Astraea was the fifth asteroid discovered, on 8 December 1845, byKarl Ludwig Hencke and named forAstraea, a Greek goddess of justice named after the stars. It was his first of two asteroid discoveries. The second was6 Hebe. A German amateur astronomer and post office headmaster, Hencke was looking for4 Vesta when he stumbled on Astraea. The King ofPrussia awarded him an annual pension of 1200marks for the discovery.[10]
Hencke's symbol for Astraea is an inverted anchor, encoded in Unicode 17.0 as U+1F778 (),[11][12] though given Astraea's role with justice and precision, it is perhaps a stylized set of scales, or a typographic substitute for one.[13][14] This symbol is no longer used. The astrological symbol is a percent sign, encoded specifically at U+2BD9 ⯙:[15] it is simply shift-5 on the keyboard, because Astraea was the fifth asteroid discovered.[11] The modern astronomical symbol is a simple encircled 5 (⑤).
For 38 years after the discovery of the fourth known asteroid, Vesta, in 1807, no further asteroids were discovered.[16] After the discovery of Astraea, 8 more were discovered in the following 5 years, and 24 were found in the 5 years after that. The discovery of Astraea proved to be the starting point for the eventual reclassification of the four original asteroids (which wereidentified as planets at the time)[16], as it became apparent that these were only the largest of a new type of celestial body with thousands of members.
Photometry indicatesprograde rotation, that the north pole points in the direction ofright ascension 115° or 310° anddeclination 55°, with a 5° uncertainty.[7] This gives anaxial tilt of about 33°.[citation needed] With an apparentmagnitude of 8.7 (on a favorable opposition on 15 February 2016), it is only the seventeenth-brightest main-belt asteroid, and fainter than, for example,192 Nausikaa or even324 Bamberga (at rare near-perihelion oppositions).
Anstellar occultation on 6 June 2008 allowed Astraea's diameter to be estimated; it was found to be115 ± 6 km.[17]
Left: A size comparison of thefirst 10 numbered asteroids profiled against Earth's Moon. Right: The orbit of 5 Astraea in white compared with those of Earth, Mars and Jupiter.
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