Definition of astern:
(v) :
beyond the stern when viewed from
aboard
"he signalled the engineroom to go astern: in other words, to run the engine in such a way as to put the ship beyond the present position of her stern, as viewed from aboard."
"if one ship is following another, the first is astern when viewed from the second."
(preposition) :
behind
"the ship left a
wake astern of her."