Background information on navigation. Includes a discussion on longitude and latitude, the nautical mile, the knot and UTC.

## The origin of the nautical mile

Upon the realization that the earth was spherical and not flat, it was the Greek philosopher and mathematician Pythagoras who was the first person to bring forward the theory in 580 B.C.

The invention of the chip log (c. 1500-1600) was a major advance that made early navigation considerably more accurate. The "chip log" can be thought of as a very crude speedometer. A light line was knotted at intervals which were 47 feet 3 inches apart from each other and weighted. This line was tossed overboard, over the stern. The measurement for speed was taken by how many of these "knots" ran out while a 28-second sand glass emptied itself. While this 28-second sand glass was emptying, a person counted the number of "knots" that were ran out on the line. Although a rather crude method, it gave the speed of the ship in nautical miles per hour. Proportionally, the length of 47 feet 3 inches to *6,080 feet is the same as 28 seconds to one hour. Obviously, the chip log has long been replaced by highly advanced, high-tech equipment; still, the "knot" is used for speed indication on the water and in the air.

*6,087.15 ft is the set standard for the geographical mile. This is equal to 1 minute of arc on the Earth's equator. This amounts to approximately 0.1819% larger than the set standard international nautical mile which is 6,076.1 ft.

## What the nautical mile represents

A Nautical mile is one-sixtieth (1/60th) of a degree of latitude-or one minute of latitude. It is based upon the circumference of Earth. This has a variation from 6,046 ft on the equator to 6,092 ft at latitude 60ºN and 60ºS. This variation occurs due to the fact that the planet Earth is not a perfect sphere; in fact, it is slightly flattened at the North and South poles. It is actually an oblate spheroid. In the need of a set standard value, the value 6,076.1 ft (1,852 meters) has been adopted internationally. There are 360 degrees around the earth. Each degree equals 60 minutes. At the equator, the distance around the earth, or any other great circle, is 21,600 nautical miles. Multiplying 360 degrees by 60 minutes will give 21,600 nautical miles as the answer: 360º x 60= 21,600nm.

• International nautical mile = 6,076.1 ft
• Statute mile = 5,280 ft
• 6,076.1 ft / 5,280 ft= 1.1507 or approximately 1.15 statute miles
• 1.15 statute miles equals 1 nautical mile.

To convert:-

• statute miles to nautical miles, divide the statute mileage by 1.15: (statute mileage) / 1.15=nautical mileage.
• nautical miles to statute miles, multiply the nautical mileage by 1.15: (nautical mileage) x 1.15=statute mileage.
• kilometers to nautical miles, divide the kilometers by 1.85: (kilometers) / 1.85= nautical mileage.
• nautical mileage to kilometers, multiply the nautical mileage by 1.85: (nautical mileage) x 1.85= kilometers.

Example Conversions:

• 100 statute miles to nautical miles: 100 / 1.15= 86.9565nm or approximately 87nm
• 100 nautical miles to statute miles: 100nm x 1.15= 115 statute miles
• 100 kilometers to nautical miles: 100 km / 1.85= 54.0540nm or approximately 54nm
• 100 nautical miles to kilometers: 100nm x 1.85=185k

## The Knot

A knot is a unit of measure for the speed at which your aircraft, ship or boat is traveling. This is equivalent to 1 nautical mile per hour (6,076.1 ft). This is approximately equal to 1.15 statute mph or 1.85 kph.

Example Conversions:

• 100 mph to knots (kts): 100 mph / 1.15 = 86.9565 kts or approximately 87 kts
• 100 kts to mph: 100 kts x 1.15 = 115 mph
• 100 kph to knots (kts): 100 kmph / 1.85 = 54.0540 or approximately 54 kts
• 100 kts to kph: 100 kts x 1.85 = 185 kph

## Latitude

The measurement of a particular location's position north or south of the equator is called latitude. The latitude is measured in degrees. The earth has what are called hemispheres. They are the northern hemisphere, southern hemisphere, eastern hemisphere, and the western hemisphere. The two hemispheres that, obviously, divide the world's northern and southern halves are the northern hemisphere and the southern hemisphere. Each hemisphere is 180º. By dividing 180º in half, locations are between 0º and 90º degrees north or south of the equator; in other words, the northern hemisphere has a latitudinal range between 0º to 90ºN, and the southern hemisphere has a latitudinal range between 0º to 90ºS. The equator's latitude is represented by 0º. The equator circumferentially extends around the world. Degrees of latitude have equal spacing; however, the oblate spheroid shape of the earth causes a degree of latitude to vary from 68.70 statute miles (59.70nm or 110.57km) at the equator to 69.41 statute miles (60.32nm or 111.70km) at the poles. The basic averages are as follows: 69 statute miles; 60nm; 111.1km, respectively.

## Longitude

The measurement of a particular location's position east or west of the Prime Meridian is called longitude. The Prime Meridian is located in Greenwich England, 0º longitude. It is an arbitrary location from which longitude is measured. A degree of longitude (meridian) at the equator is 69.17 statute miles (60.15nm or 111.32km). Unlike a degree of latitude, there is no set average distance for the degree of longitude above or below the equator. As one heads north or south of the equator, the distance between the lines of longitude become less until they finally converge at the poles.

## The Equator

The equator, located at 0º latitude, divides the Earth into the northern and southern hemispheres. The sun is located directly over the equator on approximately March 21 and September 21. On these dates, the days and nights are of equal length. These two designated days are the Vernal Equinox in the spring and the Autumnal Equinox in the fall.

## The Tropic of Cancer

The Tropic of Cancer is an identified line of latitude located at 23º 27' North. This is the northern solstice. This is the northernmost point where the sun's rays are directly overhead at least one day of the year. This occurs at approximately 12 noon on June 22 of each year. On this approximate date in the northern hemisphere, the sun will appear at its highest point above the horizon. In the southern hemisphere on this approximate date, the sun will appear at its lowest point on the horizon. This marks the first day of summer in the northern hemisphere (summer solstice) and the first day of winter in the southern hemisphere (winter solstice). During the next six month period, the days shorten in the northern hemisphere. The sun's inclination above the horizon at noon decreases. In the southern hemisphere, the opposite occurs: the days lengthen, and the sun's inclination above the horizon at noon increases.

## The Tropic of Capricorn

The Tropic of Capricorn is an identified line of latitude located at 23º 27' South. This is the southern solstice. This is the southernmost point where the sun's rays are directly overhead at least one day of the year. This occurs at approximately 12 noon on December 22 of each year. On this approximate date in the southern hemisphere, the sun will appear at its highest point above the horizon. In the northern hemisphere on this approximate date, the sun will appear at its lowest point on the horizon. This marks the first day of summer in the southern hemisphere (summer solstice) and the first day of winter in the northern hemisphere (winter solstice). During the next 6 month period, the days shorten in the southern hemisphere. The sun's inclination above the horizon at noon decreases. In the northern hemisphere, the opposite occurs: the days lengthen, and the sun's inclination above the horizon at noon increases.

## The Arctic Circle

The Arctic Circle is located on the Earth's surface at approximately 66.5ºN. This designates the point where there is at least one day per year of total light-about June 21 and at least one day out of the year of total dark-about December 21; in other words, this is the point where at and above the sun doesn't set for at least one day out of the year, June 21, or rise for at least one day out of the year, December 21. It circles above the horizon during periods of total light and never rises above the horizon during the periods of total dark. The sun's inclination on the horizon will be higher in the south and lower in the north. The curvature of the earth is the factor for this occurrence. During periods of total light and at the North Pole only, the sun appears to circle equidistant from the horizon.

## The Antarctic Circle

The Antarctic Circle is located on the Earth's surface at approximately 66.5ºS. This designates the point where there is at least one day per year of total light-about December 21-and at least one day out of the year of total dark-about June 21; in other words, this is the point where at and above the sun doesn't set for at least one day out of the year, December 21, or rise for at least one day out of the year, June 21. It circles above the horizon during the periods of total light and never rises above the horizon during the periods of total dark. The sun's inclination on the horizon will be higher to the north and lower to the south. The curvature of the earth is the factor for this occurrence. During periods of total light and at the South Pole only, the sun appears to circle equidistant from the horizon.

## North Pole (Geographic North Pole)

In the middle of the Arctic Ocean which is usually covered with pack ice, the North Pole can be found approximately 450 statute miles (725km or 391nm) north of Greenland. The depth of the Arctic Ocean, at this location, is approximately 13,410 ft (4,087m). The North Pole is the earth's northernmost point. It is located at the northern end of the earth's axis. The axis is an imaginary line that runs through the center of the earth from the North Pole to the South Pole. The earth revolves on this imaginary line (axis). The North Pole is located at 90ºN latitude, and it is the location where all the meridians of longitude converge. At this point, all directions on the earth's surface are south. The coordinates for the Geographic location of the North Pole does not change for the most part. However, the icy sea at the North Pole may move 30 feet (10 meters) or more in the matter of only one minute. To stay at the Geographic North Pole, you would have to walk northward continuously. In other words, a marker that you hammered into the ice could very well be over 30 feet (10 meters) away from the original precise location of the Geographical North Pole in the matter of only one minute.

## Magnetic North Pole

The magnetic North Pole presently lies more than 1,000 statute miles (1,600km or 870nm) south of the Geographic North Pole. The magnetic North Pole presently has the approximate coordinates of 78º 18'N 104ºW. Traditional magnetic compasses point toward the magnetic north. A problem that traditional magnetic compasses have is magnetic declination. Magnetic declination can be thought of as the angle between magnetic north (the magnetic North Pole) and true north (the Geographic North Pole) at a certain location. The variance in the magnetic fields of the earth is the cause. This is a result of complex fluid motion in the outer core of the Earth. This molten metallic region lies from 1,740 statute miles (2,800 km) to 3,107 statute miles (5,000 km) below the surface. Over a period of time, the magnetic declination changes; in fact, the the magnetic North Pole and the magnetic field shift every year. This is a phenomenon called polar wandering. Those using the traditional magnetic compass must be aware of the variance between the magnetic North Pole and the Geographic North Pole. The north magnetic pole (magnetic North Pole) actually moves on a daily basis, too: from the average magnetic North Pole's center point, it has an elliptical movement approximately 50 statute miles (80km or 43.5nm). It has been determined by the National Geomagnetic Program of Geological Survey of Canada that the magnetic North Pole is moving at approximately 9 statute miles (15km or 7.8nm ) each year.

## South Pole (Geographic South Pole)

In Central Antarctica, the South Pole can be found. The South Pole is the earth's southernmost point. It is located at the southern end of the earth's axis. The axis is an imaginary line that runs through the center of the earth. The earth revolves on this imaginary line (axis). The South Pole is located at 90ºS latitude. In East Antarctica, at the center of the continent, is the South Pole, also known as the south geographic pole. The South Pole is the point where all lines of longitude converge. It is at this point where all directions on the earth's surface are north. In January of every year, a new Geographic South Pole marker is hammered into the snow. The reason for this is because the ice sheet underneath is moving at the rate of approximately 30 feet (10 meters) per year. This comes out to approximately 1-inch per day. The new precise location is almost immediately incorrect at this rate. This, however, does not change the Geographic South Pole's actual position (coordinates) on the globe. It is only the ice sheet that has moved.

## Magnetic South Pole

The magnetic South Pole is locate off the Adélie Coast of East Antarctica. This is where the south-seeking end of a compass needle points. This is due to the Earth's magnetic fields. Interestingly, the magnetic South Pole was located on land in 1909 when it was first reached. Gradually over the years, the pole has moved out to sea with changes in the magnetic fields. This is a phenomenon called "polar wandering."

## Airport & VORs

Airport VOR is located at the airport, but the airport and VOR's magnetic variation is different.

When a navaid is originally built, the antenna is oriented physically to true north. A potentiometer adjustment is then made. This slaves the navaid with magnetic north. This matches the isogonic line with a magnetic compass. These two values are initially the same; however, over a period of years, the magnetic variation of the earth will change. This rate of change is 50.27 seconds of arc per year. An arc second is 1/3600th of a degree or 1/60th of 1 minute of arc. Navaids remain online around the clock. On an "as needed" basis, periodic maintenance is performed. To correct this magnetic variation, a re-slaving to match the isogonic value is required. This calls for a complete navaid shut down along with a re-certification flight check. The navaid needs to be out of tolerance by at least 6 degrees before a re-slaving procedure will be initiated to match the new airport and navaid magnetic variation value again.

## International Date Line (IDL)

The International Date Line (IDL) is an imaginary longitudinal line on the earth's surface. This line coincides, approximately, with the 180º line of longitude-the 180th meridian. The calendar date is separated by the International Date Line (IDL). When crossing this line traveling east, it takes the traveler back one day. Traveling west across this line takes the traveler forward one day. Confusion with the calendar would occur without this imaginary line. During the earth's counterclockwise rotation, the old day gets smaller and eventually disappears as the International Date Line reaches midnight. As the International Date Line passes through midnight, the entire world has the same day. Keep in mind that the International Date Line (IDL) is an approximate fixed longitudinal point and rotates with the earth.

The International Date Line (IDL) could have been anywhere on the globe; however, due to the nonexistence of land and civilization in the region, the 180th meridian was chosen as the approximate location for the International Date Line (IDL).

## Universal Time (UTC)

The sun travels over the earth's surface from east to west at approximately 15º of longitude per hour. At approximately every 15º of longitude, there are established time zones. This allows local times to correspond to the hours of day and night. Greenwich Mean Time, the newer term UTC or Universal Time, is a single coordinated time standard throughout the world that was brought about by space travel. For example, in the Central Time Zone of the United States, you must add 6 hours to the Central Time Zone for Universal Time (UTC). For Central Daylight Savings Time, you add 5 hours to get the Universal Time (UTC).

## Zulu Time [Z]

Zulu Time is represented by the letter "Z" and is the symbol for Greenwich Mean Time or the newer term Universal Time, UTC. Zulu time is extremely useful for aviation.

## Earth's rotational speed

The earth's rotational speed varies depending upon your latitude north or south of the equator. However, the speed of the earth's rotation at the equator is approximately 1,038 mph (1,670 kmph or 903 kts). If you were standing directly at the North Pole or the South Pole, your speed would be practically nothing at all; it would take you nearly 24 hours to complete a 360º revolution. In addition, the earth is moving around the sun at approximately 66,660 mph (107,821 kmph).

**FOR FLIGHT SIMULATION NAVIGATION USE ONLY! DO NOT USE THIS INFORMATION FOR ACTUAL REAL-LIFE NAVIGATION! The information in this document is believed to be correct; however, there is no guarantee stipulated.

Information researched, written and compiled by Mark Lawrence