Why use nautical miles and knots in sailing?

We use nautical miles and knots in sailing because a nautical mile is based on the earth’s circumference. One nautical mile equals one minute of latitude or 1/60 of a degree. It takes one hour to sail one nautical mile at a speed of one knot. We use nautical miles and knots for navigating and plotting charts.

To better grasp this concept and hone in on the “why,” we need to unpack it a bit and examine the individual components. The collaboration between these components still underpins why we use nautical miles and knots in sailing in today’s digital world.

1. What are nautical miles?
2. Why divide the globe up in 360°, minutes and seconds?
3. What is latitude (parallels)?
4. What is longitude (meridians)?
5. Why use knots?

1. What is a nautical mile?

A nautical mile, which equals 1852 meters, is used in navigation by the air, marine, and space industries. It can be described as one minute (1/60 of one degree) of latitude along any longitude line, more on latitude and longitude below.

Technically, a nautical mile is an average the brain trust of the day agreed on because of the irregularity of the globe; it is a little flatter at the poles. For all intents and purposes, we can say that a nautical mile is 1852 m or one minute of latitude.

Nautical miles can be abbreviated in several ways depending on what industry you are from. In sailing terms, we are happy with just “nm”.

2. So why 360°, minutes and seconds?

Sexagesimal or base 60 or sexagenary is a numeral system with sixty as its base.

360 is a nice number adopted since eyes were looking at the sky. It represents the 360 days in a year, which is as accurate as can be expected.

As a side note, the moon goes through one full cycle of phases in about 30 days. Three hundred sixty days divided by 30 days is 12. Thus, a year was split into 12 months.

But we digress. Our current decimal system is relatively new. Instead of a “base 10” system, the Babylonians (influenced by the Egyptians) gave us a base 60 or sexagesimal system because 60 was seen as a superior number because it has a large number of factors. I.e., you can divide it by a lot of other numbers.

So now we have 360 degrees. Each degree could be subdivided into 60 minutes, and those 60 minutes could be further subdivided into 60 seconds, etc.

This method, in essence, is what we are still using today, with the only exception that on most charts, subdivisions of seconds are in decimal. For example, 20°46′23.43454″

Note the use of the notation:

• Degree = °
• Minute = ′
• Second = ″

However, for the purposes of plotting points on your chart, you would use degrees and minutes and convert any seconds to a decimal. For example,

Convert 40° 19′ 30″ N
30″ / 60 = 0.5′
Latitude: 40° 19.5′ N

Making a grid…

We all have heard about a “grid reference.” To know where we are on a map or a chart, we need a grid system that can pinpoint our location or the location of things, so we see the relationship between where we are and where we need to go. Having criss-cross lines on a map or chart will do that job nicely if we can understand what these lines are called and the relative distance between them.

Our globe is round(ish) and not a flat map, so we need to adjust it. How we do this is another story for another time…

So, let’s put some names to these grid lines now…

3. What is meant by latitude?

Think of latitudes as “lines” that run east to west across the globe but tell you where you are on a north-south line called a longitude.

We also refer to latitude lines as parallels. In total, there are 180 degrees of latitude.

0° represents the equator, 90° represents the North Pole, and -90° represents the South Pole.

We can also look at the 5 major parallels, which are from north to south: the Arctic Circle, the Tropic of Cancer, the Equator, the Tropic of Capricorn, and the Antarctic Circle.

Latitude has a bit of a “ladder” in it (with a bit of imagination), which may remind you that we measure latitude vertically.

In a grid reference, we usually state latitude first. For example:
34° 54.5′ S 143° 35.8′ E

4. What is meant by longitude?

Conversely, think of longitude as “lines” that run north to south across the globe but tell you where you are on an east-to-west line called a latitude. Lines of longitude are also called meridians.

There are 360 degrees of longitude, the centre of which is known as the Prime Meridian. It passes through the Royal Observatory, Greenwich, England. It divides our globe into the Eastern Hemisphere (-180 degrees of longitude west) and the Western Hemisphere (180 degrees of longitude east). 

Longitude is commonly stated last, for example.

34° 54.5′ S 143°  35.8′ E

5. Why do we use knots?

A knot is an old maritime measure of speed. If you cover one nautical mile in one hour you will be travelling at a speed of one knot.

The term knot dates from the 1600s, when mariners measured the speed of their ships by deploying a tool called the “common log.” This tool was a coil of line with uniformly spaced knots attached to a piece of triangle-shaped wood. The wood was lowered into the water behind the ship and allowed to float away behind it. The line was allowed to extend from the coil as the distance between it and the ship was timed. When the specified time had passed, the line was pulled in, and the number of knots on the line between the boat and the log was counted. The speed of the ship was determined by the number of knots counted.

So there—clear as mud. Hopefully, you now know a little more about why we still use what looks at first glance like an old and antiquated system. Next time you plot a course, you may think back and thank the old guard for inventing such a great system.

If you want a better understanding, I recommend the movie Longitude with Stephen Fry as Sir Kenelm Digby and directed by Charles Sturridge. It is a great historical movie that explains the challenge of determining longitude within the British Navy.

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