Where the 112.5 degrees comes from...

Daydream believer said:
And there was me thinking that the helmsman used to stand naked with his legs apart. The ship would head North or south. When the swell changed,, indicating a different latitude , he noticed the swing change in the nether regions . At that point he would turn east or west.
Click to expand...
Are you sure that you're not confusing that with the original sundial?
 
lw395 said:
A fingerwidth is about 2% of an arm's length.
So a 10m boat being 3 fingers, 10m is 6% of the range so it's about 160m away.
I don't think degrees bring much to this particular party.

It's funny, we all use 1 in 4 or 25% for a steep hill, few people will talk about a 10degree hill (unless they mean a cold one...).
Click to expand...
We are rather keen on gradients of climb and descent in my business. The rule book is full of them :-)
 
I think that the division in 360 degrees comes from ancient astronomers. One degree is the angular way made in one day by the sun on the background of the fixed stars (though not exactly).

Sandro
 
someone said it was internationally agreed for ships and aircraft - nope, aircraft are 110 deg

and there's the rub - its all theoretical claptrap because in real life the cut-offs arent that precise and its the relationship of the lights you see that matter not the precise angle.
 
and there's the rub - its all theoretical claptrap because in real life the cut-offs arent that precise and its the relationship of the lights you see that matter not the precise angle.
Click to expand...

Agreed but its why ships have their side lights set into boxes with not only the light casing itself set to the angle - but a board at front of the box that makes double sure light does not 'bleed' into change-over ... there's also a rear board to ensure rear cut-off.
Yachts of course don't have this and rely on the light case itself.

I love who ever designed pulpits with light plates ... I have a pulpit with plates to fit side lights to ... one each side. Problem is the designer forgot to have the plates aligned with the fore-aft line of the boat and so the lights need wedge inserts to correct them. Otherwise they angle inwards based on the pulpit line. 3D printer provided them as nothing could be found on shop shelf.
 
"..Agreed but its why ships have their side lights set into boxes with not only the light casing itself set to the angle - but a board at front of the box that makes double sure light does not 'bleed' into change-over ... there's also a rear board to ensure rear cut-off.
Yachts of course don't have this and rely on the light case itself..."

Mine does..although the new LED lights are tiny compared to the old paraffin ones,IMG_3084.JPG and I rely on the light fitting itself for the aft cut-off.
The spaces previously taken by paraffin lamps are now full of 'shrooms..
 
PuffTheMagicDragon said:
Talking about fingers and angles.
Arab sailors navigating across the Indian Ocean had no octants or sextants. They had a 'Kamal', which consisted of a rectangular wooden plate, roughly the size of a cigarette packet. A length of string was fastened to it and had knots tied on it towards the end. A knot was held between their teeth and the Kamal was held vertically such that the string was stretched. Holding the bottom edge of the Kamal on the horizon, the height of Polaris was estimated in finger widths. The 'pilot book' would say, for example, to go to Calcutta "Go South until Polaris at two fingers and then turn East for five days when you will meet a change of current direction..."

Kamal_Polaris.png
Click to expand...
Probably would end up on the rocks because that is not Polaris!Ursa Major.jpg
 
I wonder how many navigation lights are fitted with such precision as to be accurate relative to the axis of the boat within a fraction of a degree ?
The split of the 360 degrees of a circle into 32 makes sense if you think of the imperial measurements of length where 1/32nd of an inch isn't such an unnatural division.
 
I wonder how many navigation lights are fitted with such precision as to be accurate relative to the axis of the boat within a fraction of a degree ?
Click to expand...

Quite !

Reminds me of an old RAF joke ...

Instructor and student up in a training biplane ...

Instructor tells student :

Ins " 1 degree port lad...."

Student " We can't steer that accurate in this heap ... "

Ins " OK then 5 degrees port, 4 degrees stbd ... smartar** "
 
MartynG said:
The split of the 360 degrees of a circle into 32 makes sense if you think of the imperial measurements of length where 1/32nd of an inch isn't such an unnatural division.
Click to expand...

It may make sense, but I doubt that one is much to do with the other. It ultimately derives, surely, from their being four cardinal points, to which there's evidently reference in the Old Testament. (Although five cardinal points were favoured in ancient China...) The 32 point concept seems to have arisen in the Mediterranean in the Middle Ages. Such multiples didn't figure in Classical Greek/Roman measurements and the Middle Eastern ones from which they derived (they liked 28, amongst others ?). Ancient Greece and Rome used a 12-point wind rose.
 
No. Alkaid is in Ursa Major. You are getting your Ursas in a twist! Puff the magic dragon's original illustration showed Polaris in Ursa Minor, different shaped "handle"
Here hoping you don'tend up on the rocks : )
 
You must log in or register to reply here.

Share this page

Top