Tables for finding True North with Pelorus

[Benz]

I seem to remember somewhere seeing a method for finding True North by sighting the sun with a pelorus, but can't find it now, either in my Pelorus manual or in Bowditch.
Anyone remember running across this, and where to find it?

[ChrisJHC]

This sounds like a variation on a morning and evening sun sight.
Taken when the sun is 1.5 diameters above the horizon.
At the same time you read the bearing on the magnetic compass.

Work out the “true” bearing of the sun at sunrise and compare with the true bearing from the compass (after allowing for magnetic variation).

You could then just add or subtract as required to work out where True North is.

Of course, if you’re on land, just put a stick in the sand and mark the progression of the sun’s shadow (particularly around noon). From that you can easily work out true north and other cardinal points.

[GordMay]

An ordinary analogue watch can also be used to determine the approximate true North.

In the North Temperate Zone [latitude 23.° to 66.5° N], the hour hand is pointed toward the sun.
A north-south line can be found midway between the hour hand, and 12 o’clock. This line will point south*.
This applies to standard time.
For daylight saving time, the north-south line is found midway between the hour hand, and 1 o’clock.

* If there is any doubt as to which end of the line is north, remember that the sun is in the eastern part of the sky before noon, and in the western part in the afternoon.



Imagine a line traveling through the 12 O'clock (green line) and another line pointing towards the sun (red line). Divide the angle that is made in half, and this point will indicate Due South (yellow line).



In the South Temperate Zone, 12 o’clock is pointed toward the sun, and halfway between 12 o’clock, and the hour hand ,will be a north-south line. If on daylight saving time, the north-south line lies midway between the hour hand, and 1 o’clock.

Using a watch, to determine direction within the tropics is more complicated, since there are two solar maxima. The sun may lie to the north or south [or at midday be directly overhead], depending upon the time of the year.

[nigel1]

I use Nories Tables, which include Azimuth Tables (ABC) and Amplitude Tables, with these you can determine the true bearing of celestial objects.

[nigel1]

Or, by using a calculator
Based on the ABC Tables
C = A +/~B ( + if Latitude and Declination are same name, difference if opposite names)


A = Cotangent LHA x Tangent Latitude
B = Cosecant LHA x Tangent Delination


Cotangent Azimuth = C x Cosine Lat


Combining all the above


Cot Azimuth = ((Cot LHA x Tan Lat) +/~ (Cosec LHA x Tan Lat)) x Cos Lat


Amplitudes are much simpler
Sin Amplitude = Sin Declination x Secant Latitude

[Benz]

Originally Posted by GordMay

An ordinary analogue watch can also be used to determine the approximate true North.

In the North Temperate Zone [latitude 23.° to 66.5° N], the hour hand is pointed toward the sun.
A north-south line can be found midway between the hour hand, and 12 o’clock. This line will point south*.
This applies to standard time.
For daylight saving time, the north-south line is found midway between the hour hand, and 1 o’clock.

* If there is any doubt as to which end of the line is north, remember that the sun is in the eastern part of the sky before noon, and in the western part in the afternoon.



Imagine a line traveling through the 12 O'clock (green line) and another line pointing towards the sun (red line). Divide the angle that is made in half, and this point will indicate Due South (yellow line).



In the South Temperate Zone, 12 o’clock is pointed toward the sun, and halfway between 12 o’clock, and the hour hand ,will be a north-south line. If on daylight saving time, the north-south line lies midway between the hour hand, and 1 o’clock.

Using a watch, to determine direction within the tropics is more complicated, since there are two solar maxima. The sun may lie to the north or south [or at midday be directly overhead], depending upon the time of the year.

Thanks Gord, I have run across this one before. Perhaps this is what I was remembering, and interpolating a pelorus.

[Benz]

Originally Posted by nigel1

I use Nories Tables, which include Azimuth Tables (ABC) and Amplitude Tables, with these you can determine the true bearing of celestial objects.

Thank you. I will try to secure Norie's tables.

[requiem]

The watch or "shadow stick" methods can include significant error, unfortunately. Here's a post discussing the problem, which includes this image showing the error over the course of a year:



(shadow stick in red, watch in blue)

[StuM]

Originally Posted by requiem

The watch or "shadow stick" methods can include significant error, unfortunately. Here's a post discussing the problem, which includes this image showing the error over the course of a year:



(shadow stick in red, watch in blue)

To say nothing of the error inherent in the difference between your longitude and the mid-point of your time zone.

In PNG Solar noon is up to 45 minutes away from 12:00, depending on location.

[ChrisJHC]

Originally Posted by requiem

The watch or "shadow stick" methods can include significant error, unfortunately. Here's a post discussing the problem, which includes this image showing the error over the course of a year:



(shadow stick in red, watch in blue)

This shows the “shadow stick” method is actually pretty accurate around local noon.
Certainly accurate enough if you’re in a situation where you’re relying on a stick in the sand to find civilisation.

[Benz]

I'm looking for something a little more accurate than a shadow stick, since I'll need to steer a ship in waters where a magnetic compass is unreliable. If I can know the azimuth of the sun in relation to my estimated position, I can then head the ship where I need to go.
This may be all academic, since failure of all onboard GPS units at once is unlikely, but I'd like to have my strategy ready in advance.

[GordMay]

Here’s an article that discusses “Celestial Navigation in High Latitudes” [for aircraft]
➥ https://www.flightliteracy.com/celes...igh-latitudes/

Bowditch: CHAPTER 33 - POLAR NAVIGATION
➥ https://thenauticalalmanac.com/2017_...Navigation.pdf

[boat driver]

Originally Posted by Benz

I'm looking for something a little more accurate than a shadow stick, since I'll need to steer a ship in waters where a magnetic compass is unreliable. If I can know the azimuth of the sun in relation to my estimated position, I can then head the ship where I need to go.
This may be all academic, since failure of all onboard GPS units at once is unlikely, but I'd like to have my strategy ready in advance.

cruising in the high latitudes we checked the compass during the sunset to confirm the variation tables, and terrestrial navigation checks. We also carried an aircraft gyro but it required as many checks as well to be sure it was working.
Today the ability to confirm variation via GPS (keeping log for reference) would suffice if not using as gyrocompass. More paramount importance is understanding the limits of all equipment and discipline to cross check when arriving into terrestrial areas/
lol, in the north many of the charts were derived only by compass.

[johnn33]

Dear Benz - I expect you have come across
Emergency Navigation David Burch

International Marine McGraw Hill ISBN 978-0-07-148184-7

second edition 2008 paperback
Fascinating.

I have used finding and keeping North using hands on a watch face

John

[Lodesman]

Originally Posted by Benz

I seem to remember somewhere seeing a method for finding True North by sighting the sun with a pelorus, but can't find it now, either in my Pelorus manual or in Bowditch.
Anyone remember running across this, and where to find it?

The nautical almanac has azimuth tables for the sun - and it describes somewhere in the book how to use it to check your compass for error.