Complex sentence: John Moberly was born in St. Petersburg, Russia, of English parentage, in 1789, and entered the navy in 1801 when only in his 12th year and became post Captain in 1815, serving on a number of ships.
Split into simpler sentences: John Moberly was born in St. Petersburg, Russia, of English parentage, in 1789. He entered the navy in 1801 when he was only 12 and by 1815 had become a post Captain, serving on a number of ships.