The Compass

It is easy enough to find your way around your home, your local neighborhood, or even your home town or city, but what if you need to go further afield? Suppose you are traversing a desert or an ocean with no familiar landmarks to guide you. It is very easy to get lost. Travelling merchants and explorers have faced these navigational challenges for thousands of years, and have developed numerous means of overcoming them. Since the dawn of history, mankind has used the Sun - the most predictable object in the sky - to determine the time of day and establish direction. Over many millennia, we have acquired knowledge about the positions occupied in the night sky by the Moon, stars and planets, and have used that knowledge for navigation.

The compass is probably the earliest form of navigational instrument. Prior to the invention of the compass, navigators could establish their position and direction of travel by looking at familiar landmarks, or by reference to the Sun and other celestial objects. If a ship was out of sight of land and beset by adverse weather conditions, navigation became virtually impossible.

The earliest compass is thought to have been invented in China as early as the third century BCE. These early direction-finding devices were made from lodestone (also known as magnetite), a naturally occurring form of iron ore with magnetic properties. Essentially, a piece of lodestone has its own magnetic field so that, when allowed to rotate freely around its axis, it will align its own magnetic field with that of the Earth. Thus, from the observer's point of view, the lodestone would always appear to "point" in the same direction.

It would be nearly a thousand years, however, before these seemingly magical properties were widely used for navigation. Instead, they were used for telling fortunes, or for determining the direction in which a new building should face in order to ensure the inhabitants enjoyed long life and prosperity. This is in keeping with the ancient Chinese art of Feng Shui, the goal of which is to achieve harmony with the invisible forces around us.

Early versions of the compass employed a pointer made from a piece of lodestone, often carved into the shape of a spoon or ladle, as shown in the illustration below. The pointer was typically placed on a square base plate made of bronze, engraved around its edges with the points of the compass and the major constellations. The spoon was allowed to rotate freely, and would always come to rest with its handle pointing towards the magnetic South Pole.

The first compasses to be used for navigational purposes are believed to have appeared in China during the eleventh century CE, and typically employed magnetized iron needles suspended on a small piece of wood floating in a bowl of water. The Chinese had discovered at some point that rubbing an iron needle in a particular direction with a lodestone gave the needle the same magnetic properties possessed by the lodestone. Because the resulting magnetic field was not permanent, a piece of lodestone was carried by early navigators, and used as and when required in order to restore the compass needle's magnetic field.

Although it is uncertain exactly when the compass made its first appearance in Europe, there are numerous references to its use in the Mediterranean and elsewhere from the twelfth century onwards. Descriptions of the most common type of compass used by European navigators suggest that it was similar in design to that used by Chinese navigators, i.e. a magnetized needle floating in a bowl of water. From the evidence available, it seems likely that the compass was brought to Europe from China by Arab traders.

By about 1300 CE the compass had developed into the kind of instrument we are familiar with today. A typical mariner's compass (sometimes called a dry compass) consisted of a small box with a glass cover containing a freely rotating compass needle mounted on a pin. A compass card (also known as a compass rose or wind rose), marked with the four cardinal points (north, south, east and west), was glued to the top of the needle. The compass would occupy a fixed position in relation to the keel of the ship. This meant that, as the ship changed direction, the compass card would turn to indicate its current heading.

Later developments included the suspension of the needle above a fixed compass card - an innovation thought to have occurred in the early fourteenth century CE and often credited to the Italian mariner and inventor Flavio Gioja (although details of his life, and even whether he actually existed, are disputed). A gimbal mounting was also introduced to keep the compass needle relatively level in rough seas. This was intended to ensure that the needle could always rotate freely by preventing it from grounding itself on the compass housing.

By the end of the seventeenth century, the dry mariner's compass had largely been superseded by the liquid compass, in which the body of the compass is filled with a clear liquid in order to dampen the movement of the compass needle. This reduces the amount of "wobble" encountered as a ship changes direction. The type of liquid involved varies, but ethyl alcohol is frequently used because it has a very low freezing point, which means that the compass can be used at very low temperatures.

As well as the four cardinal points, the markings on the compass card traditionally include the four ordinal or intercardinal points (north-east, south-east, south-west and north-west), and eight secondary intercardinal points (north-north-east, east-north-east, and so on). The cardinal and ordinal points of the compass are sometimes called the eight principal winds or main winds, while the secondary intercardinal points are similarly known as the eight half-winds. Each pair of points on the resulting sixteen-point compass card may be bisected by a point known as a quarter-wind, giving thirty-two points in total, equally spaced around the circumference of the card.

The illustration above comes from a book entitled "The Seaman's Secrets" written by the English explorer John Davis (1550-1605), published in London in 1607 by Thomas Dawson. Such cards were often decorated with ornate symbols, like the fleur-de-lys you can see at the top of the card. Different coloured inks were typically used to make it easier to distinguish the various points and lines. Most modern compasses show bearings in degrees, either in addition to the traditional compass points or instead of them. True north is represented by zero or three hundred and sixty degrees (0° or 360°), east by ninety degrees (90°), south by one hundred and eighty degrees (180°), and west by two hundred and seventy degrees (270°). The illustration below shows a First World War military compass. You can clearly see that the compass displays both cardinal and ordinal points and degrees.

Using a magnetic compass seems at first sight to be relatively easy. Hold the compass level and allow the compass needle to settle down. The direction in which the needle is pointing will be north. If using a compass in conjunction with a map or chart, simply orient the map so that north on the map aligns with the direction in which the compass is pointing. There is however a problem, which is that magnetic north is not the same thing as true north. True north is the point in the northern hemisphere at which the Earth's axis of rotation passes through the Earth's surface - otherwise known as the North Pole. On most maps, directions are defined with respect to true north.

A compass needle actually points to magnetic north, which is the point in the northern hemisphere where the Earth's magnetic field points straight down. That point is currently located well over a thousand kilometers distant from the North Pole. To make things worse, magnetic north changes its position constantly, drifting by anything up to sixty kilometers per year. The difference between the direction in which a compass needle points and the direction of true north is called magnetic variation (or magnetic declination) The amount of variation encountered will depend on where on the Earth's surface the compass is being used. At the equator, the average variation is relatively small, but the further north you go (or south, in the case of the southern hemisphere), the greater it will be.

It is of course possible to compensate for magnetic variation for any location on the Earth's surface, providing the degree of variation at that location is known. To this end, the Earth's magnetic variation has been mapped numerous times since the seventeenth century, and maps and charts used for navigation often provide information about local magnetic variation. Some modern magnetic compasses can be manually adjusted to allow for such variation, enabling the user to obtain an accurate compass bearing.

Problems can also arise due to the presence of a local magnetic field, or the occurrence of electromagnetic interference. The degree of error introduced into compass readings due to such effects is called deviation. Local magnetic fields can be due to the presence of mineral deposits having magnetic properties (iron ore deposits, for example) or the use of large quantities of iron and steel in buildings, vehicles and ships. Electromagnetic interference can be generated by various sources, including electronic equipment, large electric motors, and vehicle ignition systems. In some cases, magnets are incorporated into the compass in an attempt to compensate for externally generated magnetic fields or electromagnetic interference.

The problems of magnetic variation and deviation were overcome thanks to the invention of the gyroscopic compass at the beginning of the twentieth century. As its name suggests, the gyroscopic compass relies on a high-speed gyroscope which, once it is spinning at full speed, is always aligned in the same direction. A gyroscopic compass will typically be set to point to true north (using a suitably calibrated magnetic compass or GPS data) and is then periodically checked (again using a compass or GPS data) to ensure that its accuracy is maintained.

The advantages of the gyroscopic compass are that it always points towards true north, is not affected by local magnetic fields or sources of electromagnetic radiation, and does not depend on the shifting magnetic poles for its directional capabilities. It is also unaffected by the pitch and roll of a ship in heavy seas. The disadvantages are that a gyroscopic compass tends to be quite large and heavy, can be very expensive (tens of thousands of dollars), and requires a power supply. Most ships these days rely on GPS-based navigational systems (more about that shortly), with a magnetic compass providing a backup in the event of power failure or some problem occurring with the GPS service. Gyroscopic compasses are still used for military vessels, however - particularly submarines, for which a magnetic compass or GPS system is simply not an option.