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How to Reduce a Sunline Using Pub 229
Gather
Information
As with any celestial observation, you must gather data to reduce to an
LOP. With a sextant and recorder you will need the following:
date/GMT of sight, DR position, sextant altitude (hs), height of eye of
the observer, and IC correction.
Procedure
For our example we will use the following:
Date:
31 March 1984
Lat:
36° 32.8'N
hs:
25° 46.9’
Hgt of Eye: 50 ft
GMT:
09 15 38
Long: 018° 10.0' W
IC:
- 1.0
After applying altitude corrections we have determined Ho = 25° 53.3'.
We can now use the Pub 229 strip form to complete the process of
reducing; at this point we have completed stage 1. We can move on to
the next stage of finding LHA.
Notice that to find LHA, we follow the same steps as we did for our
azimuth of the Sun problem only slightly different. Here’s the key
difference. We want to arrive at an even number LHA. To do this, we
will use an assumed longitude.
This step will help us in interpolation
later in this problem.
There is a catch though; the following rule must
be adhered to when finding an assumed longitude.
Rule: The assumed longitude used as an assumed position must be
within 30' of the original DR longitude.
Trick of the trade: When finding your assumed longitude, simply drop
the minutes of total GHA down, then add the whole degree of longitude
that is within 30' of the DR longitude. Look at our example problem
where we dropped the 52.2' down from the total GHA. If we were to
use the 18° from the original DR long. of 18° 10.0, which would be 18°
52.2' it would be more than 30', so we changed the 18° to 17° and all’s
well.
Let’s begin working our problem on the next page.
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