Line of Position — Acrux
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 Acrux
- 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 Acrux:
- Time: 1999-03-24, 23:42:06
- Sextant Altitude: 48°40.2'
- Height of eye: 9 feet
- Index error: -2.5'
With this information, we can now fill in the first part of our
worksheet:
Object: Acrux
Hs: 48°40.2
±ie -2.5
-dip -2.9
=Ha: 48°34.8
±corr: -0.9
=Ho: 48°33.9
Find Geographic Position of Acrux
Again, we go the the Almanac and look up the GHA of Aries and the
SHA of Acrux. We go to the Increments and Adjustments table to get a
correction factor.
date, time: 1999-03-24 23:42:06
almanac:
GHA: 166°58.3 v: _______ decl: S63°05.7 d: _______ HP: ______
+corr: 10°33.2 +d: ______
+v: ___._ =decl: ___°__._
+SHA: 173°20.9
=GHA: 350°52.4
±AP: ___°__._
=LHA: ___°__._
The GHA of Acrux is 350°52.4.
Pick an Assumed Position
Set AP to 54°S, 74°52.4W
date, time: 1999-03-24 23:42:06
almanac:
GHA: 166°58.3 v: _______ decl: S63°05.7 d: _______ HP: ______
+corr: 10°33.2 +d: ______
+v: ___._ =decl: ___°__._
+SHA: 173°20.9
=GHA: 350°52.4
±AP: -74°52.4
=LHA: 276°
Use Sight Reduction Tables
We go to the page for Latitude 54°,
declination same name as latitude, find declination
63°, LHA 276°.
The table gives these values:
We next go to the correction table using d=28' and minutes
of declination=5.
Sight reduction table:
Hc: 48°29 d: 28' Z: 137°
+d: 2
=Hc: 48°31
-Ho: 48°34
=dist: -3 +away, -towards
We are 3 nm closer to Acrux than our assumed position.
Plot
Draw a vector from our assumed position in the direction 137°.
Measure 3 nm along that vector and draw a line perpendicular
to the vector. This is our Line of Position for Acrux.
Note: This new line of position is nearly parallel to the
previous one. That's not so great, actually. It's nice that they're
close to each other, since this means it's likely that we didn't make
any mistakes, but the new line doesn't tell us much that the previous
one didn't. Lesson learned: sighting on two stars in nearly the same
direction isn't very useful, except to cross-check your work.
Ahh well, we press on anyway.
Next: Line of Position — Aldebaran