The planet Mercury completes a single rotation in just under 176 days. Mercury was once thought to be tidally locked, and its rotation was not discovered until 1965.
The closest planet to the sun, Mercury has the shortest orbital period of any planet in the solar system. Mercury orbits the sun every 88 Earth days, making one day on Mercury almost twice as long as a year. The planet's rotation was first discovered in 1962 and confirmed three years later by astronomers using the Arecibo Observatory. It is possible that an asteroid impact may have been responsible for Mercury's strange rotational spin.