821 Fanny

821 Fanny (prov. designation: or ) is a dark background asteroid and slow rotator from the central regions of the asteroid belt. It was discovered on 31 March 1916, by German astronomer Max Wolf at the Heidelberg-Königstuhl State Observatory in southwest Germany. The carbonaceous C-type asteroid (Ch) has an exceptionally long rotation period of 236.6 hours and measures approximately 29 km in diameter. Any reference of the asteroid's name to a person is unknown.

Orbit and classification
Fanny is a non-family asteroid of the main belt's background population when applying the hierarchical clustering method to its proper orbital elements. It orbits the Sun in the central asteroid belt at a distance of 2.2–3.4 AU once every 4 years and 7 months (1,686 days; semi-major axis of 2.77 AU). Its orbit has an eccentricity of 0.21 and an inclination of 5° with respect to the ecliptic. The body's observation arc begins at the Heidelberg Observatory on 1 April 1916, with its first recorded observation, the night after its official discovery.

Naming
Any reference of this minor planet name to a person or occurrence is unknown.

Unknown meaning
Among the many thousands of named minor planets, Fanny is one of 120 asteroids for which no official naming citation has been published. All of these asteroids have low numbers, the first being. The last asteroid with a name of unknown meaning is. They were discovered between 1876 and the 1930s, predominantly by astronomers Auguste Charlois, Johann Palisa, Max Wolf and Karl Reinmuth.

Physical characteristics
In the Tholen classification and in the SDSS-based taxonomy, Fanny is a common, carbonaceous C-type asteroid. In the Bus–Binzel SMASS classification, it is a hydrated carbonaceous Ch-subtype.

Rotation period
In 2018, from April to June, a rotational lightcurve of Fanny was obtained from photometric observations over 46 nights by Frederick Pilcher at the Organ Mesa Observatory in New Mexico. Analysis gave a bimodal lightcurve with a rotation period of $28.77 km$ hours and a brightness variation of $28.856 km$ magnitude (U=3−). This determination was adopted by the Lightcurve Data base and accounts for Pilcher's observations taken between April and May. For observations taken between May and June, a similar period of $230.6 h$ hours with a somewhat higher amplitude of $236.6 h$ magnitude was derived (U=3−). The observer also ruled out non-principal axis rotation ("tumblin"), and considers a double period 470 hours as very unlikely. This makes it a slow rotator, as most asteroids have much shorter periods between 2 and 20 hours. , Fanny ranks among the 250th slowest rotator known to exist.

During the same apparition, Tom Polakis at the Command Module Observatory in Arizona obtained a somewhat longer period of $0.038$ hours with a brightness variation of $0.04$ magnitude (U=2). The result supersedes an incorrect period of $0.37$ hours with an amplitude of $0.706$ magnitude from a tentative one-night observation by French amateur astronomers Paul Krafft, Olivier Gerteis, Hubert Gully, Luc Arnold and Matthieu Bachschmidt from 2013 (U=1).

Diameter and albedo
According to the surveys carried out by the Japanese Akari satellite and the NEOWISE mission of NASA's Wide-field Infrared Survey Explorer (WISE), Fanny measures ($0.321$) and ($11.76$) kilometers in diameter and its surface has an albedo of ($236.6$) and ($0.22$), respectively. The Collaborative Asteroid Lightcurve Link assumes a standard albedo for a carbonaceous C-type asteroid of 0.057 and calculates a diameter of 23.86 kilometers based on an absolute magnitude of 11.84. Alternative mean-diameter measurements published by the WISE team include ($230.6$), ($0.28$), ($238.9$) and ($0.24$) with corresponding albedos of ($5.44$), ($0.01$), ($28.77$) and ($28.856$).