Mercury is the smallest planet in the Solar System and the closest to the Sun. It is one of four terrestrial planets in the Solar System, and is a rocky body like Earth. It is the smallest planet in the Solar System, with an equatorial radius of 2,439.7 kilometers (1,516.0 mi).
Mercury's surface is similar in appearance to that of the Moon, showing extensive mare-like plains and heavy cratering, indicating that it has been geologically inactive for billions of years.
The earliest known recorded observations of Mercury are from the MUL.APIN tablets. These observations were most likely made by an Assyrian astronomer around the 14th century BC. The cuneiform name used to designate Mercury on the MUL.APIN tablets is transcribed as UDU.IDIM.GU\U4.UD ("the jumping planet").Babylonian records of Mercury date back to the 1st millennium BC. The Babylonians called the planet Nabu after the messenger to the gods in their mythology.
The Greco-Egyptian astronomer Ptolemy wrote about the possibility of planetary transits across the face of the Sun in his work Planetary Hypotheses. He suggested that no transits had been observed either because planets such as Mercury were too small to see, or because the transits were too infrequent.
In medieval Islamic astronomy, the Andalusian astronomer Abū Ishāq Ibrāhīm al-Zarqālī in the 11th century described the deferent of Mercury's geocentric orbit as being oval, like an egg or a pignon, although this insight did not influence his astronomical theory or his astronomical calculations. In the 12th century, Ibn Bajjah observed "two planets as black spots on the face of the Sun", which was later suggested as the transit of Mercury and/or Venus by the Maragha astronomer Qotb al-Din Shirazi in the 13th century.
In India, the Kerala school astronomer Nilakantha Somayaji in the 15th century developed a partially heliocentric planetary model in which Mercury orbits the Sun, which in turn orbits Earth, similar to the Tychonic system later proposed by Tycho Brahe in the late 16th century.
Mercury is the ruling planet of Gemini and Virgo and is exalted in Virgo. In classical Roman mythology, Mercury is the messenger of the gods, noted for his speed and swiftness. Echoing this, the scorching, airless world Mercury circles the Sun on the fastest orbit of any planet. Mercury takes only 88 days to orbit the Sun, spending about 7.33 days in each sign of the zodiac. Mercury is so close to the Sun that only a brief period exists after the Sun has set where it can be seen with the naked eye, before following the Sun beyond the horizon.
Astrologically, Mercury represents the principles of communication, mentality, thinking patterns, rationality/reasoning, adaptability and variability. Mercury governs schooling and education, the immediate environment of neighbors, siblings and cousins, transport over short distances, messages and forms of communication such as post, email and telephone, newspapers, journalism and writing, information gathering skills and physical dexterity.
In medicine, Mercury is associated with the nervous system, the brain, the respiratory system, the thyroid and the sense organs. It is traditionally held to be essentially cold and dry, according to its placement in the zodiac and in any aspects to other planets.
In modern astrology, Mercury is regarded as the ruler of the third house; traditionally, it had the joy in the first house. Mercury is the messenger of the gods in mythology. It is the planet of day-to-day expression and relationships. Mercury's action is to take things apart and put them back together again. It is an opportunistic planet, decidedly unemotional and curious.