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I've been looking in a bit more detail at the diagram, provided with Andres
Ruiz' computation, showing his resultant "cocked hat", and it's left me a
bit puzzled.

I haven't found a way to print out that plot, so my comments refer only to
the picture as I see it on my screen.

The final "fix" (affected as it may be by an error in the time of the Pollux
observation, an error which isn't my present concern) is placed at the
centre of the plot, at (0.0, 0.0) on the graph, so its actual lat and long
are not shown on the plot, except as nearby text. But what are the x and y
scales on that plot?

The y scale is simple enough. It shown changes in latitude, in minutes and
in nautical miles , which are the same thing, positive going North. If
angles measured on that plot are to mean anything, then the x axis should be
to the same scale of miles on the plot, and so a position circle should plot
as a circle. On my screen, that doesn't happen; a position circle plots as
an ellipse, horizontally squashed. So on that picture, angles will be
distorted.

What about the units along the horizontal scale, then? Although there's
nothing to say so, those are plotted in nautical miles, also, even though
that scale of miles differs from the miles of Northing. It's only possible
to quantify shifts in longitude by doing some trig., and arithmetic.

To me, that appears to choose the worst of all possibilities. The two
scales, horizontal and vertical, should be the same in miles, the vertical
scale should be MARKED in minutes of lat. (as it is), and the horizontal
scale should be MARKED in minutes of long., so that lat and long can be read
straight off it.

==============

Additionally (and this is a separate matter) Andres' program appears to
treat all longitudes as Easterly, so that a Westerly longitude is shown as
negative. To me, this seems somewhat perverse, though I am aware that many
programs (particularly for astronomy) do so. We navigators work in hour
angles (GHA and LHA) for our positions of bodies in the sky, and those hour
angles are always measured Westwards, so that they always increase with
time. An hour angle is nothing else but the longitude of the body, measured
Westerly from Greenwich, 0 to 360. Why don't we measure our geographical
longitudes exactly the same way, so that we simply difference the longitudes
to get local hour angle? Meeus is an astronomer who sets us a sensible
example. It seems madness to measure hour-angles as positive Westwards, and
longitudes as positive Eastwards. Can anyone really justify it?

contact George Huxtable at george@huxtable.u-net.com
or at +44 1865 820222 (from UK, 01865 820222)
or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.


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