For measuring speed on land, there are two units in common use, statute-miles-per-hour and kilometers-per-hour. For measuring speed through the air or water, the unit knot or nautical-miles-per-hour is often used, in addition to the statute-miles-per-hour or kilometers-per-hour. The three different units of distance:

- statute mile
- nautical mile
- kilometer

The nautical mile was originally defined as the distance equivalent to a meridional arc length corresponding to one-minute of one-degree of latitude, which required knowing the circumference of the Earth. This is where definitions become interesting.

In creating the length unit of a meter, the fundamental definition was related to the circumference of the Earth. The distance from the North Pole to the Equator measured on a meridian passing through Paris was arbitrarily defined to be 10,000,000-meters. Since the North Pole to Equator distance was one quarter of the Earth circumference, this suggested that the circumference of the Earth would thus be 40,000,000-meters. The actual distance between the North Pole to the Equator on a meridian passing through Paris was never measured. As a substitute, a distance thought to be one-quarter of the meridian from the Equator to the North Pole was measured by a six-year-long survey that concluded in 1798, known as the Arc measurement of Delambre and Méchain. Their survey was much later revealed to have been done with errors which were concealed by a surveyor at the time.

The problem with this method of definition was the Earth was not a perfect sphere. Compensating for an increase in diameter of the sphere at the equator should have been applied.

The full great circle distance of 40,000,000-meters would be 360-degrees. Thus one minute of one degree expressed in meters would be

40,000,000-meters/(360 × 60) = 1851.851-meters

Later investigation found that the Earth circumference was actually closer to 40,008,000-meters. Thus the length of one-minute of one-degree of Latitude would then be:

40,008,000-meters/(360 × 60) = 1852.222-meters

The length of a meter was initially defined as a particular length of a platinum bar in Paris, and much later re-defined in terms of atomic wavelengths.

The relationship between the foot and the meter was defined in 1893 using inches as the unit:

39.37-inches = 1-meters

This defined the foot in terms of meter as

1-foot = 0.30480061-meter (approximately)

We can calculate the length of a nautical mile in feet from the above relationships. Using the value of one nautical mile as 1852.222-meters:

1852-meters × (39.37-inches/1-meter) × (1-foot /12-inches) = 6076.83-feet

However, in the United States and United Kingdom, the circumferential distance of the Earth used to define one-minute of one-degree of Latitude used an average for the ellipsoid distance, resulting a value of 1,853.2480-meters. This produces a value in feet for a nautical mile of approximately:

1853.2480-meters × (39.37-inches/1-meter) × (1-foot /12-inches) = 6080.1978-feet

In the United States a five-significant-digit value of 6080.2-feet was used; in the United Kingdom the "Admiralty Mile" is defined as 6,080-feet.

The nautical mile was eventually re-defined to be exactly 1852-meters, more or less losing its previous definition as a particular arc distance length.

Eventually, the United States in 1954 adopted the "international Nautical Mile" value of 1852-meters, converting it into feet with a value using the relationship between the yard and meter then in use, and came up with a value of approximately 6,076.10333-feet in one nautical mile. However, using the international definition of foot, the nautical mile was then adopted as being equivalent to 6,076.11549 International feet.

What I observe in all of this is that the definition of a statute mile has remained the same since 1593, while the definition of the nautical mile, first proposed as a distance of a meridional arc of one-second of one minute has been a moving target when converted to an equivalent length in meters or feet for 450-years.

Limiting use to the United States, the two distance units for speed are generally statute miles (6076.1-feet) and nautical miles (5,280-feet). The conversion factor between these units is thus

1-nautical mile = (6076.1 / 5280)-statute-miles

and limiting to four significant digits

1-nautical mile = 1.151-statute-miles (approximately)

For conversion a value of 1.15 gives quite acceptable accuracy.

In common use the acronym MPH means statute-miles-per-hour and NMPH means nautical-miles-per-hour. The word "knot" as a speed unit is often used ambiguously. While a knot is 1-NMPH, in many cases of casual writing, defining the speed unit as a "knot" leaves confusion about the exact unit being referenced, and, for that reason, explicit use of 1-NMPH is preferred.

The topic of which unit of speed measurement is appropriate for measuring the speed of a small recreational boat capable of speeds of up to 50-MPH will be pursued in the next installment of this article.