Mercury is the innermost of the eight planets in the Solar System. It is also the smallest, and its orbit has the highest eccentricity of the eight. It orbits the Sun once in about 88 Earth days, completing three rotations about its axis for every two orbits. Mercury has the smallest axial tilt of the Solar System planets. The perihelion of Mercury's orbit precesses around the Sun at an excess of 43 arcseconds per century beyond what is predicted by Newtonian mechanics, a phenomenon that was explained in the 20th century by Albert Einstein's General Theory of Relativity.

Mercury, being an inferior planet, appears as a morning star and an evening star, but is much more difficult to see than the other inferior planet, Venus. At its brightest, Mercury is technically a very bright object when viewed from Earth, but it is not easily seen in practice because of its proximity in the sky to the Sun.