Mercury is the closest planet to the Sun but, perhaps surprisingly, it does not have the highest temperatures. It is the second densest planet of the Solar System, but also the smallest planet. The structure of Mercury makes it the most similar planet to Earth.

Mercury's highly eccentric, egg-shaped orbit takes the planet as close as 29 million miles or 47 million kilometers, and as far as 43 million miles or 70 million kilometers from the Sun. It takes a trip around the Sun every 88 days thus 1 orbit/year is the equivalent of 88 Earth days. Mercury travels through space at nearly 29 miles or 47 kilometers per second, faster than any other planet.

Radar observations in 1965 proved that the planet has a 3:2 spin–orbit resonance, rotating three times for every two revolutions around the Sun. The eccentricity of Mercury's orbit makes this resonance stable at perihelion, when the solar tide is strongest. The Sun is nearly still in Mercury's sky. The orbital eccentricity of Mercury in simulations varies chaotically, from zero or circular to more than 0.45 over millions of years because of the perturbations of the other planets.More accurate modeling based on a realistic model of tidal response has demonstrated that Mercury was captured into the 3:2 spin–orbit state at a very early stage of its history, within 20 or 10 million years after its formation. Mercury spins slowly on its axis and completes one rotation every 59 Earth days. But when Mercury is moving fastest in its elliptical orbit around the Sun, and it is closest to the Sun, each rotation is not accompanied by a sunrise and sunset like on most other planets. The morning Sun appears to rise briefly. It then sets and rises again from some parts of the planet's surface. The same thing happens in reverse at sunset for other parts of the surface. Mercury travels in an elliptical orbit slowing down when it’s farther from the Sun, and accelerating as it draws closer.

Mercury Facts