HADLEY, JOHN (— 1744), a distinguished English astronomer, of the details of whose life almost nothing is known. He was a member and afterwards a vice-president of the Royal Society, to whose Transactions he made some valuable contributions. He was the inventor of the sextant which bears his name, or at least the first who made it practically useful at sea; for the idea had been long before struck out by Hooke and perfected by Newton. Some important changes and improvements have since been made upon it by Mayer and Borda.