Celestial Navigation...

Sailing through the Stars?

Oh, how many people get weary,
Bearing both their burdens and their scars,
And don't you think they'd love like to stop complaining,
And fly like eagles
Out among the stars.

Apologies to Merle Haggard.
I love that song. People should listen to it

Oops, was that a hijack or what? Sorry, Dave:-$

Don

Sailing through the Stars?

"All my Lines of Positions live in Texas..."

Hmm doesn't rhyme, dang it.

You can practice the computations on land by using a transit instead of a sextant, since you don't have a true horizon most places on land. Artificial horizons are a significant complication to use. But with a transit and a static 'deck', you can just take a vertical on a few things and compute your lines of position.

There are numerous programs, but it is probably one of those things that might have real survival value to know how to do manually.

There is tons of information on the web and youtube.

If you do want to try it with a sextant, there are many inexpensive Davis sextants on ebay or new that are plenty good enough for the beginner.

There are also a lot of so called 'bubble sextants' which are used for air navigation and are usually WWII surplus, but few of them are operational.

- jlw

There is a USAF version on there right now for 20 bucks plus shipping, that is if no one bids on it.

It looks like it might need to be mounted to the aircraft, though, I'm not sure.

I have learned a lot about ephemeris data and time that I didn't know. I had no idea time is so complicated, TT, Atomic Time, UTC, UT1, delta T, etc.

I have a 1' Dietzgen transit suitable for sighting the sun and a Total Station with reticle illumination for stars (5").

While you are looking on E-bay try looking up "Air Navigation Air Force Manual 51-40."

E-bay the "American Practical Navigator" (also known as the Bowditch), don't get a recent one because it has the tables separate. I have on dated 1943, it has a short bio of Bowditch. It also has reference to Captain Thomas H. Sumner. Under the lines of position it has a short story about Sumner (page 177 in my book) and his discovery of the LOP (Line of Position) still used today. The LOP was called Sumner lines for years and years. I suppose some of the old navigators still call them Sumner Lines. My Bowditch was from USS Seginus, all our navy ships had a Bowditch on board. I don't think it is published now. Bowditch had connections with surveying. I think he is the fellow that came up with the compass adjustment. Seems to me that there was a contest for an adjustment procedure and he won the contest at a young age.

If the sextant has a sort of tube looking thing sticking up on it about six inches or so long it is a periscopic sextant. That was to plug into a slot through the skin of the aircraft and used a bubble for leveling, it would have a timer that averaged the observation. There were one minute and two minute timers. In the Air Force we used the HO 249 sight reduction tables and the air almanac. Don't know if they still publish the air almanac or not it is for one year, if I remember right and was published in England. I think the surface folks used a HO 214 for sight reductions. I haven't done any of that stuff since 1967. Still have my books though and take them out from time to time and put them away thinking to myself how simple the GPS has made the art of navigation.

The book I downloaded (link above) has the story of Sumner in it.

He is going so far as to discuss the correction for the oblateness of the spheroid. The navigation formulas assume the earth is a sphere but it's really an oblate spheroid. He is now discussing the Geoid which isn't a concern at sea but is in many mountainous regions.

I did a Latitude observation of Polaris a few weeks ago which I reduced. It landed a few hundred feet north of my position as seen on the map (and north of my position derived from GPS). Now I will see if his formula corrects my astronomic derived latitude to my GPS derived latitude (or at least closer). Geocentric latitudes (e.g. GPS) are generally smaller than Geodetic (his term which I find confusing-seems like it should be astronomic).

edit-OK never mind that. His formula puts me 15 miles south (Elk Grove). My uncorrected latitude from a Polaris observation at lower culmination is better than that.

> He is going so far as to discuss the correction for the oblateness of the spheroid. The navigation formulas assume the earth is a sphere but it's really an oblate spheroid.

The oblateness correction has evidently been a concern to navigators for a long time. About 1730 the King of France sent out two expeditions, one to the arctic and one to Peru. Each party triangulated 200 miles or so in a north-south direction and took star shots at both ends to determine the ground length of a degree of latitude. Between 1802-1875 British crews triangulated north-south across India, 1500 miles or so, also taking star shots, as well as setting up a control network for mapping. In about 1850 the British crews triangulated the altitude of the world's highest mountain, which they named for George Everest, who was then in charge of the survey.

Two very good books--"The Great Arc" by John Keay, about the Indian survey; and "The Mapmaker's Wife" by Robert Whitaker, about the Peruvian triangulation and many related adventures.