Magnetic Compass



The magnetic compass is an instrument that indicates the whole circle bearing from the magnetic meridian to a particular line of sight. It consists of a needle that aligns itself with the Earth’s magnetic flux, and with some type of index that allows for a numeric value for the calculation of bearing. A compass can be used for many things. The most common application is for navigation. People are able to navigate throughout the world by simply using a compass and map. The accuracy of a compass is dependent on other local magnetic influences such as man-made objects or natural abnormalities such as local geology. The compass needle does not really point true north but is attracted and oriented by magnetic force lines that vary in different parts of the world and are constantly changing. For example, when you read north on a compass you are reading the direction toward the magnetic north pole. To offset this phenomenon we use calculated declination values to convert the compass reading to a usable map reading. Since the magnetic flux changes through time it is necessary to replace older maps with newer maps to insure accurate and precise up-to-date declination values.

How Do You Know When You’re At the Pole?

Remember that your magnetic compass would be reading magnetic north but you want to get to geographic north. What would you do to figure out where you were going, particularly when the end of your journey was on ice that was moving? If you’re standing at the North Pole, all points are south of you (east and west have no bearing). Since the earth completes a full rotation once every 24 hours, if you’re at the North Pole your speed of rotation is quite slow—almost no speed at all, compared to the speed of rotation at the equator of about 1,038 miles per hour.

Arctic explorers carried a sextant with them to take measurements as they got closer and closer to the pole. Prior to GPS, this was the only accurate way to determine if you actually made it!

Where Exactly Is the South Pole?

Like the geographic North Pole, the South Pole lies at the point where the lines of longitude converge (it’s like the part of the orange where all the sections come together). How would you know if you’ve actually reached the South Pole? As at the North Pole, you would use a sextant to measure the angle of the sun off the horizon. When Amundsen reached the South Pole he sent three men out at 90 degrees to each other with the instructions to travel 10 miles in those directions and to plant a flag. That way he knew he had encircled the pole. Amundsen stayed at the pole so that his team could take a series of hourly observations with the sextant over a 24-hour period to determine their actual position. They found they were 6 miles from the pole so they moved their camp.

What is Latitude and Longitude?

Knowing latitude and longitude is a simple way to identify location. Navigators talk about their north-south position using parallels of latitude—the lines running across the map, chart, or globe, from left to right, west to east. A latitude coordinate tells how far north or south you are from the equator, the line that goes around the middle of the globe dividing it into the Northern and Southern Hemispheres. A longitude coordinate tells how far east or west you are from the prime meridian, the line of longitude that runs through Greenwich, England. Lines of longitude, which are also called meridians, run north and south on a map and converge at the poles.

Distance is written in terms of degrees. The equator lies at 0 degrees and the parallels of latitude north of the equator are identified as north, and those south of the equator are identified as south. The North Pole lies at latitude 90 degrees north, and the South Pole at 90 degrees south. The prime meridian lies at 0 degrees longitude. Meridians of longitude east of the prime meridian are designated as east, and those west of the prime meridian are identified as west.

Where longitude 180 degrees west meets longitude 180 degrees east in the Pacific Ocean is the International Date Line, the place where the date actually changes. Fortunately the International Date Line doesn’t go through any islands—it zigs and zags along the 180-degree meridian—otherwise for people living on one side of the date line it would be today, and for their neighbors living on the other side it would be tomorrow, which could get very confusing. Without the International Date Line, travelers going westward would discover that when they returned home, they had spent one more day on their vacation than they thought, even though they had kept careful tally of the days. This happened to Magellan’s crew after their first circumnavigation of the globe. Likewise, a person traveling eastward would find that one fewer day had elapsed than he or she had recorded, as happened to Phileas Fogg in Around the World in Eighty Days by Jules Verne.

Each degree of latitude and longitude is divided into 60 minutes, and each minute is further divided into 60 seconds (think of how time is divided and you’ll never forget this). Navigators measure distance in nautical miles. One nautical mile equals one minute of one degree and has been set at 6,080 feet. So one degree of latitude or longitude equals 60 nautical miles (or 70 land miles). Any location on earth is described by two numbers—its latitude and its longitude. If a ship’s captain wants to specify position on a map, these are the “coordinates” they would use. Think of position coordinates like you think of street addresses. When position coordinates are given, it’s just a way to pinpoint a place by identifying where lines of longitude and latitude intersect. This can be particularly helpful in the middle of the ocean where there are no visible landmarks. Coordinates are always read by stating the latitude first and the longitude second. One very famous set of position coordinates is latitude 41 degrees 33 minutes north, longitude 50 degrees 01 minute west. On April 14, 1912, this is where the ocean liner Titanic struck an iceberg in the northern Atlantic Ocean and quickly sank.