Handbook of Magnetic Compass Adjustment - Maritime Safety ...

earing of the range and the magnetic heading of the ship is read under the forward lubber's line of the pelorus at the instant
of taking a sight on the range. The deviation is the difference in degrees between the compass heading of the ship and the
known magnetic heading of the ship as indicated by pelorus.
714. Deviations on compass courses may be obtained by the use of reciprocal bearings. A pelorus is set up on shore and the
south end of the dial is aligned with magnetic north. A ship can then sight the pelorus on shore, using an azimuth circle or
pelorus, at the same instant the observer on shore sights the ship. The ship's bearing from shore on the reversed pelorus is the
magnetic bearing of the shore position from the ship. Continuous communication between ship and shore is necessary and
must be so arranged as to provide simultaneous observations. Two methods of such communication are by flashing lights,
and preferably, by short range two-way voice radio.
Additional methods of determining deviations are by the use of azimuths of the moon, stars, and planets. For information
as to the calculation of azimuths of these celestial bodies, refer to Pub. No. 9, The American Practical Navigator.
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