Rotation of venus

Venus, despite being Earth's closest neighbor, remained shrouded in mystery because of its thick and featureless cloud layer. Scientists used microwave reflections to determine Venus's very slow period of axial rotation of 250 days.
 Roland L. Carpenter and Richard M. Goldstein United States 1962 Rotation of venus

Venus, despite being Earth's closest neighbor, remained shrouded in mystery because of its thick and featureless cloud layer. Scientists used microwave reflections to determine Venus's very slow period of axial rotation of 250 days.