Fundamentally, the classic compass is any magnetically sensitive device able to indicate the direction of the magnetic north of a planet's magnetosphere. Often compasses are built as a stand alone sealed instrument with a magnetized bar or needle turning freely upon a pivot, or moving in a fluid, thus able to point in a northerly and southerly direction. An early form of the compass (a magnetized needle floating in water) was invented in ancient China sometime before 1044. The dry compass was invented in medieval Europe around 1300. This was supplanted in the early 20th century by the liquid-filled magnetic compass.

Other, more accurate, devices have been invented for determining north that do not depend on the Earth's magnetic field for operation (known in such cases as true north, as opposed to magnetic north). A gyrocompass or astrocompass can be used to find true north, while being unaffected by stray magnetic fields, nearby electrical power circuits or nearby large masses of ferrous metals. A recent development is the electronic compass, which detects the magnetic directions without requiring moving parts. This device frequently appears as an optional subsystem built into GPS receivers.

Navigation prior to the compass

Prior to the introduction of the compass, position and direction at sea was primarily determined by the sighting of landmarks, supplemented with the observation of the position of celestial bodies. Ancient mariners usually kept within easy sight of land. The invention of the compass enabled the determination of heading when the sky was overcast or foggy. And, when the sun or other known celestial bodies could be observed, it enabled the calculation of latitude. This enabled mariners to navigate safely away from land, contributing to the Age of Discovery.

Olmec artifact

Based on his find of an Olmec hematite artifact in Mesoamerica, radiocarbon dated to 1400-1000 BC, the American astronomer John Carlson has suggested that "the Olmec may have discovered and used the geomagnetic lodestone compass earlier than 1000 BC". If true, this "predates the Chinese discovery of the geomagnetic lodestone compass by more than a millennium".John B. Carlson, “Lodestone Compass: Chinese or Olmec Primacy? Multidisciplinary Analysis of an Olmec Hematite Artifact from San Lorenzo, Veracruz, Mexico”, Science, New Series, Vol. 189, No. 4205 (5 September, 1975), pp. 753-760 (753) Carlson speculates that the Olmecs may have used similar artifacts as a directional device for astrological or geomantic purposes.

The artifact itself is part of a lodestone that had been polished into a bar with a groove at one end (that Carlson suggests may have been used for sighting). The artifact now consistently points 35.5 degrees west of north, but may have pointed north-south when whole. It has been suggested that the artifact was in fact used as some constituent piece of a decorative ornament. No other similar hematite artifacts have yet been found.