Thanks for the advice! I had a much better outcome today!
About yesterday's sight Frank asked "Could you post the details of that sight? Did you record the actual
angle (double the altitude) as taken from the sextant? And what was your
index correction? "
(1) With the two suns superimposed, the sextant read 60º 38.5'
(2) I detect no index error so I made no correction, simply dividing the sextant reading in half to get 30º 19.25'
(3) I subtracted this from 90º to get a ZD of 59º 40.75'
(4) I looked up the declination of the sun in the nautical almanac and got 23º 13'
(5) I subtracted the dec. from the ZD to get a latitude of 36º 27.25'
(6) I asked my Garmin 76Cx for its extimation of the latitude and it responded 36º 50.01'
(7) I subtracted my lat from the gps lat to get an approximate error of 22.26'
Thinking about problems that may have affected the accuracy of the sight, I see four:
- there was a light haze, which made one of the images a slightly different size from the other, which in turn made superposition of the images a little sloppy
- it was windy, which disturbed the artificial horizon's reflecting surface
- the glass covers of the a.h. fogged slightly
- inexperienced in manipulating the instrument, I may have missed actual noon by as much as several minutes.
After this experience and after reading Gary's advice, I did things a little differently today.
I used the lower limb observation instead of superpositioning.
I used oil in the horizon rather than water, to reduce rippling
I put the horizon outside an hour before noon so there would be no fogging
I began shooting ten minutes before clock noon to be sure to get the LAN
The result was a big improvement over yesterday's!
Sextant reading 59º 26.6'
No index correction
Halved 29º 43.3'
Lower limb correction +14.6'
HO = 29º 57.9'
ZD = 60º 02.1'
declination = 23º 09.2'
latitude = 36º 52.9'
gps latitude = 36º 50.01
The improvements to the artificial horizon plus a drop in the wind were probably the most important difference. Taking the lower limb probably had an effect, too. Finally, beginning to shoot early, I was able to follow the sun to its zenith and then detect the moment when it began to fall away from that.
I look forward to using a real horizon soon. Frank's observations about local geography put me in mind of a number of possibilities.
Best regards,
Patrick
--
Dr.. Patrick Goold
Department of Philosophy
Virginia Wesleyan College
Norfolk, VA 23502
757 455 3357
Charles Olson: "Love the World -- and stay inside it."
