Complex sentence: Ross was born in London, the nephew of noted Scottish Arctic explorer Sir John Ross (1777-1856), under whom he entered the navy in 1812, accompanying him on Sir John's first Arctic voyage in search of a Northwest Passage in 1818.
Split into simpler sentences: Ross was born in London, the nephew of noted Scottish Arctic explorer Sir John Ross (1777-1856). He entered the navy in 1812 under his uncle, whom he accompanied on Sir John's first Arctic voyage in search of a Northwest Passage in 1818.