A compass is a navigational instrument that shows directions in a frame of reference that is stationary relative to the surface of the Earth. The most common type of compass is the magnetic compass which shows direction relative to the Earth's magnetic poles.
The compass was invented in China during the Han dynasty between the 2nd century BC and 1st century AD. It was initially used for divination, geomancy, and Feng Shui. By the 11th century, it was adopted for navigation by sailors across the Mediterranean and Arabian seas.
The Earth has a magnetic field that is approximated by the field a magnetic dipole located near the planet's core. This magnetic dipole is tilted relative to the Earth's rotational axis. The magnetic needle of a compass aligns itself with these lines of magnetic force, which are oriented towards the magnetic poles.
To understand the direction, consider this basic principle: The North end of a magnetic needle is attracted to the Earth's magnetic North Pole, which is located near the geographic North Pole. Similarly, the South end of the needle points towards the Earth's magnetic South Pole, near the geographic South Pole.
Magnetic declination is the angle between magnetic north (the direction the north end of a compass needle points) and true north. The declination varies depending on where on Earth one is located and changes over time.
To adjust for declination, one must know the local declination angle. For example, if the declination is \(10^\circ\) east, rotate the compass housing \(10^\circ\) to the east before taking a reading.
To effectively use a compass with a map, one should align the map's north with the compass's north. Place the compass on the map with the edge along the desired line of travel. Rotate the compass housing until the north indicators on the map and compass align. The direction of travel arrow on the baseplate points towards your destination.
Some compasses come with an inclinometer, which measures the angle of slope. This is useful in navigation across terrains and for geologists determining the dip of rock layers.
Magnetic anomalies can cause compass errors. These anomalies might be due to magnetic minerals in the Earth or man-made objects such as cars, electrical wires, or steel structures. It's important to be aware of and adjust for these potential errors.
You can create a basic compass at home. You'll need a needle, a small magnet, a piece of cork, and a bowl of water. First, magnetize the needle by stroking it with the magnet. Then, push the needle through a small piece of cork. Float the cork in the bowl of water. The needle will align itself with the Earth's magnetic field, pointing towards magnetic north and south.
Despite advances in technology, the magnetic compass remains a fundamental tool for navigation. It's reliable, does not require batteries, and is not affected by technological failures. Understanding how to use a compass and read a map is an invaluable skill for outdoor enthusiasts, sailors, and adventurers alike.
