Sample Lesson 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. Key Facts & Summary Since Mercury can be seen without the need of a telescope, many ancient civilizations saw the planet, and as such it is impossible to determine who discovered it first. However, it was first observed with the help of a telescope in early 17th century, by Galileo Galilei. Galileo’s crude telescope didn’t manage to capture Mercury’s phases, this would be observed later by astronomer Giovanni Zupi in 1639, and thus he discovered that the planet had similar phases like Venus and the Moon. In ancient times, Mercury was taught as being two different objects in the sky: The Mourning Star and The Evening Star. In Venus’s case, it was also mistakenly believed to be two different things. Venus spends most of its time away from Earth. This paradoxically makes Mercury the closest planet to Earth, a plurality of the time. Mercury was named after the Roman messenger god, because of its fast movements around the Sun. Mercury is the closest planet to the Sun at a distance of 57.91 million kilometers / 35.98 miles or 0.4 AU away. It takes sunlight 3.2 minutes to travel from the Sun to Mercury. Despite its closeness to the Sun, it is not the hottest planet, that title belongs to Venus but Mercury is the fastest planet, completing a trip around the Sun in 88 Earth days. This also makes one year on Mercury the equivalent of 88 Earth days, the shortest year of any planet. It orbits around the Sun with a speed of about 29 miles or 47 kilometers per second. Despite being the smallest terrestrial planet from the Solar System, and in fact the smallest of all the planets, it is the second densest planet in the Solar System, with a density of 5.43 g/cm³. For a comparison, Mercury’s size is about a third of Earth, and Earth has a density of 5.51 g/cm³. Mercury has a radius of 2.439 km or 1516 mi, and a diameter of 4.879 km or 3.032 mi. Mercury’s axis has the smallest tilt of any of the Solar System’s planets at about ​1⁄30 degrees, while its orbital eccentricity is the largest of all known planets in the Solar System. Mercury’s distance from the Sun is only about two-thirds or 66%, of its distance at aphelion, at its aphelion it is 0.44 AU away from the Sun. At its closest distance or perihelion, it is 0.30 AU away from the Sun.