Today I know we have the technology to position anything accurately within a few feet. But how would sailors in old times measure distance and have maps of the sea? Why is there a nautical mile and different from the land mile?
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First, a “land mile” is the amount of distance traveled regardless of escalation. This doesn’t work at sea due to waves, the ship is moving up and down all the time, so they had to come up with a way of measuring only forward distance. At sea in harsh conditions, you might be traveling at a speed of 10 MPH, but making no forward progress at all due to the waves.
Back in the day, sailors had a couple of different ways to determine things like that depending on where they were. In the Mediterranean, for example, they would often just stay in sight of the coastline. Many of their coastal buildings were built in such a way as to align with the rising and setting of stars and equinoxes, so you could roughly tell where you were and which direction you needed to sail away from if you were headed to Egypt or something (because that was more than a day’s voyage, so you had to sail at night across open water with no landmarks).
You’ve got your celestial navigation dating back to at least Ancient Greece (as noted in Odyssey), where they’d keep a certain star at a certain angle away from the ship using a sextant to measure the angle, which could keep them moving along a straight latitudinal line.
Later on we developed the astrolabe, magnetic compasses, kamals, clear sunstone, and I’m sure I’m missing some others. All these assisted navigation in various fashions when just looking at stars wasn’t an option.
And of course, all that time we had cartographers on ships and on land, drawing everything out to a remarkably accurate degree. Making accurate maps was a profitable endeavor due to the increasing need for maritime travel.
This was a massive problem for ancient mariners – with no landmarks you were totally lost the moment you lost sight of shore.
Eventually two breakthroughs enabled open sea travel.
The first was the discovery of magnetic stones, and the concurrent discovery that the Earth itself was magnetic. This lets you build a compass and find north no matter where you are.
The second is the development of celestial navigation. There’s no *land*marks at sea, but on a clear night the sky fills with hundreds of them. If you know what day it is and approximately how big the Earth is, you can chart what stars should be visible from what points on the globe and in what direction.
Following the sun, moon, and stars with the aid of star charts and a compass was the complex but accurate method pre-industrial sailors used to reach their approximate destination across the open ocean.
Once accurate maritime clocks were developed the system became extremely accurate. If you know exactly what minute it is and know exactly when stars *should* rise and set at a certain location you can determine how close you are with high accuracy.
Of course, this all requires clear skies. Many ships lost course in stormy or overcast conditions and ended up way off target.
The simplest way to estimate distance run is to measure your speed, and multiply by the amount of time spent at that speed. This is called dead reckoning, and is still an important part of navigation, it’s the only way submarines can navigate while submerged for example.
The old way to measure speed was to have a board tied to a rope. You would drop this board over the back of the boat, and let the rope pay out. The rope had knots in it, so by counting the knots that passed through your hands in a fixed time, you found the speed. Hence the unit for speed at sea is the knot (1 Nautical Mile per hour).
A nautical mile is not an arbitrary length, it is the distance on the surface of the Earth of 1/60th of a degree of Latitude. So if a boat sails North 1 degree it will have traveled 60 NM. 1/60th of a degree is called a minute, and charts are marked in degrees, minutes and decimals of minutes. It is easy to measure distance in NM from this scale using a ruler or pair of dividers (the classic navigation tool).
The other thing to remember is that distance is often not terribly important in sailing, so long as you are going in the right direction (using a compass), you will eventually reach your destination. Of course over really long distances the errors start to add up, which is why celestial navigation and then GPS were invented.
Knots!
[https://oceanservice.noaa.gov/facts/nauticalmile_knot.html](https://oceanservice.noaa.gov/facts/nauticalmile_knot.html)
*Knot is one nautical mile per hour (1 knot = 1.15 miles per hour). The term knot dates from the 17th century, when sailors measured the speed of their ship by using a device called a “common log.” This device was a coil of rope with uniformly spaced knots, attached to a piece of wood shaped like a slice of pie. The piece of wood was lowered from the back of the ship and allowed to float behind it. The line was allowed to pay out freely from the coil as the piece of wood fell behind the ship for a specific amount of time. When the specified time had passed, the line was pulled in and the number of knots on the rope between the ship and the wood were counted. The speed of the ship was said to be the number of knots counted.*
The stars generally. You can approximate your north/south position without any tools, and most ships stayed within sight of land even when making long journeys. IE before the development of the sextant and accurate time keeping devices to plot longitude, most ships were never on open water for extended periods of time.
The reason you need accurate clocks for the plotting of east/west longitude is because you have to compare the position of the sun or stars at local time, to the time at your home port (such as GMT) and in order to keep track of the time at your home port you needed an accurate time keeping device. The problem was that early on, most accurate clocks used pendulums which did not work well with ships that bobbed and moved around. A grandfather clock only works when it’s on flat, stable ground, not on a ship that can tilt in any direction.
Another measurement that was important for determining distance was speed as well as time. If you know how fast you’re going and how long you’ve been traveling then you can approximate the distance you traveled. This wasn’t precise because both wind and water could move, so while the ship might have moved 100 miles through the water, it might not have moved 100 miles compared to a fixed point on land.
Speed was measured in several ways but a common one was to use knots tied in a length of rope at regular intervals. The rope was tossed overboard and allowed to play out it’s length and how many knots it played out in a given amount of time gave the boat it’s speed through the water. We still give speed in water in knots.
A piece of wood attached to a long rope which had knots at set distances. The rope had for four attachment points; two at the top were fixed, bottom were fixed with pegs. The board was thrown over the side and the board acted as a anchor. As the ship moved forward and the rope played out it was timed. The number of knots counted. How many knots passed determined the speed – 6 knots. When the time was expired the rope was tugged to release the pegs and the board hauled in. Which is why speed at sea is measured in knots.
Hey, I can chime in on this one. A few years back I read a relatively short, very interesting book called “Longitude: The True Story of a Lone Genius Who Solved the Greatest Scientific Problem of His Time”
With several navigation techniques sailors had been able to keep track of which latitude they were currently at, but longitude was a big issue. Before a working device to track longitude was developed, sailors destined for across the Atlantic would “sail the latitude” and basically just point themselves at the new world and keep going until they hit it, with only estimates about how much time it usually took to cross. They could estimate very roughly where they were based on how long they’d been sailing, and what conditions had been like on their voyage, but until an accurate device for measuring longitude came about they were literally “at sea” until they hit land, which is where the term comes from.
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Others had commented on astrological/celestial navigation but the real advances in exploration happened after John Harrison’s invention of the “marine chronometer”.
The Marine Chronometer accurately kept time at sea which was important since you can calculate your longitudinal (east-west) position based on the time of the clock and high noon.
Because the earth is 360 degrees and we have 24 hours a day, that means every hour of travel is 15 degrees of longitude. By measuring the gap in time between high noon in your current sea position and the clock you’ve sent to a “standard time” (Greenwich Mean Time), you can calculate how far east/west you’ve traveled.
For example, if high noon on your ship occurs at 1 PM on your clock, you know that you’ve essentially traveled 15 degrees since the difference between high noon and your clock is one hour.
Measuring speed through water is easy. You take a rope, tie knots into it at a fixed distance from each other, tie it to a bucket. Now, you can toss the bucket into the water, flip a small hourglass, and count how many knots get dragged out before the hourglass runs through. That’s why nautical miles per hour are called knots.
There are many different miles, derived and standardized in different ways. Nautical miles are standardized as an angular minute (1/60th of a degree) along the earth surface, because that makes calculations/navigation more convenient.
Now, I mentioned speed *through water*. The problem is, the water could also be moving. If you’re moving at 5 knots (through water) towards north, but the water is flowing south at 1 knot, you’re actually moving at only 4 knots relative to ground. The way to correct that is to get a *fix* – determine your position.
You can do that for example by sighting two landmarks, measuring the angle at which they are seen (if you have a sufficiently accurate compass), drawing those lines on a map, and seeing where they intersect. For example, if you see a tower straight north, you can draw a line south from the tower on a map, and you’re somewhere along the line. Draw a second line from a tower that’s to the west, and the lines will intersect and that’s where you are.
Far away from land, you can also use the stars. But what if it’s overcast? You’re kind of screwed. You have to measure your speed through water, guess the current, calculate your guessed speed over ground, and then from that and your last known/assumed position, calculate where you’re now. Say you think you’re moving north at 4 knots, then every hour (more often in practice), you’d go to the map, measure 4 miles from your last position, and mark your new assumed position. This is called dead reckoning, and is about as accurate as it sounds, especially as every mistake adds up over time.
You better be accurate… because you can’t see underwater obstacles, and if you miss an island, you could get lost until you can get good enough weather to get a fix again.
Ships sinking is bad for business, so governments invested ridiculous amounts into getting accurate maps of the coastal waters. On the ocean, it’s a bit less of a problem, but it’s good to know where you can find an island to get some fresh water, food, and much needed rest, so those were also mapped (based on whatever position information was available).
Obviously, this all royally sucks. Thus, humanity applied its collective smarts, shot atomic clocks into space where they fly around at a speed of around 14000 km/h, broadcasting the time. Your phone can receive the signal, and from the time in the signal can deduce how long the signal took to get to you, and thus how far you were from the satellite when it broadcast the time. Combine this with the knowledge of where exactly the satellite was at that time (which is a separate problem in itself), the time and position from 3 other satellites, and some relativistic math, and you have your exact position, whether the sky is clear or not, and don’t have to rely on dead reckoning anymore. That’s how GPS and similar systems (GLONASS, Galileo, BeiDou) work. (Before satellites, there was a ground radio based system called LORAN, because it’s a *really* annoying problem.)
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