Next message: Frank Reed: "Re: dip, dip short, distance off with buildings, etc."
Bill, you wrote:
"I'm not dogging Nate, it is just that times have changed, and as pointed
out, impact traditional navigation."
Just to clarify, "Nate" did not invent any tables that relate to this topic
though the rough estimates of the changes in dip caused by refraction in that
era were not far off.
The Table 15 in 20th/21st century "Bowditch" that has caused occasional
puzzlement on the list appears to have been added in the 1958 edition (which was
a MAJOR revision of the publication). At that time it was numbered as Table
9. And even then the text notes that "an error may be introduced if refraction
differs from the standard value used..." but there's no indication at all
how much error a navigator should anticipate. And what I am saying is that a
large portion of this variability is both calculable and also easy to interpret
conceptually as a change in the effective curvature of the Earth.
And you wrote:
"Back to the beach shots. Even though the adjusted T15 constants get us
closer to the 23 nm target (Sears) and 23.5 nm (Hancock) with height of eye
15 ft and assumed base above water level of 30 ft, both calculations leave
us short. Meaning the observed angles were too high by approx 0.8'. How
much of this difference be attributed to possible thermal inversion?"
Very easily, ALL OF IT could be the result of variation in the lapse rate
(called a "temperature inversion" if it goes positive). Such variations are not
merely common, they're quite normal. It is easy to find stations, where the
lapse rate is positive (linguistic purists would call this negative, so just
to clarify, by positive I mean "warming with altitude") in the morning and
negative in the afternoon, day after day. At such places, that 0.8 minute of
arc variance in the measured heights of distant objects should be expected, and
more so, too. In fact, I've set this up in a spreadsheet where I can vary
the lapse rate and see the effect on the measured heights of the three tall
structures (Sears, Hancock, and Cooling Tower) visible from the Indiana beach
that day in late September. They are all consistent with a specific moderate
temperature inversion.
-FER
42.0N 87.7W, or 41.4N 72.1W.
www.HistoricalAtlas.com/lunars
