In Europe, the first meridian telescope was made by the Danish astronomer Roemer; it was the first of its kind, which Roemer called the Rota Meridiana. The observation of a celestial body above the horizon gives its altitude; this is the complement of its zenith distance (the angle it makes with the local vertical). The value of this altitude is largest when the celestial body crosses the meridian.
Even as the sectors equipped with a telescope increased in size through the XVIIIth century thanks to progress in metallurgy, up to the large mural quadrants made by the English instrument maker John Bird and of which the Paris Observatory has two examples, the instruments descended from the Rota Meridiana started to see the light of day.
Meridian Circles and Transit Telescopes
On the one hand, instruments of the transit telescope type, for the determination of the meridian transit time of a celestial body, began to appear; they were generally small and thus accurate. On the other hand, the sectors transformed into meridian circles; a complete circle with peripheral graduations and equipped with a telescope which could pivot around its center, furnished the zenith distance of the celestial body, and thus its altitude.
The combination of the circle with a transit instrument gives what is generally referred to today as the meridian instrument.
The original sectors were equiped with cross-hairs which enabled one to note the time of transit thanks to a clock. With the advent of electricity, the instrument makers were able to equip the eyepiece grid with mobile vertical wires, such that electrical contact was made when a vertical wire crossed one of the wires of the grid. The electrical contact was registered on a roll of paper running through a chronographic pen recorder.
Repeated measurements and chronographs
The average of a succession of ten or twenty such electrical contacts gives the moment of transit. At the same time, another observer registers the readings on the two circles associated with the telescope. The single reading of the original instruments of this type was replaced by repeated measurements made with microscopic micrometers on each of the two circles...
The texts describing the transit instrument, the so-called garden instrument, indicate that it was built thanks to the generosity of the banker Raphaël Bischofscheim who offered it to the Paris Observatory during Le Verrier’s lifetime. It was used to observe again the stars in Lalande’s catalogue of 5000 stars, then for various international programs. The last time it was used was for the establishment of the AGK3 catalogue: in fact, the Observatory was the first to finish the work for the part of the sky which had been attributed to it. It was replaced by the automatic transit instrument of the Bordeaux Observatory, than by the astrometric satellite Hipparcos.
