The actual slow rotation of Venus was first found by radar observations from Earth.

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Radar observations from Earth have been used to determine the actual rotation rate of Venus. Radar observations from Earth in the 1960s revealed the slow retrograde rotation of Venus, which rotates on its axis in the opposite direction of its orbit around the Sun. Prior to these observations, it was believed that Venus had a rotation period similar to Earth's.

The Actual Slow Rotation of Venus Was First Found by Radar