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» Online Classroom   »   » Public Discussion of Cel Nav   » Celestial Navigation

Author Topic: Celestial Navigation

 - posted September 24, 2008 01:00 PM      Profile for Gandy           Edit/Delete Post 
In the text discussing Running Fixes, pg. 67B,C, D, E, you illustrate problem #6 and plot the lines of position. I do not understand how you arrive at a different value between the DR Lon, and the a-lon used for plotting. See the value at WT 1620, DR Lon is 62 deg. 54.8 min. E, and the a-lon for plotting is 63 deg. 9.7 min E. In your instructions on pg. 67C you state in step (2) "Look at the last DR position and compare it to the given GHA of the sun to pick your Assumed Longitude, and from these figure the LHA." How is this done?
From: home
David Burch

 - posted September 24, 2008 01:24 PM      Profile for David Burch           Edit/Delete Post 
The selection of the a-Lon has been expanded in the online tech support, but the minutes part of gha for east lon = min of gha-60, so that when you add they end up = 60 exactly which = 1° and they go away. The DR value has to be found from actual DR plot.

here is a review of the rules for east lon:

Eastern Longitudes
Step 1. Choose a-Lon° = DR-Lon° and choose a-Lon’ = 60’ - GHA’

Step 2. Now look at the difference between DR-Lon and a-Lon. Call this Delta

Step 3. If Delta is less than or equal to 30’ you are done. The AP = a-Lon° a-Lon’ = DR-Lon° GHA’

Step 4. If Delta is greater than 30’, then we need to change a-Lon° by 1°, either up or down according to Step 5.

Step 5. When delta is greater than 30’, if GHA’ is greater than 30, then increase a-Lon° by 1°, but if GHA’ is less than 30, decrease a-Lon° by 1°.
Again, after a few practice ones, you will not need these sort of formal guidelines.

From: Starpath, Seattle, WA
David Burch

 - posted September 24, 2008 01:28 PM      Profile for David Burch           Edit/Delete Post 
We should clarify that the rules above for finding a-LonE is a formal one.

The more informal is just "choose a-LonE as close as possible to the DR but with the minutes part of a-Lon = 60- min part of GHA." then we give the hint that the closest one will always be within 30'

From: Starpath, Seattle, WA

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