How Does a Compass Work? Pointing You in the Right Direction

How Does a Compass Work? Pointing You in the Right Direction

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My brother sent my son a compass this past Christmas. Of course, the whole family passed it around marveling how it indicated North no matter how we turned it. When my son wanted to know exactly how it did that, I realized that I didn’t remember precisely how does a compass work. I went off to the internet for a bit of research.



Image via flickr

So how does a compass work? If you open the compass, you’ll see that inside is a small magnetic pin suspended in liquid.

If you remember from science class, all magnets have two poles, north, and south. The south pole of one magnet is attracted to the north pole of another magnet.

The Earth is a magnet and has two poles. The North Pole attracts the north pole of the magnet in the compass which ironically makes it the South Pole of the planet’s magnetic field.


compass pointing north

Image via pexels

The magnetic needle of a compass, which is often red, will always point north, no matter in which directing you are facing. However, you need to know that north on the compass isn’t true north.

Unless you are using a gyrocompass, your compass will not always point exactly north because the Earth’s magnetic North Pole is not the geographic North Pole. True North, or Map North, is the point where all longitudinal lines meet on the map.

The magnetic North Pole is about 1,000 miles south of the geographic North Pole about eleven degrees from the tilt of the Earth’s axis. Believe it or not, the magnetic North Pole isn’t even stationary. Over the previous 100 years, it has moved 620 miles towards Siberia.

The difference between the two norths can be read on a compass by the declination angle. While it might not seem like much of a difference, if you happen to travel just one degree off for a mile, you’ll be 100 feet off track. Accurate navigation takes declination into account.


hand with a compass

Image via pexels

Before we talk about using a compass, let’s talk about the different components. You’ll need to know what these are and what they do in order to read the compass correctly. In this section, we will be describing a basic field compass otherwise known as a baseplate compass. It’s the most common and most affordable type of compass.

Parts of a basic field compass

  • Baseplate
  • Direction of travel arrow
  • Compass housing
  • Degree dial
  • Magnetic needle
  • Orienting arrow
  • Orienting lines

The baseplate is the plastic, clear plate that holds the compass. The direction of travel arrow is the one that points away. The turnable circle around the compass housing is the degree dial. The spinning needle is the magnetic needle that points north. The non-magnetic arrow is the orienting arrow. The lines that run parallel to the orienting arrow are the orienting lines.


map and a compass

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Now that we have a general idea of what comprises a compass let’s talk about how to read one. Most compasses are read in the same manner, unless they are marine, prismatic or electronic compasses.

Hold the compass flat on your open hand in front of your chest. Figure out which direction you are currently facing by turning the degree dial until the orienting arrow lines line up with the magnetic arrow. Both the lines and arrow should be pointing North. Look at the travel arrow and find the point where it intersects with the degree dial. That intersection point is the degree of the direction you are facing.

It’s important that you take declination into account. You’ll need a declination chart, also known as local calibrations, to determine the difference between true and magnetic north in your location. If you are traveling on the line of zero declination, you do not need to make any adjustments. If you are to the east of that line, you will need to subtract the number of degrees from your bearing as indicated on the chart. If you are to the west of the line, you will need to add the number of degrees.



Image via unsplash

If you were at the magnetic North or South Pole, the compass needle would move aimlessly, unable to settle in one direction. If you were at the geographic North or South Pole, the compass needle would continue to point to the magnetic North Pole.

In the same way, a magnet or being inside an iron ship can interfere with the accuracy of your compass. The difference between an accurate reading pointing to magnetic north and a reading that has metal or electrical influences is called deviation.



Image via flickr

For the most part, a baseplate compass will get you where you want to go provided you read it correctly. There are other types of compasses, however. Depending on your travel location, desired navigation accuracy, and even your method of transportation, you may want to consider one of the following kinds.


baseplate compass

Image via flickr

For general plotting and navigation, that’s all you need. Sometimes the baseplate will have a magnifying lens to help you read maps, glow in the dark components to allow you to read the compass when there isn’t much light and different measurement scales.


lensatic compass

Image via flickr

The U.S. military prefers the lensatic compass for their maneuvers. The base holds the needle, dial, and rotating scales. The cover has a sighting wire which gives it an advantage over a baseplate compass. You can use a sighting wire to align your direction with distant objects. The rear lens is used to read the dial.


competition compass

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A competition compass attaches to your thumb, which is why this type is also called a thumb compass. Athletes prefer this kind because it allows them to hold the compass and map in one hand while running, biking or other high movement activity.


prismatic compass

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If you need to be extremely accurate in your navigation, you’ll want to get a prismatic compass. It uses a prism to read the compass while sighting distant objects simultaneously. Surveyors often use prismatic compasses.


marine compass

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It’s no surprise that a marine compass, also known as a card compass, is used on boats. The compass card spins freely in a bowl of alcohol and water and mounted on pivots on a binnacle stand so that it remains horizontal even when the boat is rocking from side to side or pitching up and down.



Image via flickr

If you need to find true north, you’ll want to use a gyrocompass. This device uses gravity and a gyrowheel ensuring the axis always finds the north-to-south line. External magnetic fields do not affect the reading since it doesn’t use a magnet for direction. The gyroscope is a spinning wheel that continues to move in the same direction no matter which direction it is turned. Aircraft use this type of compass.



Image via flickr

If you find yourself unable to master reading a traditional compass, electronic compasses will give you a numerical readout. They also have the advantage of being about to store bearings in the memory and let you know if you wander off course with a beep or an alarm. They don’t work as well in conjunction with maps and need batteries which make them less than ideal in some situations.


astro compass

Image via flickr

If you are working in a scientific lab at the North Pole, you will want to use an astrocompass. This device uses the position of fixed points in the sky like the Sun or stars to determine true north. Since they aren’t dependant on the magnetic pull of the Earth, they are much more reliable in the North and South Pole.


antique compass

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Can you believe that humans have been navigating by the pull of the Earth’s magnetic field for more than 2,000 years?

The first directional finders to use magnetic force were developed by the Chinese between 300 and 200 BCE. They were made from lodestone, which is a piece of the mineral magnetite that is naturally magnetized.

The first official compasses had needles mounted on pins that were magnetized by striking them with a lodestone. These were developed around 1300 in Europe and Arabia. The magnetized pins were installed in a box and could move freely to indicate north. Compass cards that marked North, South, East, and West were added to the device in the thirteenth century.

The first compass that used liquid to suspend the magnetized needle was invented by Edmund Halley in 1690. Francis Crow created the first liquid mariner’s compass in 1813. The first gyroscope was made by Leon Foucault in 1852. F.O. Creagh-Osborne developed the first aircraft compass in 1909. The first portable compass designed for individual use was invented by Tuomas Vohlonen in 1936.

So there you have it! Now you know the answer to the question “How does a compass work?” and a bit more information besides. Now get out there and explore!

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