Set between two domed 1890’s observatories clad in elaborate terracotta, the simple cone mediates between them. It forms the centrepiece of a new stone-paved public court. The geometry of the planetarium reflects key astronomical concepts. Above ground it is manifested as a tilted bronze cone aligned with the north star at 51.5 degrees. The disc cut at 90 degreees through its apex is parallel to the celestial equator. This plane is clad in layers of reflecting glass in which the space of the passing sky is revealed. A groove up the south side marks the axis of the cone on the meridian line and aligns with the pole star.
Precision construction was required to create this exact geometrical form, achievable only with prefabricated metal cladding. Bronze was chosen not only for its rich looks, but also for its timeless connections with historic astronomical instruments.
The cone beneath is constructed from 250mm thick concrete to keep out sound. This is clad in a 8mm thick phosphor bronze carapace weighing 32 tonnes in total. The metalwork was prefabricated in Gateshead then brought to site in 18 curved segments, where it was site welded using highly specialised full penetration bronze welding to achieve the exact conic form. This demanding process took five welders 13 weeks to complete.
Temporary stiffening struts allowed each panel to be aligned and held in the exact position required, including an allowance for shrinkage. Only then was the whole shell bolted to the movement brackets. The resulting welded surface was then ground down to remove traces of the metal weld lines before it was patinated.
Four layers of acid solutions were applied to build up the surface of rich reds ad browns finished with a hot-applied patination of green splashes resonant of nebulae seen in images from outer space. The final waxed finish gives a matt lustre to the seamless surface.