Why Do Ships Measure Speed In Knots?

So, just a coincidence that the knot is a nautical mile per hour, equal to one minute of latitude? Or was the distance between knots (and therefore the nautical mile) adjusted after lat/lon had been invented? In other words, is a knot today the same as a knot in the 1500s?
 
DaleB said:
So, just a coincidence that the knot is a nautical mile per hour, equal to one minute of latitude? Or was the distance between knots (and therefore the nautical mile) adjusted after lat/lon had been invented? In other words, is a knot today the same as a knot in the 1500s?
Click to expand...
From Wiki:

Origin​

Until the mid-19th century, vessel speed at sea was measured using a chip log. This consisted of a wooden panel, attached by line to a reel, and weighted on one edge to float perpendicularly to the water surface and thus present substantial resistance to the water moving around it. The chip log was cast over the stern of the moving vessel and the line allowed to pay out.<a href="https://en.wikipedia.org/wiki/Knot_(unit)#cite_note-Abbott1858-8"><span>[</span>8<span>]</span></a> Knots tied at a distance of 47 feet 3 inches (14.4018 m) from each other, passed through a sailor's fingers, while another sailor used a 30-second sand-glass (28-second sand-glass is the currently accepted timing) to time the operation.<a href="https://en.wikipedia.org/wiki/Knot_(unit)#cite_note-9"><span>[</span>9<span>]</span></a> The knot count would be reported and used in the sailing master's dead reckoning and navigation.

This method gives a value for the knot of 20+1⁄4 inches per second or 1.85166 kilometres per hour. The difference from the modern definition is less than 0.02%.

Derivation of knots spacing:

1 kn=1852 m/h=0.5144 m/s, so in 28 seconds that is 14.40 metres per knot.

I would imagine that the chip log came along after the 1500s. Before that, the navigators would get their lat-long positions using astronomical observations, and the distance travelled in x hours would give x speed. Didn't we do the same sort of thing in our PPL nav training? We just had the whiz wheel (or E-6B) and a watch to make it easier. And a map, of course. We didn't have to do spherical trigonometry.
 
Last edited:
murphey said:
That's because until a clock was invented, there was no way to accurately measure latitude/longitude, altho the ancient Greeks did have some basic measurements based on distance.

en.wikipedia.org

History of longitude - Wikipedia

en.wikipedia.org en.wikipedia.org
Click to expand...
You need an accurate clock to measure longitude, but latitude doesn't require a clock (though a clock makes it easier or quicker).
 
Half Fast said:
Yet the article never explains that a nautical mile is a minute of latitude.
Click to expand...
I was kind of blown away when I learned that. Makes all other units of distance measurement/speed seem arbitrary and silly. At least on this planet.
 
Jim K said:
I was kind of blown away when I learned that. Makes all other units of distance measurement/speed seem arbitrary and silly. At least on this planet.
Click to expand...
The meter traces back to a physical property of the planet as well.
 
Jim K said:
I was kind of blown away when I learned that. Makes all other units of distance measurement/speed seem arbitrary and silly. At least on this planet.
Click to expand...
10,000,000 meters from the equator to the poles
 
And..... the earth though isn't a nice perfect sphere. So the distance between latitudes and longitudes is complicated.
xxxxxxxxxxxxxxxxxxxxxxxxxx
Distance measurement on Earth, which is an oblate spheroid (flattened at the poles and bulging at the equator), uses sophisticated mathematical models and formulas to account for its imperfect shape. The primary methods for accurate calculation involve the use of geodesics on an ellipsoid, with the most widely used being Vincenty's formulae or the even more robust Karney's method.
 
Dan Thomas said:
I would imagine that the chip log came along after the 1500s. Before that, the navigators would get their lat-long positions using astronomical observations, and the distance travelled in x hours would give x speed.
Click to expand...
Prior to the chronograph (mid-1700s) there wasn’t a practical way to find longitude aboard ship. The time component could be calculated based on the position of the moons around Venus, or something like that, but a time component is required to find longitude accurately. Latitude is relatively easy, and a chip log could give an estimate of distance traveled on a particular course so that a guesstimate of longitude could be made.
 
“Why do ships measure speed in knots?” Because the survey crew had a heck of a time keeping up while swimming with the calibrated chains… (rods, chains, furlongs: pecks, bushels, hogsheads, long tons… nobody knows how to properly measure things now days lol).
 
SlipKnot?

images
 
MauleSkinner said:
Prior to the chronograph (mid-1700s) there wasn’t a practical way to find longitude aboard ship. The time component could be calculated based on the position of the moons around Venus, or something like that, but a time component is required to find longitude accurately. Latitude is relatively easy, and a chip log could give an estimate of distance traveled on a particular course so that a guesstimate of longitude could be made.
Click to expand...
If you said "easy", it would be more correct. Lunar observations became practical at about the same time. The angle between the moon and something else in the sky also provides a time. See: https://www.starpath.com/resources2/brunner-lunars.pdf

The moons of Jupiter could also provide time, but weren't used at sea.
 
Half Fast said:
Yet the article never explains that a nautical mile is a minute of latitude.
Click to expand...
And at longitude at the equator in theory, except the earth is an oblate sphere representing an M&M.

1762494335593.png
 
Last edited:
DaleB said:
So, just a coincidence that the knot is a nautical mile per hour, equal to one minute of latitude?
Click to expand...
The nautical mile was defined as 1 minute of latitude in the begining, and then slightly adjusted. Coordinates started to be used for navigation in the 15th century, and the first logs started being used towards the end of that century. Initially they used a 5000-ft mile and no knots, before standardizing on using 1/60th of a degree of latitude as a definition for the nm and tying knots on the log line accordingly.
So the chicken to egg progression is coordinates -> nautical mile (1 minute of latitude) -> knot (nm/hr).
 
You must log in or register to reply here.
Back
Top