Compass

Compass

A compass is a device that indicates direction. It is one of the most important instruments for navigation.

Grades

9 - 12+

Subjects

Geography, Human Geography

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 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 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.

Fast Fact

Animals and CompassesMany 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.

Fast Fact

Pole SwapIf 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.

Fast Fact

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."

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Writer
Alison Ince Rodgers
Editor
Jeannie Evers, Emdash Editing, Emdash Editing
Producer
National Geographic Society
other
Last Updated

November 7, 2022

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