From:                              Bob Goethe

Sent:                               October 27, 2017 6:46 PM

To:                                   'Bill Anderson'; Carly Butler; 'Brian Hehn'

Subject:                          NASA: Why Pub. 249?

 

You may wonder why small boat sailors all seem to use Publication 249, Sight Reduction Tables for Air Navigation rather than Pub 229, Sight Reduction Tables for Marine Navigation.  There are several reasons.

 

1.    Vol 2 + vol 3 of pub. 249 weigh 4.2 pounds.  The six volumes of Pub 229 weigh 13 pounds.

2.    Pub 229 gives you a theoretical accuracy of 0.1 nm vs. Pub 249’s 1.0 nm.  However, to actually take a sextant sight that really IS accurate to 0.1 nm, you need to be standing on the stable deck of an aircraft carrier.  If you are on a 40 foot yacht, there is no way on earth you are going to get sights more accurate than 1 nm from your true position...and the truth is that you will be ecstatic if you can get a sight within 3 nm.  5 nm is good, and an error of 10 nm is acceptable.

3.    Pub. 229 is more complicated to use, and reducing a sight takes longer than it does with pub. 249.  The air navigators in the pre-GPS days needed a method of sight reduction that was QUICK.  When travelling at 300 knots, if you took very long to cook up your navigational fix, it would be out of date before you worked it out.

By the same token, small boat sailors (like me, at least) who can get seasick if they spend too much time looking at columns of tiny numbers ALSO like a sight reduction method that is QUICK.  The quicker I can do the pencil-and-paper part of celestial navigation, the healthier I am going to stay.

 

So every US Navy ship will have a copy of Pub. 229 on the bridge.  And good for them.

 

Small boat sailors all use Pub. 249.

 


I have, just for the heck of it, taken several sights and reduced them by using both Pub. 249 and 229...and I got (in the cases I tested) precisely the same results.  So while Pub. 229 has an increased theoretical accuracy, in the actual sights I took from my living room in Edmonton, I got no greater accuracy than with 249...and Pub. 249 was quite a bit easier to use.  So to heck with 229 for me!


 

You may see Pub 249 and 229 (and others) referred to as “HO 249” and “HO 229”.  Up until a few decades ago, these reference volumes were issues by the US Hydrographic Office...hence the “HO”.  Now, what used to be called the Hydrographic Office has been taken over by the US  “National Geospatial Intelligence Agency” – which is a mouthful – and all the old “HO” volumes were renamed as simple “Publication” or “Pub.” Volumes.

 

It is 6:42 PM and I am really, really quitting this for today.

 

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____________________

Bob Goethe

 

"As I sail, I praise God, and care not."
            Luke Foxe, Arctic Explorer, 1634

 

I desire no more delight
Than to be under sail and gone tonight.

           Gratiano, in Act 2, Scene 6, The Merchant of Venice

           By William Shakespeare, 1596

 


Some day you will read in the papers that D.L. Moody, of East Northfield, is dead. Don't you believe a word of it! At that moment I shall be more alive than I am now.
           D. L. Moody (1837-1899)

 

 

 

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