You have probably observed that a compass points north irrespective of your geographical position. Now, why does the compass needle of magnetic compasses point towards the Earth’s geographic North Pole?
Earth behaves like a giant bar magnet. As you may already know, a magnet is a piece of material that is capable of attracting magnetic objects placed within its magnetic field. A magnetic field can be explained as the region of space around a piece of magnet where its magnetic force is felt.
In science, the magnetic field around a magnet is usually represented by magnetic field lines. These lines are imaginary lines that are used to explain magnetism. In most science laboratories, a bar magnet is often used for magnetism experiments. The magnetic field around simple bar magnets is similar to the Earth’s magnetic field. Like a bar magnet, the Earth has magnetic North and South Poles.
A compass is a navigation tool that detects the geographic poles of a location. Under the influence of the Earth’s magnetic field, the compass needle aligns with the Earth’s magnetism and, as a result, points towards the geographic North Pole.
Earth’s magnetic poles are slightly off from the axis on which the Earth spins. Earth’s rotation axis is the basis of its geographic poles. Although the Earth’s magnetic pole is slightly off from the geographic poles, they are close enough to enhance the working of a magnetic compass as a valuable navigation tool.
Earth’s magnetic North Pole lies in the same direction as the true North Pole (which does not change concerning Earth’s magnetic core like the magnetic North Pole), while the magnetic South Pole is towards Earth’s geographic South Pole.
Magnetized objects and magnets often have two opposite poles; a North Pole and a South Pole. Compass needles are no different. A basic physical experiment shows that like poles repel while unlike poles attract. Earth’s magnetic North Pole attracts the magnetic needle’s South Pole, so the needle points towards Earth’s fixed North Pole (true north), while the magnet’s North Pole points to Earth’s south magnetic pole (Earth’s geographic south).
The North magnetic pole is the direction a compass points to when the compass needle aligns with the Earth’s magnetic field. The Earth’s magnetic north varies from time to time and from place to place in response to the Earth’s magnetic core. It takes about 960 years for the magnetic poles to complete an entire cycle of drift across the arctic ocean.
Earth’s true North Pole is, however, the direction that points directly towards the geographic North Pole.
The difference between true north and magnetic north forms an angle which is referred to as declination. Declination varies from place to place due to the non-uniform nature of the Earth’s magnetic field. It is also called magnetic variation.
According to USGS, “The deviation of the compass from true north is an angle called “declination” (or “magnetic declination”).”
Although there are many kinds of compasses, there are two major types; magnetic compasses and non-magnetic compasses.
Magnetic compasses are the most common type of compass. In fact, the term “compass” almost always refers to magnetic ones. They consist of a magnetic needle, often made of steel, and rotates on a horizontal plane so that the needle lines up with the Earth’s magnetic field. The ends of the needle of the compass point to the magnetic North Pole and magnetic South Pole.
Because it is made of lightweight lodestone, it is widely accepted to say that a magnetic compass is a magnet. The end of the magnetic needle, which is painted red, is suspected to be the South Pole as it always points towards the magnetic North Pole under normal conditions.
This type of compass does not use a magnetic material (lightweight magnet or magnetized compass needle). An example is the gyro compass. It consists of a mounted wheel or disc whose spinning axis is confined to a horizontal plane so that it assumes a position parallel to the Earth’s axis and thus points north (true north) with the influence of the Earth’s rotation. It finds true north by using a fast-spinning wheel and friction forces to exploit the rotation of the Earth. They are widely used on ships.
Magnetic vs. Gyro Compass
A magnetic compass has many advantages. It can be used in place of marine compasses because it allows mariners to find their way easily. Some sources say that magnetized compasses are also called marine compasses. Often, mariners use the north star, Polaris, to identify the Earth’s north. Identifying the northern hemisphere or true north makes it easier to find their way.
Sometimes, however, it gets difficult to locate the north star or other astronomical cues when the sky is cloudy. A magnetized compass uses a magnetic needle (which is mounted on a pivot) that spins freely so that it always points to the North Pole of our planet’s magnetic field. This mechanism works because a compass and its needle are made of lightweight material.
A light magnet can move freely and align itself in the Earth’s north-south direction. However, a giant bar magnet at rest cannot move because of the force of gravity and frictional force acting against the magnetic force.
Unlike gyro compasses, magnetized compasses do not need an external force or source of power to work, so you do not need to worry about power supply failure or batteries running down.
A magnetized compass is usually affected by ferromagnetic materials (e.g., iron, steel, cobalt, nickel, and various alloys), which is why it cannot be accurately used in ship’s hulls because they’re made of ferromagnetic materials which distort the magnetic field of the compass. Gyro compasses, on the other hand, are not affected by ferromagnetic materials.
A magnetized compass is very stable close to the equator, which is far from the Earth’s magnetic North Pole. However, as the compass is moved closer to the Earth’s magnetic poles, it becomes more sensitive to crossing the Earth’s magnetic field lines. At some point, the compass will not indicate any specific direction but will begin to drift; the needle starts to point up or down due to magnetic inclination. This experience will not occur with a gyro compass.
Place the compass flat on your palm, right in front of your chest. First, you must know your bearings to figure out where you’re headed. To do this, move your compass until the direction of the travel arrow (the arrow in the baseplate pointing away from the compass. The baseplate is the transparent plastic plate on which the compass is embedded) points in the direction you’re traveling.
Turn the degree dial (the turnable dial that houses the compass and has readings up to 360 degrees) until the orienting arrow (the non-magnetic arrow inside the compass housing) aligns with the north end of the magnetic needle. When this happens, you’ll be able to tell where your direction of travel is.
If you’re using a map, place the map on a flat surface with the compass on it so that the orienting arrow points towards the true north on the map. Slide the compass so that its edge passes through your current position on the map.
Ensure that the orienting arrow continues to point north. Draw a line along the compass edge through your current position on the map. Your new path will be along the drawn line if you maintain the bearing. This will align the compass’ orienting lines with the map’s North-South Pole.
Once the degree dial is in place, you can now remove the map and use the new bearing to navigate. Hold the compass horizontally in front of you with the direction of the travel arrow pointing away from you. Turn your body slightly until the north-seeking pole of the magnetic needle is aligned with the orienting needle.
You must find out the declination in the area to compensate for it on your compass. Turn the degree dial according to the number of degrees to the left or right depending on whether the area experiences northeast or northwest declination.
According to Historians, China may have been the first to develop and use a compass over 2000 years ago. It is said to have been discovered during the Han dynasty (202 BC – 220 AD).
Before the introduction of compasses, landmarks and the position of celestial bodies were used for navigation on land and sea. Other techniques were sampling mud from the seafloor, analyzing the flight path of birds, observing the wind, sea debris, sea state, etc.
This could happen when a compass experiences reverse polarity. Reverse polarity is a term that describes a situation where the magnetism in a compass needle becomes permanently reversed so that the north-seeking pole (North Pole) points north and the south-seeking pole (south magnetic pole) points south instead of south and north, respectively.
Reverse polarity happens when a compass comes in contact with a magnet or any device/gadget which has a magnet in it, e.g., radio speakers, walkie-talkie devices, etc. Refrigerator magnets can also reverse the poles of a magnet.
Reverse polarity can be corrected by repeatedly stroking the needle with a small magnet. Compasses can be protected from reverse polarity by keeping them away from gadgets with speakers or magnets in them.