NavList:

   

Reply

Fred Hebard said-

>It would seem then, that the altitude measurement (not especially its
>accuracy, but the measurement itself) is a key component of the overall
>determination of the cleared distance.

Yes, indeed. (In certain circumstances, it's possible to calculate the
altitude rather than measure it, but that's an additional complication that
I suggest we avoid at present). And, of course, you measure the altitudes
of BOTH bodies involved, though the Moon is by far the most important
because of its large parallax. Then this allows for correction of the two
refractions as well, not very significant compared with the parallax
corrections, but it's still important to get them right.

Because the amount of the Moon's parallax correction is about 1 deg x cos
alt, then to achieve an accuracy of 1 arc-minute in the parallax correction
only calls for an accuracy of 1 deg in the altitude in the worst case, so
it's an undemanding observation. That is why I have presumed that any error
in the parallax correction itself is quite negligible.

>The cleared distance is a
>function of _both_ the altitude and the observed distance.  In the
>hypothetical case here, of completely stopped motion due to the earth
>spinning twice as fast, it would be the magic ingredient that allows
>one to map two identical observed distances to two different cleared
>distances.

You've got it!

>I suppose right around meridian passage of the moon might be a bad time
>to take a lunar, especially where it's altitude is about 90 degrees?

Why do you say that, Fred? It isn't so, though it's what I have argued
until the last few days. Although the AMOUNT of the parallax changes little
around meridian passage of the Moon, the DIRECTION of the displacement of
the Moon by parallax is changing very fast, and so, therefore, is the
component, along the Moon's path, of the displacement between true and
apparent Moons. The "clearing" procedure is clever enough to work all this
out, and get from the uneven apparent motion to the regular true motion, at
all altitudes of the Moon.

George.

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

   

Reply