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» Online Classroom   »   » Public Discussion of Cel Nav   » Problem 9.3b

Author Topic: Problem 9.3b

 - posted April 22, 2010 05:00 AM      Profile for Terry           Edit/Delete Post 
In working through Problem 9.3b, I have a question. The HP for the moon in this is 54.8. Since the problem lists the sighting as using the lower limb of the moon, would not the value of the additional altitude correction be 1.4 instead of 1.6 as stated in the answer key? As I read it 1.6 would be the value for an upper limb sighting.
From: Spanaway WA

 - posted April 22, 2010 06:41 PM      Profile for HHEW           Edit/Delete Post 
Just to be sure we're on the same page (literally), we're looking at the moon altitude correction table (0°-35°) on page 219 of the text.

The sight reduction form on page 199 shows a LL sight of the moon, an Ha of 24° 46.5'; HP of 54.8'.

From the table on 219, the closest tabulated HP value is 54.9 (row 4). The value on that row for L in the 20°-24° column is 1.6'.

The value for HP 54.6, L, is 1.3'. So, as the HP goes up .3' , the L correction goes up .3' also. Then, HP 54.7 would be 1.4'. and HP 54.8 would be 1.5'.

At this stage of a moon reduction many navigators (me included) would just take the nearest HP value and read off the correction without interpolation, which is evidently what was done in this example.

When you're new to celestial, it is worrisome, though, not to find your result agreeing exactly with an example. You've probably run into the same thing in using HO 249, where you can often get variations of .3'-.5' depending on whether or not you round the declination increment.

-Hewitt Schlereth

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