A compass is a device that indicates direction. It is one of the most important instruments used for navigation. Magnetic compasses are the best-known type of compass. While the design and construction of the magnetic compass have changed significantly over the centuries, the concept of how it works remains the same. A magnetic compass consists of a magnetized needle that rotates to line up with Earth's magnetic field. The ends point to what are known as the north magnetic pole and the south magnetic pole.

History of Compasses

The principle of magnetism has been observed by humans for thousands of years. Ancient Greeks observed the principle of magnetism, but they did not understand its relationship to the Earth or that a magnetized metal would point north. People in Ancient China also recognized magnetism. They learned that a magnetized bar of lodestone tied to a string would always point in the same direction. This observation became part of their spiritual beliefs as religious leaders used a magnetic spoon balanced on a plate to predict the future. Some consider this to be the earliest form of a compass, although compasses are more accurately defined as instruments devised for navigational purposes. A unique aspect of these early compasses in China is that they were oriented to the south and were referred to as “south pointing spoons” or “south pointers.” Later Chinese compasses were similarly oriented to point south rather than north like today’s compasses.

There is evidence that explorers from China and Europe were using compasses to navigate the seas as far back as the 1100s. In fact, many historians believe that people in China were using compasses to navigate long before that time. Miners in search of jade, for instance, appear to have used “south pointing spoons.” Some historians also believe that compasses originated in China and traveled to Europe through trade routes, but others think that Europeans developed the technology independently.

Early Compasses

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 settled, 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. In time, 32 points of direction were added to the compass card.

In their earliest use, compasses were likely used as a backup navigational tool for when the sun, stars, or other landmarks could not be seen. As compasses became more reliable and more explorers understood how to use them, compasses became an essential tool for travelers.

Adjustments and Adaptations

By the 15th century, explorers realized that the “north” indicated by a compass needle 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 it varies depending on location. Because of this variation, a compass could lead a novice user many kilometers off-course. Navigators learn to 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, help 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 mirror that allows the user to see the landscape and compass reading at the same time, or markings in Braille for people with a visual impairment. 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. It was used in the 19thand 20thcenturies to survey land. Because a solar compass is not affected by iron metal deposits or location relative to the poles, a solar compass can be more accurate than a magnetic compass, particularly near the poles. The most common method is to use a compass card and the angle of the shadow of the sun to indicate direction.

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 12 o'clock mark 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 Earth and monitoring stations on Earth to pinpoint the receiver's location. GPS receivers can plot latitude, longitude, and altitude on a map. In open areas and optimal conditions, standard GPS is accurate to about 6 meters (20 feet), and researchers have developed a so-called “SuperGPS” that is accurate within 10 centimeters (3.9 inches).

Many people throughout history have used knowledge of the stars and constellations as a kind of compass. For example, Polynesians have been using patterns in the sky to navigate the ocean for at least 2,000 years. A native Hawaiian historian, Charles Nainoa Thompson, developed a Hawaiian star compass in the mid-2000s to illustrate how Polynesians use the constellations for navigation. Much like a magnetic compass, there are 32 stars situated around a center point on a star compass.

Impact of the Compass

The compass had a major impact on the world, particularly for people who were navigating the sea. Because compasses were more accurate than other naval navigational tools, explorers used them to explore parts of the world that were unknown to them during the so-called Age of Exploration that began in the early 15th century, a development with both positive and negative impacts. European exploration contributed to trade and the circulation of knowledge, but it would also lead to the spread of disease, the colonization of new lands, and the enslavement of Africans and other indigenous populations. Because today’s global relationships have emerged from these relationships, the legacy of colonization—and the invention of the compass—continues to have a profound impact on the world today.