Mercury

Planet Mercury

Mercury is the closest planet to the Sun and the smallest of the eight planets of the Solar System. It is named after Mercury, the Roman name of Hermes, the messenger of the gods, because it moves quickly across the sky and completes its orbit of the Sun in only 88 days.

The Planet Mercury

Mercury is about 4,880 kilometres across, roughly 40 percent wider than the Moon, and has no moons of its own. It is never far from the Sun in the sky, and so it is seen only briefly, low on the horizon, before sunrise or after sunset. The ancient astronomers knew this. Pliny the Elder writes that the star Mercury, which some called Apollo, is carried in a lower orbit than Venus and moves quicker, shining sometimes before sunrise and sometimes after sunset, and never going far from the Sun (Pliny, Natural History, Book 2). Hyginus gives the planet the Greek name Stilbon, “the gleaming one”, calls it small but bright, and says that it was given to Mercury because he was the first to set up the months and to observe the courses of the stars (Hyginus, Astronomica 2.42).

Mercury turns on its axis once in about 59 Earth days, and for every two orbits of the Sun it spins exactly three times, so that one solar day on Mercury, from one sunrise to the next, lasts about 176 Earth days. It has almost no atmosphere. Its surface is covered in craters, and because it cannot hold the heat, the temperature rises to roughly 430 degrees Celsius by day and falls to about minus 180 at night.

Exploration

The first spacecraft to reach Mercury was Mariner 10, which flew past the planet three times in 1974 and 1975 and mapped about half of its surface. MESSENGER, launched by NASA in 2004, entered orbit around Mercury in 2011 and mapped the whole planet before it ended its mission in 2015. A joint European and Japanese mission, BepiColombo, was launched in 2018 and is due to enter orbit around the end of 2026.

Mercury in Myth

The Romans identified their god Mercury, patron of merchants, with the Greek Hermes. Livy records that the temple of Mercury at Rome was dedicated on the fifteenth of May (Livy 2.21.7). The consuls disputed which of them should dedicate it; the senate referred the question to the people, and the one chosen was to preside over the corn market and form a guild of merchants (Livy 2.27.5).

Hermes, Son of Zeus and Maia

The Homeric Hymn to Hermes sings of Hermes as the son of Zeus and Maia, lord of Cyllene and of Arcadia rich in flocks, and the messenger of the immortals. Maia, a nymph with lovely hair, bore him in a shadowy cave after joining Zeus in love (Homeric Hymn 4.1–6). Born at dawn, he played the lyre at midday and in the evening stole the cattle of far-shooting Apollo on the fourth day of the month, the day on which his mother bore him (Homeric Hymn 4.17–19). He made the first lyre from the shell of a tortoise he found at the door of the cave (Homeric Hymn 4.24–26).

The Messenger of the Gods

In the Odyssey, Zeus calls Hermes his messenger and sends him to the nymph Calypso with the order to let Odysseus go (Odyssey 5.28–31). Hermes puts on his beautiful golden sandals, which carry him over the sea and over the boundless earth with the breath of the wind, and takes the wand with which he charms the eyes of the men he chooses and wakens others from sleep (Odyssey 5.43–48). The Iliad describes the same sandals and wand when Zeus sends Hermes to guide Priam to Achilles (Iliad 24.339–345). In Virgil, Jupiter sends Mercury to Carthage to remind Aeneas that he has to sail for Italy (Virgil, Aeneid 4.222–278).