Today is the anniversary of the death, at Oslo on 15 Apr 1873 (some sources state 11 Apr), of the Norwegian astronomer and physicist Christopher Hansteen. Noted for his research into geomagnetism and his mapping of Earth’s magnetic field, Hansteen established the Christiania Observatory (pictured) in 1841. The 45 km diameter lunar crater Hansteen, located near the south western edge of Oceanus Procellarum, is named in his honour.