# NavList:

## A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

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Re: Star - Star Observations
Date: 2010 Mar 10, 09:42 -0500

Gentlemen

When I posed my initial question, I was looking for a general case solution.  In my usual obtuse fashion, it came across as a special case question, and at that, poorly stated.

A search through the archives provided this clearing method.  http://fer3.com/arc/m2.aspx?i=015102&y=200404   Would this still be considered a good general case solution?  I have implemented it, since the corner cosine factors are a trivial matter to calculate, given the nominal inputs of refraction correction, altitude and calculated distance.   I am going to give George’s general case equation a try, to see how it compares to the one provided in this link.

Frank, you wrote: “Just to repeat, from my own experience, using star-star distances to test a sextant's arc error is worthwhile but somewhat limited in accuracy unless a high-power telescope is used” The minimum power I use for this exercise is 10x.  Perhaps this is backwards, but I calculate what the observed distance should be and then set the sextant to that distance.  I observe the object to my left in the direct path, the object to my right using the reflected or index path.  With a vibratory motion (there’s that 1800’s Bowditch language again!), I check to see if the objects pass thru one another.  That is difficult enough, with the contorted positions and my bad eyesight, so I have not yet moved to the point of correcting small observed discrepancies.  That’s for another day, once I have complete confidence in the constructs, equations and my facility with the sextant.

Having used the Skyscout for Mars – Sirius distances, I can clearly state that the right ascension provided for Mars is sadly lacking in accuracy, making any attempt to use the Skyscout probably not too useful for this exacting exercise.  The star data, again provided in right ascension, has been converted to arc and then into SHA.  I have spot checked star data against the Nautical Almanac day page and have seen consistently good results.  Is it accurate for all days, all stars and all cases?  I can’t make that statement but Frank’s caution regarding the Skyscout data is well placed.

However, using the Skyscout has made this a fun exercise.  Point at object one, get RA and declination.  Enter into spreadsheet.  Point at object two, get RA and declination.  Enter into spreadsheet.  Read observed distance from spreadsheet, set the sextant to that distance and perform all kinds of contortions in a hysterical attempt to observe the bodies pass through one another.  Twist and bend like a crazy maniac!!  All the tabular lookups are removed, none of the long reductions which are prone to human error are present.  I just point and shoot.

Best Regards

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