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» Online Classroom   » Celestial Navigation   » Public Discussion of Cel Nav   » Choosing the wrong A-Lon?

Author Topic: Choosing the wrong A-Lon?

 - posted May 31, 2020 03:31 PM      Profile for bruce           Edit/Delete Post 
From the discussion in another thread, I got curious about the impact of choosing an A-Lon which is *not* within 30' of DR-Lon.

Decided, as long as it is gray and rainy outside, it might be interesting to play with it. So from Chapter-7 I plotted Star #1 (Altair) normally, and plotted Star #5 (Arcturus) with three different A-Lons

"A" is the correct A-Lon (minutes from the minutes part of GHA, degrees from the degrees part of DR-Lon, checked to ensure the resulting A-Lon is within 30' of DR-Lon)
-- A-Lon = 126* 55.9'W
-- LHA = 35*
-- a = 31.2 A
-- Zn = 240*

"B" is an a-Lon with the right minutes part, but one full degree too large
-- A-Lon = 127* 55.9' W
-- LHA = 34*
-- a = 67.2 A
-- Zn = 238*

and "C" has the right minutes part, but is one full degree too small
-- A-Lon = 125* 55.9' W
-- LHA = 36*
-- a = 5.8 T
-- Zn = 241*

When plotted, it was interesting to see that the three resulting LOPs essentiall merged at a single point, and that point turned out to be really really close to where the Altair LOP crossed. At least within my ability to plot.

So... it appears that as long as the minutes-part is chosen correctly (to produce a whole-degree LHA) choosing the degrees part so that the resulting A-Lon is within 30' of DR-Lon may not be super critical. I don't know if it is generally true or not, but in this case an A-Lon off by a degree in either direction - but with the resulting LHA used correctly to obtain Hc and Z - still appears to produce a valid fix.


From: Everett, WA

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