A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Brad Morris
Date: 2012 Sep 14, 15:52 -0400
Brightness does indeed matter. That's why my list used the navigational stars and the "Bright Stars" from the Astronomical Almanac.
The smart lads will point out that its hard to find a star that appears only in the "Bright Stars" list. It was, right up until I got a Sky Scout.
Enter the star name and the Sky Scout takes you there. Its nearly idiot proof (lucky for me, snicker)! Once I could see both stars of the pair, I could begin. Set the circle of reflection, and bend like a crazy maniac. Since it didn't matter which star took which path, I would try all kinds of approaches to the sight.
Camera calibration does benefit by using star distances. This way dip, height of eye, and a fuzzy horizon are eliminated as error sources. The time needed to do a set of observations is reduced as well. The only problem in my way is the sensitivity of my camera to the lower magnitude stars. Planets and bright stars work great.
[NavList] Star - Star Distances
From: Brad Morris
Date: 26 Apr 2010 12:08
After a rather lengthy and exhausting data manipulation effort, I have finally completed an interesting list. You will find this list to be similar to the list found in Cmdr. Bauer's "The Sextant Handbook". Cmdr Bauer provides a few star-star pairs in an appendix.
The list pairs every one of the 57 navigational stars, plus polaris, with each other. Once the pairing is made, I have computed the distance, ignoring aberation and refraction. I used the right ascension and declination from the USNO Bright Stars list. Aberation and refraction values cannot be pre-computed as they are a function of the time and your position on the planet.
I was frustrated by randomly trying pairs, searching for the right pair in the range I wanted. This list solves that little dilemma.
So what good is a list of interstellar angles that does not include those aberration and refraction? The value is that the list permits you to find a star - star pair in the desired range. Example: By trial and error, you have determined your arc error except for values in the 50 degree range. By looking at this list, you will find 111 pairs whose nominal range is between 50 and 60 degrees. One of them might just work for you!
Not every star - star pair will be visible from your location. For example, my latitude is 40N. I can never expect to use the southern cross (Acrux, Gacrux) for star - star pairs from my latitude. They will be below my horizon. Further, even if the stars may be visible from my latitude, the pair will only be visible at certain times of the year.
However, since the list is an exhaustive pairing of all navigational stars, plus Polaris, we have 1626 pairs to play with. The list begins with a small distance (Hadar, Rigel Kentaurus) at 4.399 degrees and ends with a large distance (Schedar, Gacrux) at 178.598 degrees.
As a practical matter, we would want to keep the altitudes above 10 degrees so the nominal maximum angle would be about 160 degrees. Of course, very few sextants can measure in this range. That's a silly remark! A much more practical range would be about 120 degrees. Keeping in mind Cook's spectacular 155 degree sun - moon lunar, however, the list is not truncated.
In actual practice, I have used the list to select the star pair based upon the desired angle. Then I use my 2102-D Rude Star Finder to see if the pair is visible from my location. If so, I then check to see if it will be visible at NIGHT (hahaha). Then, using the Bright Stars right ascension and declination for both objects, I calculate the apparent distance, accounting for the aberration and refraction. Naturally, this is done by spreadsheet, not by hand. Having the apparent distance calculated, I preset my sextant to the angle and measure.
Hopefully, this list survives the critical inspection of my colleagues here on the NavList.
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