The scheme has just picked up £3 million of crucial cash to ensure that it will be finished on time in spring 2007.
Allies and Morrison's planetarium - part of a wider £15 million development - will be clad in bronze, while the geometry of the building reflects 'basic astronomical ideas'. The 120-seat facility will be tilted at 51.5° so that it points towards the North Star.
The overall redevelopment of the Royal Observatory will include new galleries, a new science and astronomy education centre, and the planetarium.
The observatory has claimed that its plans will double the areas open to the public when they are complete.