traugott Leberecht von Ertel, Monaco di Baviera, 1842
Brass, steel, bronze and wood
141 x 74 x 200 cm
INAF-Astronomical Observatory of Cagliari, Selargius (Cagliari)

The circle of the meridian of Ertel was one of the best instruments of the Roman College at the time of entrusting the direction of the latter to Angelo Secchi. It was donated in 1842 by the Superior General of the Society of Jesus, Jan Roothan, the same who later authorized the transfer of the Observatory on the roof of the Church of St. Ignatius.
Secchi, who made a thorough examination of it in 1850 (Figure 1), used it for the usual observations of the transit of the stars on the meridian, in particular for that of the Sun, which gave the signal of the Noon, in the project of reform of the measure of the time, strongly wanted by Secchi. This envisaged the switch to the European system (duration of the day measured from noon to noon, and divided in twenty-four hours), abandoning the system in use in the Papal States, which complied with the liturgical one, with the beginning of the day following the Sunset. To make the signal of the Noon public, Secchi adopted the system of the time-ball: A wicker ball, covered by black canvas, and inserted in a pole, was lowered down at the moment of noon, with a telegraphic signal, from the roof of the Church of St. Ignatius: at the sight of this signal, a cannon shot was fired from the Gianicolo.
To align the instrument, Secchi replaced the old target on the Aventine with a target placed on the Pincio Hill, where today a bust of the scientist is placed.
After Secchi’s death, the instrument had a singular destiny (Figure 2), passing from hand to hand, between different institutions. In 1910 it was give in by the Roman College Observatory to the Filotecnica of Milan in exchange for a larger instrument; here it was irretrievably modified and renewed in some parts, so as to be still usable. The instrument was then acquired by the Astrophysical Observatory of Catania where it came into operation in 1913. In 1971 he was given a permanent loan to the astronomical observatory of Cagliari, where it is still preserved and musealized.

Pino Calledda, INAF-Osservatorio Astronomico di Cagliari
Ileana Chinnici, INAF-Osservatorio Astronomico di Palermo

Bibliography
A. Secchi, “Esame del Circolo Meridiano”, Memorie dell’Osservatorio del Collegio Romano, 1850, pp. 9-23;
E. Paci, “Studio del Circolo Meridiano di Ertel -I”, Memorie della Società degli Spettroscopisti Italiani, serie 2, vol. 3, pp. 121-122.