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» Online Classroom   »   » Public Discussion of Cel Nav   » Working a backsight of the Sun problem

   
Author Topic: Working a backsight of the Sun problem
WJohntri


 - posted June 10, 2008 10:58 AM      Profile for WJohntri           Edit/Delete Post 
I need some help on how to work a back sight of the sun.
From: Mississippi
Capt Steve Miller


 - posted June 10, 2008 11:15 AM      Profile for Capt Steve Miller           Edit/Delete Post 
When working a back sight it is very important to be looking exactly 180* from the body being sighted. Swing your body right and left to see if the altitude gets any higher when doing the swinging. All this can be a bit difficult. Once done you subtract your sextant reading from 180 and use that result as the Hs of your sight. Then handle the reduction in the normal manner.
Here in FL in order to do a noon sight at the beach I have to face North where there is a water horizon due to the coastline angle being NW - SE. Due South is over the land, therefore I have to do a back sight to get my Latitude from a Sun sight. At around mid June (like about now) the altitude of the Sun at LAN is about 83*, which makes it hard to be facing directly away from the Sun. Good Luck with your sights.

From: Starpath
Capt Steve Miller


 - posted June 10, 2008 11:28 AM      Profile for Capt Steve Miller           Edit/Delete Post 
A follow up on my last posting. During the reduction process you have to reverse the sign of the Index Correction (IE or IC) and the height of eye (Dip). The rest of the calculations are as normal.
From: Starpath
WJohntri


 - posted June 10, 2008 12:09 PM      Profile for WJohntri           Edit/Delete Post 
Thank you for the help. Still have a problem.
The problem I am tring to work out.Here is the problem. HS upper limb of the sun 118*41.4, Index error 2.0' on the arc,height of eye 24'.
The answer I get is 62*18.3 HO. The answer the book gives is 61*40.8'HO.Maybe the book is wrong.

From: Mississippi
WJohntri


 - posted June 10, 2008 12:12 PM      Profile for WJohntri           Edit/Delete Post 
Date June 2 1981
From: Mississippi
David Burch


 - posted June 10, 2008 02:33 PM      Profile for David Burch           Edit/Delete Post 
please see the previous discussion on backsights to see if that might help....i moved it here from the online cel nav discussion, which is not visible to the public.

* * * please see Lecture on Back sights in Lesson 11 of the online cel nav course. You are still registered for that course and should be able to get to it.

From: Starpath, Seattle, WA
Capt Steve Miller


 - posted June 10, 2008 05:13 PM      Profile for Capt Steve Miller           Edit/Delete Post 
I just worked the problem with the data you gave. the 118* 41.4' converts to 61* 18.6'. Then add the dip correction of 4.8 and add the IE correction of 2' gatting an Ha of 61* 25.4. Next your backsight of an upper limb is really a lower limb sight. Therefore you add the altitude correction lower limb of 15.5'. This gives you an Ho of 61* 40.9'.
Hope this helps

From: Starpath


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