An Ertel's meridian circle was in the Observatory of the Navy Hydrographic Institute already at the end of the nineteenth century; it was mounted on two granite monoliths that rested on the most solid part of Fort St. George's wall, dating from the sixteenth century, where the Institute was operating.
The instrument was built at the Mathematical-Mechanical Institute of Munich, whose owner was Traugott Lebrecht Ertel (Fribourg 1778 - Munich 1858); he had completed his scientific studies there, and became maker of popular instruments for astronomical research, also in in collaboration with his sons Georg and Gustav.
The instrument, however, was not sufficiently stable, so it was replaced first by the transit telescope by Cook and later by the one by Bamberg.
The circle remained unused and therefore, in 1924, was loaned to the Observatory of Brera, at the opening of the branch of Merate.
In 1942 the Department of the Navy decided to dismiss the instrument without charge. Today the instrument is in the museum of that Observatory.
The meridian circle is a tool to detect transits, it can therefore detect the instant of meridian transit by celestial objects.
It is also featured with graded circles that allow to determine the zenith distance of celestial objects. Using the meridian circle one can thus obtain both coordinates (right ascension and declination).
The instrument consists of a telescope supported by a shaft perpendicular to its length, the telescope and the shaft intersect each other halfway.
Both the telescope and the shaft are shaped as two truncated cones: the four parts are connected at a cube located at the intersection, forming a cross (the two parts of the shaft and the cube are integral, while the two parts of the telescope are screwed).
The shaft was oriented in the east-west direction, so that the telescope, rotating around the horizontal axis, could go through the meridian from South to the North (the local meridian).
At both ends of the shaft are two equal brass rings. Both are divided every 3 degrees, with numerical indicators for each division engraved on a silver plate. One of the rings is fastened with screws, and the other one can rotate around the axle and be locked in the desired position by a screw.
The shaft rests on two pillars of granite. Instead of counterweights, to decrease the load there are two arms with springs screwed on two projections of the pillars. With a mechanism based on rails (in the figure it is the cart with two vertical arms that form a kind of fork), the instrument can be lifted from its supports and rotate around the vertical axis so as to exchange the supports of the shaft. Through observations made before and after the inversion, it is possible to detect some of the deviations of the instrument.
To check the levelness of the shaft, a long spirit level was used, supported by an arm. On the shaft there are two smaller rings, equidistant from the center: one of these, by means of a vise, allowed to fasten the telescope.
In front of each graded ring another ring is fastened to the granite pillars. On each one of these there were four micrometric microscopes at equal angles for reading the divisions, and another one for approximately pointing the telescope prior to an observation (almost all of these microscopes are currently lost). On one of the rings there are supports for a spirit level. The eyepiece was equipped with an impersonal micrometer.
In the center of each of the two free faces of the cube that forms the intersection of the telescope with the shaft there is a hole. The two holes are closed with lids; at the center of one of them is the support of the mirror for illuminating the micrometer wires. The two holes were done for using two collimators to determine the angle between the optical axis of the telescope and the geometrical axis of rotation, ie the error of collimation, though it seems that in Merate they didn't follow this approach.
Inside the cube there is a little dimming device, externally controlled by means of a rod ending with a screw.