What’s so special about the orbit of the planet Mercury?
Mercury is tidally locked with the Sun in a 3:2 spin-orbit resonance, and rotates in a way that is unique in the Solar System.
As seen relative to the fixed stars, it rotates on its axis exactly three times for every two revolutions it makes around the Sun. As seen from the Sun, in a frame of reference that rotates with the orbital motion, it appears to rotate only once every two Mercurian years. An observer on Mercury would therefore see only one day every two Mercurian years.
Mercury’s axis has the smallest tilt of any of the Solar System’s planets (about 1⁄30 degree). Its orbital eccentricity is the largest of all known planets in the Solar System;[ at perihelion, Mercury’s distance from the Sun is only about two-thirds (or 66%) of its distance at aphelion.
This is an extract from the Wikipedia page on Mercury. I was encouraged to look this up following my viewing of Part 1 of the fascinating new television series “The Planets” presented by Professor Brian Cox.