Astronomical symbols are symbols used to represent various celestial objects, theoretical constructs and observational events in astronomy. The earliest forms of these symbols appear in Greek papyri of late antiquity. The Byzantine codices in which the Greek papyri were preserved continued and extended the inventory of astronomical symbols.^[2][3]New symbols were further invented to represent many just-discovered planets and minor planets discovered in the 18th-20th centuries.

All these symbols were once commonly used by professional astronomers, amateur astronomers, and astrologers. While they are still commonly used in almanacs and astrological publications, their occurrence in published research and texts on astronomy is relatively infrequent,^[4] with some exceptions such as the Sun and Earth symbols appearing inastronomical constants, and certain zodiacal signs used to represent the solstices and equinoxes.

Unicode has formally assigned codepoints to most symbols, mainly in Miscellaneous Symbols Block (2600-26FF)^[5] and Miscellaneous Symbols and Pictographs Block (1F300-1F5FF).^[5]

The use of astronomical symbols for the Sun and Moon dates to antiquity. The forms of the symbols that appear in the original papyri of Greek horoscopes are a circle with one ray () for the Sun and a crescent for the Moon.^[3] The modern sun symbol, a circle with a dot (☉), first appeared in Europe in the Renaissance.^[3]

These symbols are also known to be used in alchemy texts, sun was representing gold, moon representing silver and so on.

In modern academic usage, the sun symbol is used for astronomical constants relating to the sun.^[6] The luminosity, mass, and radius of stars are often represented using the corresponding solar constants as units of measurement.^[7]

Symbols for the classical planets appear in the medieval Byzantine codices in which many ancient horoscopes were preserved.^[2] The written symbols for Mercury, Venus, Jupiter, and Saturn have been traced to forms found in late Greek papyri.^[26] The symbols for Jupiter and Saturn are identified as monograms of the corresponding Greek names, and the symbol for Mercury is a stylized caduceus.^[26]According to A. S. D. Maunder, antecedents of the planetary symbols were used in art to represent the gods associated with the classical planets; Bianchini's planisphere, produced in the 2nd century,^[27] shows Greek personifications of planetary gods charged with early versions of the planetary symbols: Mercury has a caduceus; Venus has, attached to her necklace, a cord connected to another necklace; Mars, a spear; Jupiter, a staff; Saturn, a scythe; the Sun, a circlet with rays radiating from it; and the Moon, a headdress with a crescent attached.^[28]

A diagram in Johannes Kamateros' 12th century Compendium of Astrology shows the Sun represented by the circle with a ray, Jupiter by the letter zeta (the initial of Zeus, Jupiter's counterpart in Greek mythology), Mars by a shield crossed by a spear, and the remaining classical planets by symbols resembling the modern ones, without the cross-mark seen in modern versions of the symbols. These cross-marks first appear around the 16th century. According to Maunder, the addition of crosses appears to be "an attempt to give a savour of Christianity to the symbols of the old pagan gods."^[28]

The symbols for Uranus were created shortly after its discovery. One symbol, , invented by J. G. Köhler and refined by Bode, was intended to represent the newly discovered metal platinum; since platinum, commonly called white gold, was found by chemists mixed with iron, the symbol for platinum combines the alchemical symbols for iron, ♂, and gold, ☉.^[29][30] This symbol also combines the symbols of Mars (♂) and the Sun (☉) because in Greek Mythology, Uranus represented heaven, and represents the combined power of Mars' spear and the Sun.^[31] Another symbol, , was suggested by Lalande in 1784. In a letter to Herschel, Lalande described it as "un globe surmonté par la première lettre de votre nom" ("a globe surmounted by the first letter of your name").^[32]

Several symbols were proposed for Neptune to accompany the suggested names for the planet. Claiming the right to name his discovery, Urbain Le Verrier originally proposed the name Neptune^[33] and the symbol of a trident,^[34] while falsely stating that this had been officially approved by the French Bureau des Longitudes.^[33] In October, he sought to name the planet Leverrier, after himself, and he had loyal support in this from the observatory director, François Arago,^[35] who in turn proposed a new symbol for the planet ().^[36]However, this suggestion met with stiff resistance outside France.^[35] French almanacs quickly reintroduced the name Herschel forUranus, after that planet's discoverer Sir William Herschel, and Leverrier for the new planet.^[37] Professor James Pillans of the University of Edinburgh defended the name Janus for the new planet, and proposed a key for its symbol.^[34] Meanwhile, Struve presented the nameNeptune on December 29, 1846, to the Saint Petersburg Academy of Sciences.^[38] In August 1847, the Bureau des Longitudes announced its decision to follow prevailing astronomical practice and adopt the choice of Neptune, with Arago refraining from participating in this decision.^[39]

The International Astronomical Union discourages the use of these symbols in journal articles. In certain cases where planetary symbols might be used, such as in the headings of tables, the IAU Style Manual permits certain one- and (to disambiguate Mercury and Mars) two-letter abbreviations for the names of the planets.^[40]