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Dan Allen said-

>...the question is, how does one calculate refraction at higher >altitudes
>on land?  Do I just use the actual barometric
>pressure or do I somehow factor in elevation explicitly?
>
>It is my understanding that the more atmosphere, the more refraction,
>>hence higher elevations have less pressure and thus less
>refraction.  Bowditch and other sources do not say much about >refraction
>as a function of elevation above sea level.  I did notice
>one comment about Air Almanacs having extra columns in their refraction
>>tables to allow an elevation to be entered, but I do not
>have an Air Almanac to check on this.
>
>My guess is that if refraction is stricly due to atmospheric thickness,
>>the barometric pressure is sufficient, but if there are
>geometrical aspects to refraction (which I know there are since there >is
>no refraction for a body directly overhead), then elevation
>much above sea level needs to be accounted for separately from >barometric
>pressure.  The question then is, by how much?

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

George Huxtable responds-

As I understand it, refraction is, rather strictly, proportional to
atmospheric pressure. So all that Dan has to do is to reduce his
refraction, from the standard tables, by an (off-the-cuff) amount of about
3% for every 1000 ft of elevation. So at 1000 ft the refraction at 10 deg
angular altitude would be reduced from 5.25 minutes to about 5.1 minutes.
For greater angles of altitude the reduction would be correspondingly less.
So not very significant, unless the observer is working to a very high
level of accuracy.

As Dan points out, refraction varies with the angle of altitude, becoming
zero when straight overhead. But that doesn't complicate the allowance for
altitude as above. Whatever the sea-level refraction was, just reduce it in
proportion to the pressure.

Presumably a reflective pool is being used as an artificial horizon, in
which case high accuracy is indeed possible. Several errors, that are
inseparable from using the real horizon as a reference, vanish in this
case.

The only other effect that Dan would need to keep an eye on is that of
temperature, because in his mountain retreat Dan may experience abnormally
cool air.

Elevation is not normally a concern of mariners, so its effects are not
considered in the navigation textbooks. I wonder if astro-navigation ever
played an important role in navigation on the North American Great Lakes?
Even there, the elevations don't exceed 600 feet, so the impact on
navigation was negligible.

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

In another mailing, Dan asks the inverse question-

Does anyone have any clever ideas about determining one's elevation above
sea level using a sextant?  What if one knows one's
latitude and longitude exactly -- would that help?

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

Comment from George-

I have no clever ideas about this. Dan doesn't say if he has a view of the
sea from his mountain. If so, that would make a big difference.

Because the elevation has so little effect on the observed position of
objects in the sky, then presumably such observations are a very bad way to
determine elevation. An aneroid barometer would do the job much better. But
if latitude and longitude are accurately known, surely a map would give a
good answer. Here, I am thinking of the UK, every square inch of which has
been surveyed to death. Perhaps things are different in the vast expanses
of the American NorthWest.

George Huxtable.




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