large equatorial

One sees from everywhere around Nice, this imposing building, capped of its dome, which is reproduced on the last former banknotes of 200 francs realized in homage to Eiffel. It became today, the symbol of the Observatory, for the Nice inhabitants.
Raphael Bischoffsheim, keen of Egyptology wanted its principal instrument sheltered inside a temple of Astronomy placed under the sign of sunlight.


*"Apollo who emerges from the zodiac."*

The god, covered after the manner of ancient Egyptians, haloed of solar rays and holding up the torches of knowledge and intelligence, symbolizes science illuminating the world.

The dome rests on a circular wall inserted in the square monument, built out of ashlars of La Turbie (26,40m wide and 10m high). Each frontage presents four columns with Greek Ionic order capitals.
One penetrates in the sanctuary by an ashlar staircase and a bronze folding doors, inspired by sketches that Garnier drawn when he was studying the Pantheon of Roma, and which costed a fortune to Bischoffsheim.

In order to support the weight of the doma and to permit a perfect stability to the huge moving mass, the building was supported on a level of 7 depth meters foundations, directly posed on the rock.

In spring 1885, the dome was exhibited in the workshops of Eiffel in Levallois-Perret, in the Parisian suburbs. Then it was dismounted, shipped to in Nice and installed atop the Mont Gros at the end of 1885.

The general shape coupole-eiffel_coupe

of the dome is a half-sphere of an internal and outed diameters respectively of 22,40m and 23,90m.

The innovation of this construction was due to the simple application of the Archimedes' principle: any body plunged in water loses part of its weight, equivalent to that of the water which it moves.

The dome weighing 95000kg, it was built a circular tank intended to support it, being able to contain more than 95000 liters of water.

"It is simply posed on water, floats like a boat, so slightly that the hand of Parisian can make it turn using a small hand-winch. " (Camille Flammarion)

The annular tank had the same diameter as the cupola, with a height of 1,50m and a width of 1,20m. a steel reinforcement forming the base of the frame of the mobile dome was fixed on the float.

Eiffel did not choose pure water: it would have frozen below zero degrees, and that can happen atop Mont Gros exposing the dome to the risk to be immobilized during beautiful nights. He prefered a dissolution of magnesium chloride, which freezes only 40° below zero.

Garnier modified the project so that the dome can be operated either by means of the float alone, or by means of rollers.
Thus, it was possible to combine the two systems in order to make support with the rollers a gradual weight and to obtain while increasing or by decreasing this weight, resistance required to have a perfect stability. Moreover without blocking the observations, one could repair one of the two systems while using the other.

coupole-restauration The dome made a full rotation on itself in 4 minutes, which was remarkable! (The dome of the refractor of 38cm of the Observatory of Paris was so hard to turn, that one needed several men to put it moving, and forty five minutes of work were necessary to make it achieve only one turn).

The dome outside was furnished with an absolutely tight sheet envelope. And the system of opening was clever, the trap door of observation was closed by two external shutters, doubled of a series of small interior shutters opening independently and according the observer request.
This system prevented the wind from being engulfed on all the height of the trap door and held the interior of the dome sheltered against bad weather, after completion of the observations.

At the origin the movement of the dome was done using a hand-winch. In 1888, an electric motor was installed and made it possible to turn the dome at a variable speed.

Since 1969, date of restoration of the unit (which was delayed by an attack of the Eiffel family which refused to see flame-cutting the rusted cupola!) the repaired cupola, rests on a track by carrying wheels.

Mounting is by Paul Gautier, the objective, of 76cm of diameter and of 17,89m of focal distance was made by the Henry brothers, the magnification going up to 2250 times.
It was brought into service in 1887. It was then the largest in the world with the one of PoulkovoObservatory, in Saint Petersbourg, ( also of 0,76cm and brought into service in 1885).
It remains the second for the focal distance and the fourth by the diameter of its objective (76cm). Bischoffsheim invented himself the mobile system of observation: an escalator on the floor and a platform moving, laterally and in height, on rails fixed in the wall, to allow the observer to follow the refractor in its various positions.

The year of its installation marked the beginning of the systematic observations of double stars and research of nebulae. One also adapted to the instrument photographic rooms and cameras for the study of atmospheric turbulence.

References:
ANNALES de l'OBSERVATOIRE de NICE, t. 1.
C. FLAMMARION, L'observatoire de Nice et l'astronomie en France, in l'Astronomie, Bulletin de la Société d'Astronomie Française, 1885 (206).
A.FOLLI et G. MERELLO, Charles Garnier e la Riviera, Erga Edizioni, Genova 2000, (155).