What is "to be astern of the reckoning"

Astern \A*stern"\, adv. [Pref. a- + stern.] (Naut.)

1. In or at the hinder part of a ship; toward the hinder part, or stern; backward; as, to go astern.

2. Behind a ship; in the rear. ``A gale of wind right astern.''
   --De Foe. ``Left this strait astern.''
   --Drake.

   To bake astern, to go stern foremost.

   To be astern of the reckoning, to be behind the position given by the reckoning.

   To drop astern, to fall or be left behind.

   To go astern, to go backward, as from the action of currents or winds.