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"George Huxtable responded:

> The rise,
> and the fall of the Sun are indeed quite symmetrical, which is why his
> folding-paper trick will work. It comes down as a mirror-image of the way it
> went up. It's just that the centre of that symmetry is displaced from Local
> Apparent Noon, exactly as I explained, when there is a North-South component
> of the vessel's velocity, or a changing declination.

Darn it George, it is my turn to be pedantic.  If the declination of the
body, the sun in this case, changes during the observations it cannot be
literally a mirror image (even from the same location).  You did cover this
under, "... or a changing declination." Common knowledge is--or at least as
I understand it--the declination of the Sun is constantly changing at a
rapid hourly/daily rate (as opposed to stars). I believe I addressed this in
my infamous "sea monkey" post. 

NavList 2453, 22 March, 2007

"Why in the name of all that's holy do you do that yourselves?  There are
only two times a year, and two longitudes on earth that the sun will be 0
declination at LAN. It will have a lower declination before LAN and a higher
declination after LAN now and vice versa in September, so Hc and the
slope/curve will be affected and not symmetrical.

I went through the trauma of the sea monkey kit being brine shrimp in my
youth.

Then, before cel nav, the belief that the sun would rise exactly in the east
and set exactly in the west; and the day (sunrise to sunset) would be
exactly 12 hours long. I would check out my topo map, find a farm field(s)
with equal elevation miles away, adjust my Silva Ranger for declination
(variation) and wait with my binoculars for sunrise and sunset. Always
disappointed.

Now I know unless there is a bizarre and serendipitous shift in refraction
both at sunrise and sunset it will not happen. Also that I will not get a
pony for my birthday."

Glib moments behind, from my beginning days--which included a preoccupation
with LAN for reasons already posted--I have been interested in elegant
(simple on the water) method(s) of accounting for the vessel's motion. As I
now understand my memory is not as acute as I believed it was, and as list
turnover results in many new readers, please (George et al) recap the
methods of accounting for vessel motion. Or point me to the archive's
thread(s).

> when there is a North-South component
> of the vessel's velocity.

What about east/west displacement if trying to determine longitude from LAN?

Thanks

Bill B.


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