Where the 112.5 degrees comes from...

Kukri

Kukri

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Spin off from the jargon thread...

As we all know , sidelights are intended to be visible over an arc of 112.5 degrees or ten compass points, and there are 32 points on the old style compass card. So one point is 11.25 degrees. Why?

Well, I remember from a French sailing textbook that if you hold your arm out straight and make a fist with your hand the angle between your thumb and the other side of your hand is 11.25 degrees.

And the French for “a fist” is “un poing”.
 
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I'm clearly not in the Trump league, but my horizontal fist and thumb conceal 18 degrees of the village hall across the green. The tips of four fingers make about 11.25 though, so that's something to try and remember. (it's now too dark to try and find some combination that makes 10. Although just by chance I checked yesterday and my forefinger is about one degree.

Big hands or short arms?
 
Kukri said:
Spin off from the jargon thread...

As we all know , sidelights are intended to be visible over an arc of 112.5 degrees or ten compass points, and there are 32 points on the old style compass card. So one point is 11.25 degrees. Why?

Well, I remember from a French sailing textbook that if you hold your arm out straight and make a fist with your hand the angle between your thumb and the other side of your hand is 11.25 degrees.

And the French for “a fist” is “un poing”.
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That particular fist is 1.25 degrees greater than the one used in a 1940s army field craft book I had as a child. I no longer have the book but I seem to recall it said that, a spread hand was 20 degrees, fist 10, closed hand 5(?) and a single finger (index) 1.

Do the French have a measurement for 2 spread fingers? (Un Boris aux un Nigel Peut être ;) - sorry, I'll get me coat ).
 
Angular measurement is actually to do with time. If you check out Lat - Long and Time - you will see they all inter-relate.

The Points of the Compass and the saying 3 points on the bow etc. are to do with the strange degrees for lights.
It goes further in fact ... where does the term Port 1/4 for example come from .... its to do with points again ... as the 1/4 is centred on 4 points abaft the beam ... 4 >> 1/4

Anyway - that's Seamanship Classes told us when I was Cadet !
 
JohnGC said:
That particular fist is 1.25 degrees greater than the one used in a 1940s army field craft book I had as a child. I no longer have the book but I seem to recall it said that, a spread hand was 20 degrees, fist 10, closed hand 5(?) and a single finger (index) 1.

Do the French have a measurement for 2 spread fingers? (Un Boris aux un Nigel Peut être ;) - sorry, I'll get me coat ).
Click to expand...

The seaman’s closed fist when measuring an angle has the thumb outside and crooked, which may add the missing 1.25 degrees. It was certainly used in British ships but the dérivation of the word clearly comes from the French.

conversely I recall the Breton yachtsmen whom I knew as a boy always measured their boats in « pieds” and their depths in “brasses”.
 
It all arises from repeated halving of the intervals between the cardinal points on a compass. Nothing to do with the size of a fist, though that is a handy way of estimating angles.

And we have 360 degrees in a circle because the Ancient Babylonians used a number system with a base of 60. Base 60 gave you a lot of "easy" relationships between numbers; 60 has a LOT of common factors.
 
When we're racing and want to describe where the mark is, we will often say things like it's a thumb or a hand to the left or right of some obvious landmark.
I can't remember who introduced me to that.
But it's one of those tihings that didn't come from a book.
 
Kukri said:
Well, I remember from a French sailing textbook that if you hold your arm out straight and make a fist with your hand the angle between your thumb and the other side of your hand is 11.25 degrees.
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No sign of that etymology in the OED,

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and you'll see from the Herbert quote (1694) that eight points to the compass was also possible.

pcVpse9.png
 
When I have to go visit odd ships ... I have Supers I employ to normally go ... but occasionally I have to ... I find it sad that many of the old sayings / terms are lost ...

In my day - we always used points when describing the direction of anchor chain when anchoring ... via the radio to bridge. Lookout on bridgewing would report sightings using points.
Now you hear it in degrees.... or the worst is in terms of 2 o clock ... 10 o clock etc.
 
The 112.5 degrees is half of 45 degrees plus 90 degrees, i.e. just aft of directly abeam. It's an arbitrarily selected but internationally agreed viewing angle for the coloured arc cut-off for navigation lights on ships and aircraft.
 
Daydream believer said:
If you draw a circle with a compass then subscribe the radius around the circumfrence you will get 8 equal spaces of 45 degrees. if you then divide this by 2 ( use the compasses to form 2 arcs from the 2 points on the circumfrence ) it is 22.5 & again by 2 it is 11.25. these divisions can easily be formed by a simple set of dividers or compasses & does not need a complicated tool.
So presumably that was the reason for the subdivisions
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I get 6 segments with my compass. Maybe it's metric.
 
Refueler said:
When I have to go visit odd ships ... I have Supers I employ to normally go ... but occasionally I have to ... I find it sad that many of the old sayings / terms are lost ...

In my day - we always used points when describing the direction of anchor chain when anchoring ... via the radio to bridge. Lookout on bridgewing would report sightings using points.
Now you hear it in degrees.... or the worst is in terms of 2 o clock ... 10 o clock etc.
[/ I quite liked the clock language, but recently I had a teenager crew who had only ever used digital clocks!
Click to expand...
 
Kukri said:
I still think I am onto something. For at least a thousand years, illiterate seamen had to describe the bearings of things sighted by reference to the heading of their vessel, and they certainly didn’t have hand bearing compasses.
Click to expand...
They must have started by using phrases like, 'fine on the port bow', ' dead astern' which are still in use, by me anyway..
 
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