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There has been some discussions lately about using the technique of
graphing a number of sextant observations on the same body and comparing
them with a line having a slope representing the change in altitude of
the body over time to allow discarding erroneous observations so as to
keep them from distorting an average of the good observations. If you
are going to do this then, if you are not taking the sights from the
beach but from a moving vessel, you must adjust the slope of the line to
account for the movement of the vessel. Back in February I posted a
table I computed for adjusting sextant observations for the movement of
the vessel and this same table can be used for adjusting the slope of
the line for "manual slope plotting" technique. The table is here:

http://fer3.com/arc/img/111994.moo.pdf

For even  more precision, and for speeds in the range of .5 to 9, knots
you can use the Motion of the Observer (MOO) table found in H.O. 249
simply by dividing the tabulated speeds, 50 to 900 knots, by 100 and at
the same time dividing the tabulated values for the correction by the
same factor of 100. There are two tables, one for a one minute period
and one for a four minute period. I posted excerpts of H.O. 249
including the MOO tables here:

http://fer3.com/arc/img/105924.ho%20249%20extracts%20.pdf

You can review my previous posts on how to use these tables at these links:

http://www.fer3.com/arc/m2.aspx?i=111885&y=201002

http://www.fer3.com/arc/m2.aspx?i=111994&y=201002

http://www.fer3.com/arc/m2.aspx?i=105937&y=200807

http://www.fer3.com/arc/m2.aspx?i=105938&y=200807


You can also use the "Motion of the Body" (MOB) tables in the excerpts
of H.O. 249 for actually plotting the sloping line representing the
change in altitude of the body over time. The two MOB tables show this
for a one minute period and for a four minute period and are equally
valid standing on the beach, sailing at 6 knots or flying at 900 knots.
When you study this table you will see that the slope of the line is
determined solely for the azimuth of the body and the latitude of the
observer. You can simply refer to this table and then plot the sloping line.


Think these tables through and you can figure the sign to use for
plotting the line, (although this should be obvious, bodies to the east
are rising and those to the west are descending.)

Give em a try.

gl

   

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