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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.
* Mercury spins slowly on its axis and completes one rotation every 59 Earth days. One Mercury solar day or one full day-night cycle, equals 176 Earth days—just over two years on Mercury. (Reminder, one year on Mercury is 88 Earth days)
* Mercury does not have any known satellites or ring systems.
* Its surface is very similar to Earth’s Moon, implying that the planet hasn’t been geologically active for many years.
* Instead of an atmosphere, Mercury possesses a thin exosphere made up of atoms blasted off the surface by the solar wind and striking meteoroids. Mercury's exosphere is composed mostly of oxygen, sodium, hydrogen, helium and potassium.
* Temperatures on the surface of Mercury are both hot and cold. During the day, temperatures on the surface can reach up to 800 degrees Fahrenheit / 430 degrees Celsius. Because the planet has no atmosphere to retain that heat, nighttime temperatures on the surface can drop to -290 degrees Fahrenheit / -180 degrees Celsius. These changes in temperature are the most drastic in the entire Solar System.
* Mercury's magnetic field is offset relative to the planet's equator. Though the magnetic field at the surface has just 1.1% the strength of Earth's, it interacts with the magnetic field of the solar wind to sometimes create intense magnetic tornadoes that funnel the fast, hot solar wind plasma down to the surface of the planet.
* Mercury rotates in a way that is unique in the Solar System. It is tidally locked with the Sun in a 3:2 spin-orbit resonance.
* Mercury and Venus orbit the Sun within Earth’s Orbit, this makes them inferior planets.