The Earth is almost a perfect sphere. In order to define where we are, on the surface of this sphere, we use a system of coordinates called latitude and longitude. In the real world our Earth is spinning about its rotational axis at around 1,000 miles per hour (at the equator). This imaginary line we call the axis defines two points on the Earth's surface where it exits. These are the north and south poles, also called true north and south (for an understanding of true and magnetic north see True & Magnetic). Interestingly, the north axis line points almost exactly at the star Polaris; that's why we also call this the north star. Mariner's in the northern hemisphere, who can see Polaris at night, know their latitude simply by measuring the angle between the horizon and this star. Unfortunately, there is no corresponding 'south star' for the southern hemisphere.
Half way between the north and south poles we define the line encircling the Earth as the equator, represented by the large green circle above.
In the image at left we can see the north pole, equator and lines latitude. These lines of latitude are also called parallels because they are always parallel to each other. Lines of latitude start at the equator, which would be 0°, and are measured north or south to the corresponding pole. Each pole would be 90° away from the equator. So, halfway between the equator and the north pole we have 45° north latitude and so on.
Just as we subdivide hours of time into minutes and seconds we can subdivide each degree into 60 minutes and then subdivide each minute into 60 seconds. The convention is to use a slash mark to indicate minutes and two slash marks to indicate seconds. We would write 45° 9 minutes and 45 seconds north latitude as: Lat 45° 09' 45" N. Mariner's usually do away with seconds and instead use decimal minutes so we would actually write it as: Lat 45° 09.75' N.
Minutes of latitude have a very special place in the art of navigation. One minute of latitude is defined as one nautical mile. A nautical mile corresponds to a little more than a statute mile. If we know latitude, as it is written on the sides of a nautical chart, then we have a very handy and easy way to measure distance. This works on mercator projection charts which are the norm for navigation (see later). Taking this just a bit further we define our speed in knots; One knot equals one nautical mile per hour.
In the image at left we see the north and south poles, the equator, and lines of longitude. Lines of longitude (or meridians) are lines that run from the north pole to the south pole. Longitude forms the other set of coordinates so we can now define every spot on Earth by specifying both latitude and longitude. Longitude is measured from the meridian that runs through Greenwich, England. We call the line of longitude that runs through Greenwich the Prime Meridian. We can also call any line of longitude a meridian.
just as with latitude that starts at the equator and is measured both north and south, longitude starts at the Prime Meridian and is measured east and west. The meridian that is on the exact opposite side of the globe from Greenwich is known as the International Date Line and is 180° from Greenwich.
The largest number that can be associated with latitude is 90° which would be at one of the poles. The largest number that can be associated with longitude is 180°. We specify longitude in a similar fashion to latitude. For instance, 75° west longitude is a meridian that runs through the northeastern US; We would write 75°, 39 minutes, 30 seconds west longitude as: 075° 39.5' W Lon.
We learned above that minutes of latitude have a special meaning in navigation and are defined as one nautical mile. This is possible because a minute of latitude is equal to every other minute of latitude on the globe. However, you will notice that since meridians, or lines of longitude, are not parallel then minutes of longitude change their length as we move north or south. In fact, minutes of longitude are always smaller than minutes of latitude except right at the equator where they are equal.
Now, we can put this all together with a grid system for the Earth as in the below Google Earth image:
Above you can see the Prime Meridian running through England and how we label meridians east and west. Just to the west is W 10° while to the east of the Prime Meridian is E 10°. We also see, just north of the equator the line of latitude, or parallel, labeled N 10°. There is one and only one spot on Earth where these two lines of latitude and longitude intersect. We would write this as: Lat 10° 00.0' N; Lon 010° 00.0' E. And, this is exactly what we would put into our GPS unit if we wanted to go there.
The chart at left is a Mercator projection of the world. You can see the distortion by noticing how large Greenland and Antarctica are. The horizontal blue line is the equator while the vertical one is the Prime Meridian. Almost all nautical charts are Mercator projection. This is a simple way to represent an area on the surface of the Earth but causes distortions especially at higher latitudes (both north and south). With a Mercator projection all lines of longitude are drawn parallel to each other. This makes navigation so much easier since a straight line on the chart is now the shortest distance between two points. When long east-west distances are involved this brings too much distortion but for everyday navigation Mercator charts are the norm and are quite accurate over shorter distances.
Above is a typical Mercator projection nautical chart (credit goes to NOAA) that we might very well use for navigation. You can see the meridians (lines of longitude) are parallel with the sides of the chart and the lines of latitude are perpendicular to them and parallel with the top & bottom of the the chart.
Let's take a closer look at the edges of the chart where the latitude and longitude scales are.
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