By Shelley Moore | Updated Mar 24, 2022
On Earth, a day is 24 hours long, defined by one full rotation on its axis. Mercury, however, takes 176 Earth days to complete a single rotation from sunrise to sunrise. That means a day on Mercury lasts twice as long as its entire year.
Mercury’s orbit around the Sun is remarkably swift, completing one revolution every 88 Earth days. Its rotation is sluggish, taking 176 Earth days to turn once. The result is that Mercury experiences one year of daylight followed by one year of darkness.
Venus also has a day that exceeds its year. One Venusian day lasts 243 Earth days, while its year spans about 225 Earth days. Mercury, however, has the shortest rotation period in the Solar System.
Due to Mercury’s elliptical orbit and slow spin, the Sun’s apparent motion is dramatic. It can appear to pause, reverse direction briefly, then resume its path, creating a looping sunrise that can last days. The Sun’s apparent size fluctuates, sometimes shrinking enough to reveal background stars. When the Sun sets, the sky brightens with a rapid burst of starlight.
Mercury is the planet closest to the Sun, at an average distance of 58 million kilometers. From Earth, it is visible only during twilight, appearing as a bright point near the horizon after sunset or before sunrise.
Mercury is the smallest planet in the Solar System, with a diameter of 4,879 kilometers—slightly larger than Earth’s Moon (3,475 km). It has a very thin exosphere and a heavily cratered surface, resembling the Moon more than any other planet.
