Comments on: The knotty truth

By: David Consbruck, CFII

David Consbruck, CFII — Sat, 02 Jul 2011 15:58:57 +0000

Yes, the nautical mile is 1 minuted of latitude. That is so easy to remember. It also why I use the vertical scale markers on a Sectional Chart for measuring distance, even if I am plotting a course that is not exactly N-S.

The horizontal scale is skewed based on location and is therefore almost never completely accurate. One will notice it varies in length from the vertical, in an attempt to show the distances more accurately. Does all this matter? Not really that much, but it sure is nice to know.

Thanks to Chip, Tom, and Thomas for sharing their knowledge. Understanding is an important factor in developing that key ingredient in being a good pilot, “judgement”.

By: Buster Frysinger

Buster Frysinger — Sat, 02 Jul 2011 01:04:28 +0000

My students faces always light up when they realize the relationship between knots, nautical miles, and minutes of latitude when dealing with the sectional chart. There is almost a sense of pride instead of dreading learning a new unit of measurement!

Thank you for the great piece and comments!

By: Thomas Boyle

Thomas Boyle — Thu, 30 Jun 2011 10:46:43 +0000

Chip,

You’ve explained where the term “knots” came from, but not where the unit itself came from, nor why it’s preferred in navigation to either the mph or the kph.

The reason, of course, is that the north-south distance covered by 1 minute of latitude is a nautical mile (and the knot is 1 nautical mile per hour). Minutes of longitude vary depending on how far one is from the poles, and are close to a nautical mile somewhere not far from the equator, but minutes of latitude are approximately the same distance everywhere. Thus, the nautical mile is a “relevant” distance for the navigator.

Interestingly, the meter was originally defined as 1/10,000,000 of the average distance from the pole to the equator. Since that is also 90 x 60 = 5,400 nm, there would be 1,852 meters in a nautical mile – and that is the modern definition of a nautical mile.

By: Tom

Tom — Wed, 29 Jun 2011 17:59:14 +0000

However, you miss the main reason it is a standard- a minute of lattitude is one nautical mile, and it is a universal standard in part due to the fact its length is tied to the size of the earth. The history gives it its name; its current length is more defined by the earth, and who is to say that the orignal unit may have been so as well, lost in time? Why else would someone have chosen knots so spaced on a rope, except it was easy to link the resultant speed with distance on charts?