Direction
Knowing your direction is essential for navigating -- if you don't know where you are heading, you won't be able to predict where you will end up. This section presents two different tools for measuring direction: the compass and the lodestone.
Compass
Description: Compasses, at their simplest, are magnets suspended in such a way that their north pole points toward Earth's north pole. Larger compasses were made of a single piece of lodestone hung on a thin rope. Smaller ones are magnetized iron or steel, either balanced on a small point or floated in oil or water.
Compasses are most reliable near the Earth's equator. Earth's magnetic field is not perfectly aligned with the planet, and geomagnetic north is located about 10 degrees from the physical north pole. Local differences in the strength and direction of the magnetic field can also cause issues with navigation.
Related Instruments: Lodestone
Usage Dates: 11th century to modern times.
Images:
 Boxed CompassA boxed mariner's compass, showing how the face would tilt when the ship moves on the waves. Taken at the Harvard Museum of Historical Scientific Instruments. |  Compass Close-UpA close-up view on the face of the boxed compass. The magnetized portion is actually beneath the face. |  Boston-made CompassAnother boxed compass, this one made in Boston. |
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 Telltale CompassA "telltale" compass, which would be hung from the ceiling above the captain's bunk so that he could see the ship's heading at any time. |  LodestoneA lodestone -- a piece of magnetized iron ore -- that would be hung from a hook to determine the direction to the North Pole. |
Video:
More:
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How to use a Compass at Camping Cooks.
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Compass at the New World Encyclopedia.
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GIS Geography on the difference between Magnetic North and Geographic North
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A Telltale Compass properly mounted on the ceiling, from the Smithsonian.
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Pages on Compasses and Earth's Magnetic Field on Wikipedia.
Lodestone
Description: Lodestones are naturally magnetized rocks: specifically, a mineral known as magnetite.
Lodestones have two uses in navigation. First, they may be used as compasses in and of themselves. If they are hung from a rope, they will naturally turn until the north pole of the magnetite points towards Earth's north pole. Second, and more common, they can be used to magnetize iron or steel needles. By rubbing a piece of ferrous metal with a lodestone one can magnetize the smaller metal, which is useful for replacing compass needles when they rust or lose their magnetism.
Related Instruments: Compass
Usage Dates: The magnetic property of lodestone was known by the 6th century BCE. Its use as a compass dates to the 11th century CE in China, and about 1300 in Europe.
Images:
LodestoneA ship's lodestone, complete with metal case and hanging hook. Shown at Harvard's Museum of Historical Scientific Instruments. |  Lodestone Close-UpA close-up of the same lodestone from a different angle. |  Galileo's LodestoneA lodestone (center) holds up a metal box via magnetism. Coincidentally, Galileo's military compass is also visible in this picture. |
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Video:
More:
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Lodestone and the Needle from Ocean Navigator. An article on the history of magnetic navigation.
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Navigation before Netscape from History Magazine. Includes multiple types of navigational aids, but focuses on compasses and lodestones.
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Wikipedia's pages on lodestones and magnetite.
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An Armed Lodestone at the Museo Galileo - the same shown in the images above.