Astronomical symbols

Symbols for the Sun and Moon

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. The modern sun symbol, a circle with a dot (☉), first appeared in Europe in the Renaissance.

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. The luminosity, mass, and radius of stars are often represented using the corresponding solar constants as units of measurement.

Symbols for the planets

Symbols for the classical planets appear in the medieval Byzantine codices in which many ancient horoscopes were preserved. The written symbols for Mercury, Venus, Jupiter, and Saturn have been traced to forms found in late Greek papyri. The symbols for Jupiter and Saturn are identified as monograms of the corresponding Greek names, and the symbol for Mercury is a stylized caduceus. 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, 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.

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."

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, ☉. 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. 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").

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 and the symbol of a trident, while falsely stating that this had been officially approved by the French Bureau des Longitudes. In October, he sought to name the planet Leverrier, after himself, and he had loyal support in this from the observatory director, François Arago, who in turn proposed a new symbol for the planet (). However, this suggestion met with stiff resistance outside France. French almanacs quickly reintroduced the name Herschel for Uranus, after that planet's discoverer Sir William Herschel, and Leverrier for the new planet. Professor James Pillans of the University of Edinburgh defended the name Janus for the new planet, and proposed a key for its symbol. Meanwhile, Struve presented the name Neptune on December 29, 1846, to the Saint Petersburg Academy of Sciences. 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.

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.

Symbols for minor planets

Following Giuseppe Piazzi's discovery of Ceres, a group of astronomers ratified the name, which Piazzi had proposed. At that time, the sickle was chosen as a symbol of the planet.

The symbol for 2 Pallas, the spear of Pallas Athena, was invented by Baron Franz Xaver von Zach, and introduced in his Monatliche correspondenz zur beförderung der erd- und himmels-kunde. In a letter to von Zach, discoverer Heinrich Wilhelm Matthäus Olbers (who had named the newly discovered asteroid) expressed his approval of the proposed symbol, but wished that the handle of the sickle of Ceres had been adorned with a pommel instead of a crossbar, to better differentiate it from the sign of Venus.

Karl Ludwig Harding created the symbol for 3 Juno. Harding, who discovered this asteroid, proposed the name Juno and the use of a scepter topped with a star as its astronomical symbol.

The symbol for 4 Vesta was invented by German mathematician Carl Friedrich Gauss. Dr. Olbers, having previously discovered and named 2 Pallas, gave Gauss the honor of naming his newest discovery. Gauss decided to name the new asteroid for the goddess Vesta, and also designed the symbol ⚶ (): the altar of the goddess, with the sacred fire burning on it. Other contemporaneous writers use a more elaborate symbol () instead.

The next two asteroids, 5 Astraea and 6 Hebe, were both discovered by Karl Ludwig Hencke. Hencke requested that the symbol for 5 Astraea be an upside-down anchor; however, a pair of balances was sometimes used instead. Gauss named 6 Hebe at Hencke's request, and chose a wineglass as the symbol.

As more new asteroids were discovered, astronomers continued to assign symbols to them. Thus, 7 Iris had for its symbol a rainbow with a star; 8 Flora, a flower; 9 Metis, an eye with a star; 10 Hygiea, an upright snake with a star on its head; 11 Parthenope, a standing fish with a star; 12 Victoria, a star topped with a branch of laurel; 13 Egeria, a buckler; 14 Irene, a dove carrying an olive branch with a star on its head; 15 Eunomia, a heart topped with a star; 16 Psyche, a butterfly wing with a star; 17 Thetis, a dolphin with a star; 18 Melpomene, a dagger over a star; and 19 Fortuna, a star over Fortuna's wheel.

Johann Franz Encke made a major change in the Berliner Astronomisches Jahrbuch (BAJ, Berlin Astronomical Yearbook) for the year 1854, published in 1851. He introduced encircled numbers instead of symbols, although his numbering began with Astraea, the first four asteroids continuing to be denoted by their traditional symbols. This symbolic innovation was adopted very quickly by the astronomical community. The following year (1852), Astraea's number was bumped up to 5, but Ceres through Vesta would be listed by their numbers only in the 1867 edition. The circle later became a pair of parentheses, and the parentheses sometimes omitted altogether over the next few decades.

A few asteroids were given symbols by their discoverers after the encircled-number notation became widespread. 26 Proserpina, 28 Bellona, 35 Leukothea, and 37 Fides, all discovered by R. Luther, were assigned, respectively, a pomegranate with a star inside; a whip and spear; an antique lighthouse; and a cross. 29 Amphitrite, discovered by Albert Marth, was assigned a shell for its symbol.

Pluto's name and symbol were announced by the discoverers on May 1, 1930. The symbol, a monogram of the letters PL, could be interpreted to stand for Pluto or for Percival Lowell, the astronomer who initiated Lowell Observatory's search for a planet beyond the orbit of Neptune.

Symbols for zodiac constellations and signs

The zodiac symbols have several astronomical interpretations. Depending on context, a zodiac symbol may denote either a constellation, or a point or interval on the ecliptic plane.

Lists of astronomical phenomena published by almanacs sometimes included conjunctions of stars and planets or the Moon; rather than print the full name of the star, a Greek letter and the symbol for the constellation of the star was sometimes used instead. The ecliptic was sometimes divided into 12 signs, each subdivided into 30 degrees, and the sign component of ecliptic longitude was expressed either with a number from 0 to 11 or with the corresponding zodiac symbol.

In modern academic usage, all the constellations, including the twelve of the zodiac, have dedicated three-letter abbreviations. The zodiac symbols are also sometimes used to represent points on the ecliptic, particularly the solstices and equinoxes. Each symbol is taken to represent the "first point" of each sign, thus, ♈ stands for the vernal equinox, ♋ the summer solstice, ♎ the autumnal equinox, and ♑ the winter solstice.

Other symbols

Symbols for aspects and nodes appear in medieval texts, although medieval and modern usage of the node symbols differ; ☊ formerly stood for the descending node, and ☋ for the ascending node. In describing the Keplerian elements of an orbit, ☊ is sometimes used to denote the ecliptic longitude of the ascending node, although it is more common to use Ω (capital omega), which was originally a typographical substitute for the old symbol.

The symbols for aspects first appear in Byzantine codices. Of the symbols for the five Ptolemaic aspects, only the three displayed here—for conjunction, opposition, and quadrature—are used in astronomy.

Symbols for a comet (☄) and a star () have been used in published astronomical observations of comets. In tables of these observations, ☄ stood for the comet being discussed and for the star of comparison relative to which measurements of the comet's position were made.