Behind or towards the rear of a ship or aircraft:the engine rooms lay astern...
The ship was met by a warm welcome from the Brazilian Navy, being berthed in a prominent position astern of the sail training ship Cisne Branco.
Using powerful floodlights to attract the delicacy known to the Japanese as ‘ika’ (squid), the boats employ sails which are rigged astern to reduce the ship's roll in the ocean swells.
The ship won't point into the wind, you have to have the wind astern or at least abeam.
Origin
Late Middle English: from a-2 (expressing position or direction) + stern2.