NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Alexandre Eremenko
Date: 2012 Mar 19, 17:58 -0700
Here I post a series of observations that puzzles me very much.
On March 17, in an unusually warm weather for this season, we spent an
afternoon with Bill Burchell in St. Joseph harbor on lake Michigan,
N 42d06'8, W 86d29'5, with his Astra sextant and my TS (pocket sextant).
We made 7 series of 3-5 observations each of Sun and Venus altitudes over the lake
from a jetty, approximate height of the eye 12 ft.
The weather was sunny and warm (80 degrees F by Bill's thermometer).
The horizon looked very sharp, though there was a lot of glare under the Sun.
We took a great care in determination of the Index error (from the Sun, from
the horizon and from a remote roof), in the beginning and in the middle of
observations. Our determinations varied between 0 and 0'.5, but most were
less than 0'2.
Astra had a perceptible side error (about 1/5 of the Sun SD).
In the following
I do not correct for the Index or side error.
My TS has zero Index error to the best of
my knowledge. I had excellent results with this pocket sextant which I
posted on the list in the past.
We observed Sun (lower limb) and Venus altitudes, corrected for
the height of the eye, semidiameter and refraction, and compared the results with computed altitudes.
By the "error" I mean the observed and corrected altitude minus the computed
altitude.
GMT 21:34, Astra, Sun. Errors: -11'5, -14'2, -9'9, -9'2, -6'3 (Aver.-10.2,SD=2.6)
GMT 21:46, Asrra, Sun. Errors: -9'9, -10'5, -10'6, -10'5, -10'5 (Bill)
(Aver.-10.4,SD=0'25)
GMT 21:54, Astra, Sun. Errors: -11'1, -12'2, -8'7
(Aver.-10.66,SD=1'5)
GMT 22:38, Astra, Sun. Errors: -11'4, -8'4, -9'7, -10'5, -9'5, -10'3
(Aver.-10.0, SD=0'9)
GMT 23:46, Astra, Venus. Errors: -9'7, -10'0, -9'4, -12'3, -11'4
(Aver.=-10.6, SD=1.1)
Sun set in the middle of this observation.
GMT 23:58, Astra, Venus. Errors: -6'3
Now pocket sextant observations (w/o telescope)
GMT 22:14, TS, Sun. Errors: -11.3, -13.8, -10.8, -13.7, -13.4
(Aver.=-12.6, SD=1'3)
GMT 00:03 (March 18) TS, Venus. Errors: -11'0, -9'8, -7'8.
Can anyone propose any plausible explanation of these enormous NEGATIVE
systematic errors? I never had anything of this magnitude before.
And as you see, they are pretty stable, about 10-11 minutes.
I understand there can be a dark strip on the water, mistaken for the
horizon. But this will lead to POSITIVE errors (overshots). Refraction which
DECREASES the apparent altitude???
We are talking of the errors about 1/3 of the diameter of the Sun!
What could possibly cause this?
Alex.
P.S. Bill promises to find the water temperature of water on this day,
but it is really
hard to believe in this kind of abnormal refraction.
----------------------------------------------------------------
NavList message boards and member settings: www.fer3.com/NavList
Members may optionally receive posts by email.
To cancel email delivery, send a message to NoMail[at]fer3.com
----------------------------------------------------------------