Here's why:

* Orbital Order: Mercury is the innermost planet in our solar system, meaning it orbits closest to the sun.

* Planetary Orbits: While the planets' orbits are not perfectly circular, they are fairly consistent, meaning Mercury remains the closest planet to the sun throughout the year.

It's important to note that the planets' distances from the sun do vary slightly due to their elliptical orbits, but Mercury always stays closest.