Author

Topic: Celestial Navigation

Gandy

posted September 24, 2008 01:00 PM
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 alon used for plotting. See the value at WT 1620, DR Lon is 62 deg. 54.8 min. E, and the alon 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
The selection of the aLon has been expanded in the online tech support, but the minutes part of gha for east lon = min of gha60, 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 aLon° = DRLon° and choose aLon’ = 60’  GHA’
Step 2. Now look at the difference between DRLon and aLon. Call this Delta
Step 3. If Delta is less than or equal to 30’ you are done. The AP = aLon° aLon’ = DRLon° GHA’
Step 4. If Delta is greater than 30’, then we need to change aLon° 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 aLon° by 1°, but if GHA’ is less than 30, decrease aLon° 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
We should clarify that the rules above for finding aLonE is a formal one.
The more informal is just "choose aLonE as close as possible to the DR but with the minutes part of aLon = 60 min part of GHA." then we give the hint that the closest one will always be within 30'
From: Starpath, Seattle, WA


