Calculating longitude and the equation of time - intro videos

[SimonFa]

I know there's lots of serious sailors on here and this will be sucking eggs. I knew the theory of longitude but I still found these two videos fascinating, especially the second about the equation of time, which I didn't know.

Hope others find them just as useful.

https://www.youtube.com/watch?v=b7yoXhbOQ3Y

https://www.youtube.com/watch?v=rRHhO7Wj4Ik

 

[capnsensible]

Excellent! :encouragement:

 

[zoidberg]

Interesting and lucid vids, thanks.

The coordinate system we, the narrator, and the Ancient Greeks use for our mapping grids - Latitude/Longitude - is, of course, a 'Cartesian co-ordinate system'

It's not the only form of co-ordinate system available to us. Another important one for our use is called the 'Polar co-ordinate system', which is essentially range and bearing from a chosen point of significance e.g 25 miles from Start Point 160 degrees. Early maritime charts, or 'rutters', showed a number of such chosen points, with various significant paths, or 'routes', radiating from them. Many of these also show the distance to the destination.

These chosen points, or Poles, were often located at positions important to a sea voyage, such as just off the SW corner of Portugal, and off the NW corner of France, by Ouessant. Early European and North African mariners would sail/row to one such point or Pole, then steer towards the next, then the next, making allowances for being blown to one side or other of the intended course and adjusting expectations of arrival for speed changes.

That's very similar in principle to the techniques used by the Micronesian inter-island voyagers who colonised the whole of the Western Pacific, demonstrated again by young Micronesians and Hawaiians in recent decades. They use the concept of 'Home ( Pole ) Star' which, when overhead, is directly above their destination or their home island, and they follow radial directions from their 'Home Star', or the reciprocal, to sail hundreds of miles accurately between distant island groups..... using a 'Polar Co-ordinate' system.

It cannot be called 'primitive', for those navigators in their big double canoes were routinely making ocean voyages hundreds, likely thousands, of years before Vespucci, Columbus, and the others famed in our age of exploration as innovative navigators. Despite having no written language, no 'American Manual of Navigation', and no chronometers or backstaffs, the Micronesians committed the necessary knowledge of 'Star Paths' and judgement skills to memory during a long apprenticeship.

They also had maps of the island groups relevant to their passages, made not out of waterproof paper nor even from vellum, but from palm fronds and twigs, shells and feathers. Nevertheless, they were able to construct such with a remarkable accuracy.

It is humbling to consider that these people of the seas were trading, visiting, exploring, fishing all across the Western Pacific for hundreds of years before Europeans first managed to cross the Atlantic. They were truly 'ancient mariners'.

 

[SimonFa]

zoidberg,

Interesting, but we don't know how many failed to make a judgement of how accurate they were. I suppose that they kept doing it meant a reasonable success rate.

 

[johnalison]

This is one of their 'maps', seen at the 'Oceania' show last year.

 

[newtothis]

This is one of their 'maps', seen at the 'Oceania' show last year.

We're going to need a bigger chart table.

 

[AndrewB]

The book by David Lewis "We the Navigators" (1972) is a fascinating first-hand account of how the Polynesian navigators did this without instruments, from the last years before these skills were finally obsolete and lost.

I was struck by the fact that these traditional navigators had to serve something like a 25 year apprenticeship before being 'qualified'. These days we complain if we have to spend 10 minutes reading the manual before turning on the GPS!

 

[AntarcticPilot]

Interesting and lucid vids, thanks.

The coordinate system we, the narrator, and the Ancient Greeks use for our mapping grids - Latitude/Longitude - is, of course, a 'Cartesian co-ordinate system'

I'm afraid I have to disagree with this - latitude and longitude are not Cartesian coordinates, they are spherical coordinates and have important differences from Cartesian coordinates. The main ones are that 180E and 180W are in the same place and that latitudes are only meaningful within the range +90 to -90! Using latitude and longitude as if they were cartesian coordinates results in the Plate Carrée projection. I was on standards committees that had to consider this and other coordinate related problems, and was even the instigator of one such standard (ISO19111-2; completely irrelevant to yachting, but useful for meteorologists)

As you say, Polar coordinates are important for early navigation, and of course we all use them implicitly when we express our position as something like "I am a mile SW of Pye End Buoy". However, they rapidly become unwieldy over long distances - a few hundred miles (as you mention for Polynesian navigators) is probably as far as they are practical, and at those distances relies on a method of terminal navigation, such as bird flight or stationary clouds over islands. The Polynesians also used many other navigational inputs, such as swell patterns, which are fairly consistent in the Pacific over long distances. The Polynesian "maps" depict swell patterns in relation to the position of islands , as well as radial routes between islands.