For a long time, scientists thought Mercury rotated on its axis once every 88 days. This would make its day the same length as its year. If this were so, one side would always face the Sun, just as one side of the Moon always faces the Earth. Scientists couldn't prove this because they couldn't see any surface features. Tracking surface features is one way to determine how long it takes a planet to rotate. Even with powerful telescopes they couldn't see any features on the surface. Radar observations in the early 1960's showed that Mercury rotates about once every 59 days. In 1965, the exact number was later found to be 58 days and 16 hours. It rotates three times for every two times it circles the Sun.
