The Mount Gros observatory is not used any longer to watch the stars. The lights from the city prevent it. It hosts a calculation center. It was united with the Calern Observatory, not far from Grasse to form the Riviera observatory ( Observatoire de la Côte d'Azur. )
But the equipment still exist and among them the large white dome which you can see from the whole city. It was built at the end of the 19th century by Garnier, the Paris opera architect, who also built the Monte-Carlo Casino. The dome was built by Eiffel, (you know, the tower in Paris...) Inside is the great telescope. (A telescope with lenses, not with a mirror, the same kind of telescope Galileo used.) This 18m long monster was the largest in the world when it was built.
There are other instruments in the observatory, smaller but more specialised and which will be of interest to all astronomy lovers.
The sighseeing is breathtaking and often you can see as far as the Esterel mountains, West of Cannes.
You can visit the observatory, but you have to book your visit ahead. It is open to the public on sundays, wednesdays and saturdays à 2:45pm. You'll walk more than 2km on uneven ground, so wear comfortable shoes. If you have problems walking, it is not advised to visit the observatory.
Entrance tickets cost 6€ for an adult.
To go to the observatory from Nice you have to use the "High Corniche", Bvd Bischoffsheim. The entrance is behind the Mount Gros, when the road has made a curve and you don't see the city any longer. You'll find it on the map below.