In 1965, scientists fired radar at Mercury: The returning echo revealed its Day was 59 Earth days, not the 88 days long believed

Astronomers initially estimated that Mercury completed a rotation every eighty-eight days, which appeared plausible at the time. However, radar observations in nineteen sixty-five uncovered that its rotation period is actually fifty-nine days. This remarkable discovery questioned previous visual assessments of surface features. Subsequent analysis established a 3:2 spin-orbit resonance, revealing that Mercury rotates three times for every two revolutions around the Sun.
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