Ship's Compass
MMBC - 2949 - A Pelorus, used to take bearings of celestial objects

Navigation is the skill and science of determining the position of a vessel at sea.  Navigators also chart a course to their next destination.  When ships leave sight of land, navigation must rely on observations of the Sun and stars taken with navigational instruments

The astrolabe was used to determine latitude, the north-south position on the globe, by measuring the height or altitude of celestial bodies over the horizon and making a calculation using the known declination of the star. Declination is measured in degrees north and south of the equator: a star known to sit over the equator on a given day in the year has a declination of 0 degrees.  Almanacs were published for navigators to give accurate information about declination for these calculations.  

An astrolabe is really any instrument used to measure altitudes above the horizon, but the “mariner’s astrolabe,” popularized around 1460, was designed specifically for use at sea. A ring cast in heavy brass was marked with degrees along its edge, and an inner disc was rotated to take sightings of celestial bodies.  The degrees were measured from the Sun or stars to the observer. 

MMBC 998.038.001 - 16th Century Astrolabe, Reproduction. Used for sighting altitude of a celestial object to determine latitude

In 1590, John Davis invented the backstaff.  This instrument was a long staff with a movable cross-bar.  Like the astrolabe, the backstaff measured latitude.  The navigator would hold the staff above his shoulder next to the side of his face to take the altitude of the Sun over the horizon.  The backstaff worked with shadows instead of direct sightings, allowing the navigator to work with his back to the Sun to protect his eyes. The sextant began to replace these instruments in 18th century navigation.

The Earth has two versions of “north” that can be used to determine direction and travel. True north is the position directly over the northern pole. An instrument called a compass can measure magnetic north, which is the northerly direction in the earth’s magnetic field.  The mariner’s compass holds a rotating, magnetized needle set in a case that shows the cardinal directions of north, south, east, and west.  The needle will respond to the Earth’s magnetic pull so that when the instrument is turned, magnetic north can be located.  The pelorus functions as a compass without cardinal direction markers so that navigators can take relative bearings.  For example, expeditions could determine their direction relative to stars or points of land instead of magnetic north.  The pelorus can also be used to support readings taken with a magnetic compass.