13 Egeria reaches opposition in the constellation of Libra, the Scales

Minor planet 13 Egeria reaches opposition on 4 May when it can be found shining at 10th magnitude in Libra, the Scales, less than a degree to the west-southwest of mag. +2.7 Zubenelgenubi (Alpha² (α² ) Librae). Zubenelgenubi is the name given to the brighter, eastern component of a double star. The fainter companion, Alpha¹ (α¹ ) Librae, shines at mag. +5.2 and is located 3.8 arcminutes to the northwest of Alpha² .

At 01:00 BST (00:00 UT) on 1 May, 13 Egeria is located about 20 arcminutes southwest of Zubenelgenubi, shining at mag. +10.1. It reaches its peak opposition brightness of mag. +10.0 on 4 May, remaining that bright for a couple of days before dimming again. By the month’s close, 13 Egeria will have faded to mag. +10.8. Its monthly track has it heading west, curving and tilted slightly south. It crosses the border from Libra into Virgo on 28 May.

Egeria was discovered by Annibale de Gasparis on 2 November 1850. It is a main belt asteroid, taking 4.14 years to complete its orbit around the Sun. Its orbital distance varies from 2.79 AU at perihelion to 2.36 AU at aphelion, and it spins on its axis once every 7.05 hours. Egeria was named after a mythological nymph and is a dark object with an albedo of just 8.25 per cent, a figure which indicates how much light is reflected by its surface. It’s an uncommon type of carbonaceous asteroid known as a G-type, which accounts for just 5 per cent of the asteroid population.

Its occultations of stars have given us details about the asteroid’s physical size. One such event on 8 January 1992 gave us dimensions of 217km x 196km. A second opportunity, on 22 January 2008, meant its size could be refined to 214.8km x 192km.