Mode for position derived from sextant sightings

[Martin Lewicki]

I have been practicing with a sextant for a few months using the Nautical Almanac (NA). With this one uses NA tables for a date to reduce altitude star sightings (also of Sun ,moon planets) to pin down a geo latitude and longitude for vessel location.

To practice dry runs I use Stellarium to get accurate altitudes for target stars and run the manual calculations with the NA. But it occurred to me that just as we can input geo lat long to find altitudes and azimuths of stars for a given date and UT it must be possible to input altitude and azimuths of two or more selected stars for the  given date  and UT, and derive instantly a geo long lat position of vessel. It is a matter of just reversing the standard calculations in Stellarium! Would that be so?

One would go out on deck. Note UT. Make two or three or more  altitude and azimuth sightings of navigation stars then enter these values into a Stellarium dialog and out pops the position in Long and Lat in an instant. Stellarium after all already has the option to mark navigation stars. This way the process is still independent of GPS. A sextant, a laptop with Stellarium, with computer clock calibrated before departing is all that will be needed.

Now that stellar navigation is being reintroduced by USA Navy and queries we have been getting at the planetarium it could be useful feature.

Worth posting request to Github?

Martin

[Alexander Wolf]

Hi!

ср, 9 окт. 2019 г. в 12:27, Martin Lewicki <martin....@gmail.com>:

Yes, please do it. This is good task to enhancement of navigation plugin.

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With best regards, Alexander

Phone: +7 903 957 3596

Skype: alex.v.wolf

[Georg Zotti]

Sure, go for it. However, I had assumed the US Navy has re-introduced sextant/table navigation as emergency/fallback solution if disaster really strikes badly. Think of total power outage, and use a candle to read the tables. Maybe you have a pocket calculator, but maybe just a slide rule. No PC, no Stellarium to help you. In any case, the dialog should not be a simple black box, but should at least have the option to show intermediate results, write out what they mean, and help understanding the process of data reduction. And a chapter on this very process of data reduction should be written into the Stellarium User Guide by somebody who has navigated successfully already.

[Martin Lewicki]

Yes Geog, exactly. We should be able to do this with candle power! I have already slaved over  manual calculations with at most a pocket calculator for the trig parts. I'll get my hands dirty with A-B-C haversine tables next.  One day I may get an opportunity to do this on a vessel out in the water!  Meanwhile I practice sighting sun/stars on our local coast and use water pan reflection method in the back yard for noon sightings because our coast only faces west.

However it is an advantage to have available all portable auxiliary methods to fix location indepentent of GPS  whether be it tables, pocket calculator or Stellaruim. At least using Stellarium you have an essential clock built into the onboard copmuter/laptop too.(solar powered?)

Fascinating skill to develop.

Martin Lewicki
Adelaide Planetarium - Astronomy Educator
University of South Australia
Mawson Lakes
GPO Box 2471, Adelaide SA 5001
Ph: +618 830*2 3027*
email :martin....@unisa.edu.au
web: www.unisa.edu.au/planetarium/

On Wed, Oct 9, 2019 at 8:07 PM Georg Zotti <georg...@univie.ac.at> wrote:

Sure, go for it. However, I had assumed the US Navy has re-introduced sextant/table navigation as emergency/fallback solution if disaster really strikes badly. Think of total power outage, and use a candle to read the tables. Maybe you have a pocket calculator, but maybe just a slide rule. No PC, no Stellarium to help you. In any case, the dialog should not be a simple black box, but should at least have the option to show intermediate results, write out what they mean, and help understanding the process of data reduction. And a chapter on this very process of data reduction should be written into the Stellarium User Guide by somebody who has navigated successfully already.

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