Line of Position — Aldebaran
See Rigil Kentaurus for a detailed explanation.
Computing a Line of Position takes these steps:
- Measure the object's altitude with a sextant, note exact time of
sight, correct sextant altitude for various errors.
- Use almanac to find GP of Aldebaran
- Pick an Assumed Position
- Use sight reduction tables to compute an azimuth (Z) to
the object and an expected altitude (Hc).
- Plot
Measure the object's altitude with a sextant
Leg 57 gives this information for the sighting
of Aldebaran:
- Time: 1999-03-24, 23:43:12
- Sextant Altitude: 13°51.6
- Height of eye: 9 feet
- Index error: -2.5'
With this information, we can now fill in the first part of our
worksheet:
Object: Aldebaran
Hs: 13°51.6
±ie -2.5
-dip -2.9
=Ha: 13°46.2
±corr: -3.9
=Ho: 13°42.3
Find Geographic Position of Aldebaran
date, time: 1999-03-24 23:43:12
almanac:
GHA: 166°58.3 v: _______ decl: N16°30.3 d: _______ HP: ______
+corr: 10°48.0 +d: ______
+v: ___._ =decl: ___°__._
+SHA: 291°02.2
=GHA: 108°48.5
±AP: ___°__._
=LHA: ___°__._
Pick an Assumed Position
Set AP to 54°S, 74°48.5W
date, time: 1999-03-24 23:43:12
almanac:
GHA: 166°58.3 v: _______ decl: N16°30.3 d: _______ HP: ______
+corr: 10°48.0 +d: ______
+v: ___._ =decl: ___°__._
+SHA: 291°02.2
=GHA: 108°48.5
±AP: -74°48.5
=LHA: 34°
Use Sight Reduction Tables
Note: Aldebaran's declination is North. This means we look for a
table labeled
DECLINATION (15°-24°) CONTRARY NAME AS LATITUDE
The table gives these values:
- Hc: 14°12
- d': 56 (careful: read below)
- Z: 326°
Careful: the tables don't give the signs of the differences. Notice
that the Hc value to the right of the 14°12 entry is less than
14°12. This means that the d' term is a negative number. Use -56
here.
Again, Hc is within a degree of our Ho, which is a good sign.
Again, we go to the correction table and enter d (-56) and the minutes
of declination (30). This gives -28
Sight reduction table:
Hc: 14°12 d: -56' Z: 326°
+d: -28
=Hc: 13°44
-Ho: 13°42
=dist: +2 +away, -towards
We are 2 nm further from Aldebaran than our assumed position.
Plot
Draw a vector from our assumed position in the direction 326°.
Measure 2 nm along that vector and draw a line perpendicular
to the vector. This is our Line of Position for Aldebaran.
Yet again, this line is almost exactly parallel to the line we drew for
Rigil Kentaurus. This serves to confirm that we're on the right
track, but it's of very little use to us in terms of determining
a fix. Memo to self: next time, don't choose stars so closely in
line with each other.
Next: Line of Position — Peacock