In the early 1930s Calder's desire to create abstract paintings that moved through space led to motorized works such as A Universe, in which the two spherical shapes traveled at different rates during a 40-minute cycle. Interested in astronomy, he compared his works' discrete moving parts to the solar system. These works were an important step towards his non-motorized mobiles, as well as forerunners to his Constellation series of the 1940s.