"Wally" wrote in message
...
The exercise of plotting a Noon Sight is one of the first things a
novice learns. Neal's lack of understanding shows he has never
actually done this.

Neal seems to be contending that it's possible to line up a limb of the sun
with the horizon, such that there's a perceptible difference between limb
and horizon being 'in contact' and being separated. While this may well be
possible when everything is nice and stable, I can't help feeling that it
would be rather difficult in a bobbing boat.


Its really more difficult than that - as I said, the altitude of the Sun varies
less than 1 arc-second for roughly a minute before and after LAN; a few
arc-seconds for the minutes before and after that. The diameter of the Sun is
about 30 minutes, or 1800 arc-seconds, so we're talking a teeny, tiny, itsy,
bitsy amount here. If the Sun appears to be about the size of a pea at arm's
length, then this distance is about a tenth the thickness of a human hair (if my
napkin math is still good).

If that weren't enough, Neal claimed he just presets his sextant to the right
altitude, but even a good sextant can't be set better than about 12 arc-seconds,
most have errors much greater than that. Neal's plastic probably has completely
random errors more like 5 arc-minutes. And then there's things like the dip
correction, where a change in the height of the viewer of just a few inches
throws it off by more than an arc-second. On top of that, knowing the altitude
to preset means knowing one's Latitude - in this case to about 100 feet for each
arc-second. If my dead-reckoning were this good I wouldn't have need for a
sextant!

And remember, the Earth is moving 15 miles per minute (at the Equator) so there
isn't much room for error when making this determination. Neal claims he can
get 10 mile accuracy; its probably more like 100 miles with this method. In
reality, assuming Neal actually owns a sextant and has taken it out of the box,
what he has probably done is preset (as best he could) based on his GPS
position, and then verified that local noon occurred as predicted, within a few
minutes.

-jeff