‘The underlying sense of form in my work has been the system of the Universe’, Calder wrote in 1951. ‘The idea of detached bodies floating in space, of different sizes and densities… some at rest, while others move in peculiar manners, seems to me the ideal source of form.’

A Universe is a motorised work, in which a complex pattern is traced by two spheres, moving at different speeds along the looping wire paths. When the sculpture was first exhibited at the Museum of Modern Art, New York, Calder was told that Albert Einstein stood watching it for forty minutes, waiting for the mechanism to work through the ninety cycles of movement before it began to repeat itself.

In Calder’s Constellations, pieces of painted wood are linked together into fixed positions by steel wires. The title for the series was suggested by Marcel Duchamp and the curator James Johnson Sweeney. ‘I was interested in the extremely delicate, open composition’, Calder recalled, also noting that ‘it was a very weird sensation I experienced, looking at a show of mine where nothing moved.’