Saint Helena is a volcanic tropical island in the South Atlantic Ocean, 4,000 kilometres (2,500 mi) east of Rio de Janeiro and 1,950 kilometres (1,210 mi) west of the mouth of the Cunene River, which marks the border between Namibia and Angola in southwestern Africa. It is part of the British Overseas Territory of Saint Helena, Ascension and Tristan da Cunha. Saint Helena measures about 16 by 8 kilometres and has a population of 4,534. It was named after Saint Helena of Constantinople.