Why knots rather than kilometres per hour (KPH) or miles per hour (MPH)?

Posted by _Starter@reddit | sailing | View on Reddit | 7 comments

[-]

kentschele@reddit

The name comes from the logging rope but the background is way more scientific. A knot is a nautical mile per hour. One nautical mile is the length of a minute on a great circle (largest circumference of the earth). 60 minutes make a degree. Distances on earth in nautical miles (assuming earth is a sphere which it is really not, my nav teacher always used to say it’s more like a potato) can be used in the math for celestial navigation without conversion.

Edit: Inland barges on the European water ways do use km/h

[-]

Level_Improvement532@reddit

Oblate spheroid. A Coast Guard question in Nav1. 4 license exams later and I still don’t forget it 😉

[-]

reflUX_cAtalyst@reddit

And for anyone who can't picture what an oblate spheroid is, it's what you get if you sit on a basketball.

[-]

foolbox@reddit

Earth’s diameter at the equator is ~40km longer than the diameter at the poles— hence the oblateness— but this is only a 0.3% difference. Even Mt. Everest is only 0.07% the diameter of the Earth. The earth is pretty darn spherical.

[-]

Zephyrs_rmg@reddit

I remember Tyson saying that if you shrunk earth down to the size of a billiards ball it would be smoother than any billiards ball ever made.

[-]

nayuki@reddit

A billiard ball is about 60 mm in diameter. The Earth is about 12700 km in diameter. That's a ratio of 1 : 210 million.

If you assume that the difference from the highest mountain to the lowest ocean trench is 20 km, then if you scale it down to the billiard ball, then that would be 95 μm. From some Google searching though, the roughness of a billiard ball is something in the order of 5 μm, so this proposition is false.

Also, there are other threads on this topic: https://www.reddit.com/r/theydidthemath/comments/ejhomq/self_is_the_earth_really_smoother_and_rounder/

[-]

beatmalls3@reddit

Loved reading this.