Mercury is very hot and closest planet to the Sun. Mercury is visible to the naked eye and it is associated with Roman messenger of the gods. Mercury moves very quickly in the sky. Italian astronomer Giovanni Zupi observed Mercury through his telescope and discovered its phases just like moon.
Mercury orbits the sun at a distance of roughly 58 million km. Since it is very close to Sun, its orbital velocity is also faster and it orbits the Sun in 88 days. Since its movement is between Sun and us we see its phases just like Moon. The position of Mercury in the sky is always low and is found lower at the horizon before sunrise and after sunset. Mercury is about 4900 km in diameter or about a third the size of Earth.
Mercury stands out from other planet because it has the most elliptical orbit and during its orbit its distance from Sun range from 46 million km to 70 million km. This elongated orbit mean that it receives twice the light and heat from the sun when it is closest than when it is at its far point.
Doppler radar was used to measure its spin in 1965 and it was found that its day was only 59 earth days. To be more precise, actual Mercury year is 87.97 days and actual day is 58.65 earth days. The ratio of these two numbers is exactly . This number signify that the planet is tidally locked to its orbit.
With this weird orbital pattern, this are very weird in Mercury. If one could stay in a spot in Mercury, it would take the Sun 176 days to go around the sky once.
The first mission to planet Mercury was Mariner 10 and made 3 flybys in 1970s. Mariner mapped half the surface and it also helped to determine that Mercury had almost no atmosphere.
Another probe Messenger, entered Mercury orbit in 2011 and send back a lot of images revealing great details about the planet. Mercury has measurable magnetic field and it is assumed that this field is due to bigger iron core despite its slow spin.
The temperature on the surface of Mercury can reach 430°C.