Question

If it takes Mercury 0.21 years to orbit the Sun. How long (in years) will it take Mercury to come back to its original place in our night sky?

Answer 1

As per given data, since the Mercury takes 0.21 years to complete a single orbit of the sun. So a year on Mercury lasts almost as long as 0.21*365=76.65 days=77 days. Mercury has slow rotational period, it makes 3 complete rotations on its its axis for every two orbit it makes around the sun. So Mercury will take 77*2=154 days=0.42 years to come back to its original place in our night sky.