At sea, the distance between two points is measured in nautical miles (nm) not statute miles. The ancient Roman’s placed a statute at the start and finish of one thousand (1,000 or a “mil”, hence the term “mile”, a statute mile), double strides of marching Centurions. As recorded, a full Centurion double stride was considered to be about 63 inches or five (5) feet three (3) inches in length. Given this fact, one-thousand Centurion double strides amounted to about 5,250 feet in distance. Approximately equal to our modern statute mile of 5,280 feet (.87 of a nautical mile). Unlike the Roman’s statute mile, a nautical mile is not dependent on a Centurion stride. It’s based on earth’s girth at its equator not its poles, (East to West = 7,925.77sm, North to South = 7,899.09sm).
The distance or length of one (1) degree of longitude measured at the earth’s equator (and only at the equator) or 1/360th of the distance of the earth’s circumference (21,600nm) is 60 nautical miles. Therefore 1/60th of a degree or one (1) minute of longitude equals one (1) nautical mile (1nm = 6,076 feet – which is 1.15 times longer than a statute mile). At the earth’s equator, the planet’s east-west circumference is more-or-less equal to its north-south circumference. Therefore, in the real world and only at the earth’s equator does the length of one (1) degree longitude (more or less) equal the length of one (1) nautical mile. Traveling north or south away from the earth’s equator, the east-west distance between lines of longitude eventually decrease to zero as they converge at the poles. Because meridians of longitude are not parallel lines, they do not lend themselves to accurate distance measurements. Lines of latitude, are commonly referred to as parallels of latitude and therefore do not converge at the earth’s poles or anywhere else. Latitude is our guide to accurate distance measurements.
Since most nautical charts are based on Mercator projection, with its inherent distortion of all earthly surface features, we do not use degrees of longitude for distance measurements.
- To measure distance in nautical miles: Use the vertical margins ONLY, do not use the top or bottom horizontal margins. One (1) nautical mile equals one (1) minute on the vertical margins.
Ø One degree of indicated latitude is equal to sixty (60) nautical miles in distance. Each degree is divided into sixty (60) minutes, with each minute equal in distance to one (1) nautical mile. As already mentioned, each minute is then divided into tenths. Each tenth of a minute is equal to 1/10th of a nautical mile.
Learning Exercise: Distance Measurement
Using your dividers – measure the distance in nautical miles (nm) between the following positions – use your Light List to find the following charted objects.
1. What is the distance from Cape Henry Light to Cape Charles Light?
2. What is the distance from Chesapeake Channel Y “NCA” Whistle buoy to Chesapeake Light?
3. What is the distance from New Point Comfort Spit Light “2” to York River Entrance Channel R “18” QR Gong buoy?
4. What is the distance from Horn Harbor Entrance Light “HH” to Wolf Trap Light?
5. What is the distance from abeam of Trestle “A” and Trestle “B” of the Chesapeake Bay Bridge and Tunnel to GR “CBJ” Gong buoy?
Answers: 1. C 2. B 3. D 4.C 5.A