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BS XM DS Daytime Venus, Redux
From: Jeremy C
Date: 2011 Jan 18, 09:55 EST
From: Jeremy C
Date: 2011 Jan 18, 09:55 EST
Okay, I was a bit more prepared, and had better conditions so I tried this
again!
I also did a bit more research by tapping into the 1984 Bowditch in the
captain's library. The current Bowditch tells you what a back
sight is, but not how to correct it to Ho. I was using the wrong
procedure which was part of my problem. To do this correctly, we apply
negative IC and dip (in my case adding both of them) to the compliment of the Hs
(180-Hs). You must do the subtraction first, and then add the
correction.
Then we apply the body correction and then ex-meridian correction to get
what I call Ho' Then we determine ZD and then apply to Declination to get
Latitude (in this case subtract ZD from Declination).
I started out the math with the "easy" sight. At due north from our
anchorage I can see over the trees and use the true sea horizon. So I took
a "plain" old back sight transit shot of Venus. I was doing this just to
nail down the back sight portion of this problem.
Data:
January 17, 2011
GPS Lat 07-16.3' S
Long 072-26.9' E
ZD-6
HoE 106' for all sights
Hs 101-35.1
IE 0.8' on the arc
Time 10h 00m 54s Local
This gave me an error of 0.1' of latitude so I was doing the back sight
correctly.
I then moved on to the ex-meridian back sights I had taken before
transit. This had the added advantage over yesterday of a more distant
shoreline that was also more perpendicular to the azimuth than the post-transit
shore. The shore varied from 2.0 to 1.9 nm away which isn't great, but
less prone to error. I also made 6 observations. I only reduced
two of them (I don't have programs to do these things, so I do it all with
tables with the exception of using the dip short formula). Both of my
ex-meridians came out about 3 nm off so I suspect that the shoreline is closer
than the radar indicates. These take me 15-20 minutes each to do with
tables.
here is the data:
1) 09h 47m 44s
Hs 102-21.6
Distance 2.0nm
2) 09h 48m 38s I reduced this one
Lat: 07-19.6 S
Hs 102-16.6nm
Distance 2.0nm
3) 09h 49m 58s
Hs 102-12.6
Distance 2.0nm
4) 09h 51m 38s
Hs 102-07.8
Distance 2.0nm
5) 09h 54m 43s
Hs 102-01.2
Distance 2.0nm
6) 09h 55m 52s I reduced this
one. Lat 07-19.3 S
Hs 101-59.0
Distance 1.9nm
The good thing is that the small change in declination of 0.1 during the
sights doesn't seem to affect "a" to the 10ths place. The small distance
change in the shoreline does affect dip however so this is where my error is apt
to be.
I got an "a" of 9.4' by double interpolating the table in Bowditch and my
dip short for 2.0nm was 30.8' and at 1.9nm it was 32.4'
For Kermit: I was applying my -0.3 "v" correction for GHA today.
As a shooting note. These are VERY difficult observations to
make. Finding Venus is difficult enough and when you are trying to
shoot at over 100 degrees, any slight movement of the sextant causes to body to
skate out of view, especially with the 7x scope I'm using. I used my least
dense horizon shade to provide better contrast with Venus and the
shore/horizon. The body also moves in a parabola with a positive slope
(opposite of the normal sight) so you are looking for the apex of the arc as you
swing. Since the body is so high to begin with, the slope is quite small
as you shift azimuths, so it is difficult to find the exact azimuth of the acme
and then trying to hold the sextant perpendicular offers another challenge for
the aforementioned reasons.
This sight is as difficult to shoot as a high-altitude circle of equal
altitude observation of the moon and the math is far more intricate.
In total, this ranks as the most difficult observation I've even made in
Celestial Navigation. The ONLY thing easy about this observation is
plotting the resultant LOP.
Jeremy