Question #22 – Mercury is the smallest planet in our solar system. It is also the closest planet to the Sun. How long is a year on Mercury? How long is a day?
One year on Mercury, the time it takes to orbit the Sun once, is 88 Earth days!
Mercury's day, the time it takes to rotate once on its axis, was thought to be longer than its year. Scientists thought Mercury was tidally locked with the Sun, much like our Moon is to the Earth with one side always facing the Earth. Radar observations in 1965 showed that Mercury's day is actually about 59 Earth days.
Read more about Mercury on NASA's Solar System Exploration site.