A day on Mercury lasts twice as long as its year.

Mercury is the closest planet to the Sun. It travels at 47 kilometers per second to stay in orbit. This high speed allows it to complete a full trip around the Sun in about 88 Earth days.Mercury has a unique rhythm called spin-orbit resonance. It rotates on its axis three times for every two orbits it completes. This timing creates a very long solar day. One solar day is the time it takes for the Sun to return to the same spot in the sky.On Mercury, a solar day lasts 176 Earth days. This means the daytime lasts for 88 days and the nighttime lasts for 88 days. Because the planet has almost no atmosphere, temperatures change quickly. The side facing the Sun reaches 430 degrees Celsius. The dark side drops to -180 degrees Celsius.NASA's MESSENGER mission studied these movements between 2011 and 2015. The spacecraft confirmed that Mercury's orbit is not a perfect circle. At certain points, the planet moves faster than it rotates. If you stood on the surface, the Sun would appear to stop and move backward in the sky.