13 Egeria

Egeria was discovered on the evening of 2 November 1850 by Italian astronomer Annibale de Gasparis at the Astronomical Observatory of Capodimonte.^{[11][12]: 128 } Whereas de Gasparis's previous two discoveries—10 Hygiea and 11 Parthenope—were identified by comparison with the Berlin Academy star charts, Egeria was identified using his own ecliptic charts intended for finding new celestial objects.^{[13]: 119 [12]: 128 } Egeria's discovery was announced in December 1850 by astronomer Benjamin A. Gould through The Astronomical Journal.^[14] Following the discovery, de Gasparis delegated the naming rights of the asteroid to Urbain Le Verrier. Le Verrier chose to name the asteroid after Egeria, a mythological nymph and councillor of Numa Pompilius.^{[12]: 128 }

Upon its discovery, both de Gasparis and Gould labelled Egeria as a new planet.^[11][14] However, by the mid-1800s, the classification and terminology of the asteroids were quickly evolving.^[15] Soon after Egeria's discovery, other astronomers described it as an "asteroid"^[16] or a "minor planet".^{[17]: 25 } Throughout the latter half of the 19th century, the terms "asteroid" and "minor planet" became favored,^[15] although some publications continued to label Egeria and other asteroids as planets.^{[15][18]: 42 }

In 1851, de Gasparis announced his chosen symbol for Egeria—that of a buckler. However, Gould had apparently not seen the announcement, and in an 1852 table of asteroid symbols he left Egeria's blank.^{[19]: 8 [16]} No mention of a star was made, though in 1852 John Russell Hind included one in his drawn symbol for Egeria: (U+1CEC6 𜻆 in Unicode 17.0). The symbol is sometimes depicted with a round form (),^{[19]: 8 } though only Hind's form was encoded.^[20] As the number of asteroids assigned a symbol grew in number through the mid-1800s, the practicality of assigning each a unique astronomical symbol was questioned. In 1851, astronomer Johann Franz Encke proposed an alternative system of a number—denoting the object's order of discovery—inscribed in a circle. For Egeria, this would be ⑬.^[15][16] This system was quickly adopted by astronomers, though eventually astronomers switched from an inscribed circle to parentheses and eventually a bare number^[15]—hence (13) Egeria or 13 Egeria in modern notation.^[1][3]

Egeria orbits the Sun at an average distance—its semi-major axis—of 2.58 astronomical units (AU), placing it within the main asteroid belt and near the 3:1 mean-motion resonance with Jupiter.^{[3][21]: 323 } Along its orbit, its distance from the Sun varies between 2.36 AU at perihelion to 2.80 AU at aphelion due to its orbital eccentricity of 0.09. Its orbit is inclined by 16.5° with respect to the ecliptic, and it takes 4.14 years to complete one orbit.^[3] Egeria is classified as a background asteroid, as it does not belong to an asteroid family.^[22]

Egeria occulted a star on 8 January 1992. Its disc was determined to be quite circular (217×196 km). On 22 January 2008, it occulted another star, and this occultation was timed by several observers in New Mexico and Arizona, coordinated by the IOTA Asteroid Occultation Program.^[4] The result showed that Egeria presented an approximately circular profile to Earth of 214.8×192 km, well in agreement with the 1992 occultation.^{[citation needed]} It has also been studied by radar.^[23]

In 1988 a search for satellites or dust orbiting this asteroid was performed using the UH88 telescope at the Mauna Kea Observatories, but the effort came up empty.^[24] Spectral analysis of Egeria shows it to be unusually high in water content, 10.5–11.5% water by mass.^[25]

• Former classification of planets