One year on Mercury is 88 Earth days long. This is significantly shorter than Earth's year, which is 365.25 days long.

Here's why:

* Orbital Period: A year is defined as the time it takes a planet to complete one orbit around its star.

* Mercury's Orbit: Mercury orbits much closer to the Sun than Earth does, and the closer a planet is to the Sun, the faster it needs to travel to stay in orbit.

* Kepler's Laws: This difference in orbital period is explained by Kepler's Laws of Planetary Motion. One of these laws states that the square of a planet's orbital period is proportional to the cube of its average distance from the Sun.