Mercury revolves once about the Sun every 87.97 days. Radar observations of the planet show that it rotates only once every 58.65 days, two-thirds of its period of revolution. Only three of the planet’s days, therefore, occur during every two of its years. The side facing the Sun gets very hot, while the side facing away quickly cools to frigid temperatures. Mercury was named for the messenger of the gods in Roman mythology.

The point in Mercury’s orbit at which the planet is closest to the Sun (called the planet’s perihelion) moves a tiny amount every orbit, too much to be accounted for by the gravitational influence of other planets. The observation of these changes in Mercury’s perihelion was one of the first confirmations of Einstein’s theory of relativity, which predicted their existence.