Astro nav - what is the accuracy of this method

[bbg]

I think you are missing the point of noon bracketing. It is simple. As I recall, I did all my workings without a chart. I just worked everything out on a scratch pad and that gave me Lat and Long, which I could then plot on a chart if I wanted to.

Edit. It was enough of a challenge taking sights from a 21 foot boat going 8 knots - I wanted to keep the working simple!

 

[snowleopard]

All astro work depends on crossing two or more position lines. The finer the angle between the lines the less the accuracy. Bracketing noon is more accurate as you approach the equator. In high latitudes it becomes less useful.

 

[AndrewB]

All astro work depends on crossing two or more position lines. The finer the angle between the lines the less the accuracy. Bracketing noon is more accurate as you approach the equator. In high latitudes it becomes less useful.

The method BBG describes (as I understand it) is really based on a single observation, taken at noon. Then the sun's maximum altitude provides latitude, while the exact moment this happens provides longitude, when compared with GMT. Unfortunately it is very difficult to tell the exact moment of noon to any accuracy with a sextant, because the sun's altitude is changing so slowly at that point. This is the reason for bracketing: estimating the exact time of noon as the average between two more easily measurable times.

It isn't intuitively obvious to me why bracketing should be more accurate at the equator than in high latitudes. The position lines of latitude and longitude always cross at 90 degrees.

Of course, we know the sun-run-sun method is problematic when the sun is near directly overhead at noon, which only happens within the tropics (i.e. around the equator), for the reason you give: position lines will have a fine angle.

 

[Sybarite]

Well, I am a theoretician rather than a practical navigator; others will far exceed my practical skills! However, it will entirely depend on how long the interval is, how fast you're moving and in which direction. At sailing boat speeds (at least, those most of us encounter ) I can't imagine it making much odds. Your method is, I think, the method of equal altitudes, which is a venerable and well-known technique to make the determination of "noon" more precise. You need to remember that you're not the only thing that is moving; the Earth has also moved on in it's orbit round the sun in the interval between your sights, and that could be a significant effect near the equinoxes when day-length is changing most rapidly. I've never heard of that being regarded as a problem for this method. I think you will find that the way you are doing it averages all the various effects so that you do indeed get the nominal position at the moment of noon. Of course, you probably could stymie it by doing a radical change of course and speed during the period of the sights, but as long as you're making a steady speed and constant course, I think that your method will average it out.

If they can do it....

http://www.creartisto.com/sansboussole/expedition_en.html

 

[estarzinger]

As an aside . . . Re the OP's method . . . . It is perhaps more accurate to throw a best fit curve thru all the points and then pick the maximum point in the curve. This also means you can just take random sights before and after noon and do not need "matching" ones. Best fit curves are easy on a laptop, and not so hard on most scientific calculators.

Regarding accuracy of the method . . . In my experience, from a 37' boat, ~5nm on a good day.

 

[Mariner69]

The method you describe is 'Equal Altitudes'; there is a good description in Admiralty Manual of Navigation Vol III Ed 31 DEC 1951.

I used to use the method when at sea with the Merchant Navy in the 1970' & 80's and found it 'interesting' in the Chinese meaning of the word. You would often find the second sight was clouded and difficult to be sure of the time. You need to be in low latitudes to have a sharp curve.

It was not considered as an option with the Royal Navy in the 80's onwards where they favoured sun run mer alt and considered star sights as exotica. There was a strong urge to use the Hewlitt Packard hand held programmable computer designated PDQ. Having come from the MN I favoured my Casio calculator and the back of a chart for my plotting sheet. Then came Transit, Sylidis, and GPS.