The closest planet to the Sun and one of five planets visible with the naked eye. The greatest elongation of Mercury is about 28°, making it visible at most about 112 minutes after sunset or before sunrise. It lies at a mean distance of about 0.39 astronomical units from the Sun. Mercury is just 4,879 km in diameter, about 2.6 times smaller than the Earth. Its orbital period is 87.97 Earth days. Mercury has a high density, 5.4 g cm^-3, with only the Earth having a higher density among the planets. This is largely due to Mercury being composed mainly of heavy metals and rock. One solar day on Mercury lasts the equivalent of 176 Earth days while the sidereal day (the time for 1 rotation in relation to a fixed point) lasts 59 Earth days. Mercury is nearly tidally locked to the Sun and over time this has slowed the rotation of the planet to almost match its orbit around the Sun. Mercury also has the highest orbital eccentricity of all the planets with its distance from the Sun ranging from 46 to 70 million km. Mercury has just 38% the gravity of Earth, this is too little to maintain an atmosphere against solar winds, which blow it away. The surface of Mercury which faces the Sun has temperatures of up to 427°C, whilst on the alternate side this can be as low as -173°C. Mercury's core has more iron than any other planet in the solar system. This has to do with its formation and early life. If the planet formed quickly, increasing temperatures of the evolving Sun could have vaporized much of the existing surface, leaving only a thin shell.