A compass is a device that indicates direction. It is one of the most important instruments for navigation.
 
Magnetic compasses are the most well known type of compass. They have become so popular that the term “compass” almost always refers a magnetic compass.  While the design and construction of this type of compass has changed significantly over the centuries, the concept of how it works has remained the same. Magnetic compasses consist of a magnetized needle that is allowed to rotate so it lines up with the Earth's magnetic field. The ends point to what are known as magnetic north and magnetic south.
 
Scientists and historians don’t know when the principles behind magnetic compasses were discovered. Ancient Greeks understood magnetism. As early as 2,000 years ago, Chinese scientists may have known that rubbing an iron bar (such as a needle) with a naturally occurring magnet, called a lodestone, would temporarily magnetize the needle so that it would point north and south.   
 
Very early compasses were made of a magnetized needle attached to a piece of wood or cork that floated freely in a dish of water. As the needle would settle, the marked end would point toward magnetic north. 
 
As engineers and scientists learned more about magnetism, the compass needle was mounted and placed in the middle of a card that showed the cardinal directions—north, south, east, and west. A spearhead and the letter T, which stood for the Latin name of the North Wind, Tramontana, signified north. This combination evolved into a fleur-de-lis design, which can still be seen today. All 32 points of direction were eventually added to the compass card
 
Historians think China may have been the first civilization to develop a magnetic compass that could be used for navigation. Chinese scientists may have developed navigational compasses as early as the 11th or 12th century. Western Europeans soon followed at the end of the 12th century.        
 
In their earliest use, compasses were likely used as backups for when the sun, stars, or other landmarks could not be seen. Eventually, as compasses became more reliable and more explorers understood how to read them, the devices became a critical navigational tool.
 
Adjustments and Adaptations
 
By the 15th century, explorers realized that the “north” indicated by a compass was not the same as Earth’s true geographic north. This discrepancy between magnetic north and true north is called variation (by mariners or pilots) or magnetic declination (by land navigators) and varies depending on location. Variation is not significant when using magnetic compasses near the Equator, but closer to the North and South Poles, the difference is much greater and can lead someone many kilometers off-course. Navigators must adjust their compass readings to account for variation.
 
Other adaptations have been made to magnetic compasses over time, especially for their use in marine navigation. When ships evolved from being made of wood to being made of iron and steel, the magnetism of the ship affected compass readings. This difference is called deviation. Adjustments such as placing soft iron balls (called Kelvin spheres) and bar magnets (called Flinders bars) near the compass helped increase the accuracy of the readings. Deviation must also be taken into account on aircraft using compasses, due to the metal in the construction of an airplane.  
 
Magnetic compasses come in many forms. The most basic are portable compasses for use on casual hikes. Magnetic compasses can have additional features, such as magnifiers for use with maps, a prism or a mirror that allows you to see the landscape as you follow the compass reading, or markings in Braille for the visually impaired. The most complicated compasses are complex devices on ships or planes that can calculate and adjust for motion, variation, and deviation.   
 
Other Types of Compasses
 
Some compasses do not use Earth’s magnetism to indicate direction. The gyrocompass, invented in the early 20th century, uses a spinning gyroscope to follow Earth’s axis of rotation to point to true north. Since magnetic north is not measured, variation is not an issue. Once the gyroscope begins spinning, motion will not disturb it. This type of compass is often used on ships and aircraft.
 
A solar compass uses the sun as a navigational tool. The most common method is to use a compass card and the angle of the shadow of the sun to indicate direction. 
 
Even without a compass card, there are techniques that use the sun as a compass. One method is to make a shadow stick. A shadow stick is a stick placed upright in the ground. Pebbles placed around the stick, and a piece of string to track the shadow of the sun across the sky, help a navigator determine the directions of east and west. 
 
Another type of solar compass is an old-fashioned analog (not digital) watch. Using the watch’s hands and the position of the sun, it is possible to determine north or south. Simply hold the watch parallel to the ground (in your hand) and point the hour hand in the direction of the sun. Find the angle between the hour hand and the 12 o’clock mark. This is the north-south line. In the Southern Hemisphere, north will be the direction closer to the sun. In the Northern Hemisphere, north will be the direction further from the sun.
 
Receivers from the global positioning system (GPS) have begun to take the place of compasses. A GPS receiver coordinates with satellites orbiting the Earth and monitoring stations on Earth to pinpoint the receiver's location. GPS receivers can plot latitude, longitude, and altitude on a map. Unless large objects block signals, readings are usually accurate to within about 15 meters (50 feet).   
 
Despite advancements with GPS, the compass is still a valuable tool. Many airplanes and ships still use highly advanced compasses as navigational instruments. For casual observation—for navigators on foot or in a small boat—a pocket compass or a basic compass mounted on a dashboard remains a practical and portable tool.
compass
One of the reasons people doubt Robert Peary reached the North Pole is because he failed to provide compass observations.
Animals and Compasses
Many animals—such as certain types of ants, fish, and birds—use the sun as a compass to help them find direction. They use their internal biological clock to compensate for the sun shifting in the sky and maintain a straight course. Other animals—like pigeons—are able to navigate using the Earth’s own magnetic field. Their brains function like an internal magnetic compass to follow the Earth’s magnetic field.

Spiritual Orienteering
The Chinese first used compasses not for navigation, but for spiritual purposes. They used the magnetic devices to organize buildings and other things according to feng shui, the ancient practice of harmonizing an environment according to the "laws of Heaven."

Pole Swap
If you were using a compass 800,000 years ago and facing north, the needle would point to the south magnetic pole. Why? Even though the Earth acts like a giant magnet, it is not stable. Both the north and south magnetic poles are slowly shifting. Since the magnetic north pole was discovered in the early 19th century, it has drifted northward by more than 966 kilometers (600 miles) and it continues to move about 40 miles per year. The north and south magnetic poles have also switched places many times in the Earth’s history. 
accuracy
Noun

condition of being exact or correct.

Noun

a modification of an organism or its parts that makes it more fit for existence. An adaptation is passed from generation to generation.

aircraft
Noun

vehicle able to travel and operate above the ground.

Noun

the distance above sea level.

axis of rotation
Noun

single axis or line around which a body rotates or spins.

calculate
Verb

to reach a conclusion by mathematical or logical methods.

cardinal direction
Noun

one of the four main points of a compass: north, east, south, west.

casual observation
Noun

something that is seen, heard, touched, smelled, or tasted without being studied in depth.

Noun

complex way of life that developed as humans began to develop urban settlements.

Noun

instrument used to tell direction.

compass card
Noun

freely rotating circular card with magnets attached to its underside, its face marked with up to 32 compass points, degrees clockwise from north, or both.

complex
Adjective

complicated.

concept
Noun

idea.

consist
Verb

to be made of.

coordinate
Verb

to work together or organize for a specific goal.

critical
Adjective

very important.

deviation
Noun

error of a magnetic compass due to the effect of local magnetism, such as materials used in the construction of a ship or aircraft.

device
Noun

tool or piece of machinery.

Noun

the way in which somebody or something goes, points, or faces.

discrepancy
Noun

difference or conflict in sets of data.

engineer
Noun

person who plans the building of things, such as structures (construction engineer) or substances (chemical engineer).

Noun

imaginary line around the Earth, another planet, or star running east-west, 0 degrees latitude.

evolve
Verb

to develop new characteristics based on adaptation and natural selection.

explorer
Noun

person who studies unknown areas.

fleur-de-lis
Noun

stylized design, often associated with France or French royalty, representing three petals of a flowering iris encircled by a band.

Flinders bar
Noun

bar of soft iron, mounted vertically beneath a magnetic compass to compensate for vertical magnetic currents.

Global Positioning System (GPS)
Noun

system of satellites and receiving devices used to determine the location of something on Earth.

GPS receiver
Noun

device that gets radio signals from satellites in orbit above Earth in order to calculate a precise location.

gyroscope
Noun

device consisting of a rotating wheel mounted so that its axis can turn freely in any direction, and capable of maintaining the same absolute direction in spite of movements of the mountings and surrounding parts.

indicate
Verb

to display or show.

instrument
Noun

tool.

iron
Noun

chemical element with the symbol Fe.

Kelvin sphere
Noun

one of two iron balls placed next to a magnetic compass to compensate for horizontal magnetic currents. Also called a Kelvin ball or navigator's ball.

landmark
Noun

a prominent feature that guides in navigation or marks a site.

Noun

the geographic features of a region.

Latin
Noun

language of ancient Rome and the Roman Empire.

Noun

distance north or south of the Equator, measured in degrees.

lodestone
Noun

natural magnet.

Noun

distance east or west of the prime meridian, measured in degrees.

magnet
Noun

material that has the ability to physically attract other substances.

magnetic declination
Noun

difference between a local magentic field, or the direction a compass points, and true north, or the direction of the geographic North Pole.

magnetic field
Noun

area around and affected by a magnet or charged particle.

magnetic north
Adjective

direction that all compass needles point.

magnetize
Verb

to turn something into a magnet.

Noun

symbolic representation of selected characteristics of a place, usually drawn on a flat surface.

marine
Adjective

having to do with the ocean.

mariner
Noun

sailor.

monitoring station
Noun

facility with instruments and tools for measuring and keeping track of conditions in an area.

Noun

art and science of determining an object's position, course, and distance traveled.

Northern Hemisphere
Noun

half of the Earth between the North Pole and the Equator.

Verb

to move in a circular pattern around a more massive object.

pilot
Noun

person who steers a ship or aircraft.

plot
Verb

to form a path based on calculations.

portable
Adjective

able to be easily transported from one place to another.

principle
Noun

rule or standard.

prism
Noun

device for distributing light into different colors of the spectrum.

reliable
Adjective

dependable or consistent.

rotate
Verb

to turn around a center point or axis.

satellite
Noun

object that orbits around something else. Satellites can be natural, like moons, or made by people.

significant
Adjective

important or impressive.

Southern Hemisphere
Noun

half of the Earth between the South Pole and the Equator.

steel
Noun

metal made of the elements iron and carbon.

true north
Noun

direction of the geographic North Pole.