The circular room, with a diameter of almost 20 metres, and the perimetric rooms are contained in a building with an octagonal plan, faced in ceppo di Albino stone engraved with decorative stars. A hemispherical dome roofs the entire octagonal structure.

Ionic columns in Crevola marble hold up the portico that gives accesses to the atrium, where daylight was dimmed by a dark blue ceiling and walls studded with silver stars.

The planetarium, donated to the city by publisher Ulrico Hoepli in May 1930, possessed a Zeiss model II that projected the night sky onto a canvas covering the underside of the dome. After damage sustained during bombings in August 1943, the canvas was replaced in 1956 by the current metal plating, which features a reproduction of Milan’s skyline around the edge of the lower rim.

Other significant buildings by Piero Portaluppi:

Restoration of Atellani House, 1919-1921, Corso Magenta 65
Società Filatura Cascami Seta Headquarters, 1921-1924, Via Santa Valeria 1/3/5
Società Metallurgica Italiana Headquarters, 1924-1927, Via Giacomo Leopardi 18
Radici-Di Stefano House, 1929-1931, Via Ulisse Aldrovandi 3; Via Giorgio Jan
Residential Building in Via Foppa, 1933-1934, Via Vincenzo Foppa 4
Wassermann Houses, 1935-1935, Viale Lombardia 17
Milan Fascist Federation Headquarters (with G. Albicci, L. Mattioni, G. Reggio, M. Salvadè, M. Tevarotto, M. Zanuso), 1937-1940, Piazza San Sepolcro; Via Valpetrosa 2; Via Fosse Ardeatine 4; Via Zecca Vecchia 1
Loggia and porticos of Palazzo Reale (with E.A. Griffini, P.C. Magistretti, G. Muzio), 1937-1942-1956, Piazza del Duomo; Via Guglielmo Marconi 1/3; Via Rastrelli 1