A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: David Pike
Date: 2015 Mar 28, 17:22 -0700
In some books you read determ the bearing off a celestial object.
How can you do that reliable?
Is there a special compass or so for this purpose?
With a normal magnetic bearing compass you can look only a few degrees upwards
before the compassrose stalls.
A compass with pelorus may works perhaps, but who has that, with a free sight
around a yacht?
So who has the solution for this?
Did you mean determine your heading using a celestial object or determine the bearing of a celestial object? The only difference is whether you lock your rotating device to the azimuth of the body or not.
With a periscopic sextant it’s easy. To get your true heading, lock the sextant to the calculated azimuth of the body on the azimuth ring. Set Hc on the sextant and swing the sextant to your approximate true heading. Identify the body and place it in the centre of the sextant’s aiming tramlines. At the appropriate time, flick the locking switch to its heading locking position (up = head = heading) and read off your true heading from the heading/azimuth ring. To get the bearing of a body do the opposite. Start off locked to true heading and at the appropriate time lock to the azimuth position. In the RAF, most people were happy with one degree accuracy. At least one Victor squadron fitted a Vernier scale to their sextant mountings and claimed to be able to complete airborne compass swings to 0.1 of a degree, but I rather took that with a pinch of salt. I’m not sure they could line up their mountings with the aircraft’s fore and aft datum that accurately or if their aircraft flew that straight.
With an astro compass you do more or less the same thing.
I understand it’s also possible to something similar with a proper marine compass binnacle and pelorus, but I’ve never had the chance to try it.
If you can wait until sunrise or sunset, you can calculate the sun’s true bearing using amplitude tables from a set of nautical tables such as Norries, and use that to check your compass. With a good horizon, cleverer men than me could no doubt do the same for the moon, the planets, or the navigational stars.
For less accurate heading checks there’s also Polaris. I once sailed roughly NNW up the east coast of England keeping the starboard shroud in line with Polaris.
If all else fails, there’s the Boy Scout trick using a wrist watch.
For a serious land navigator, I suppose you could glue one of those circular spirit levels to the top of your walking stick and take the bearing of the shadow cast by your walking stick placed vertically in the ground. Dave