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On 11 Jan 2005 at 15:02, Frank Reed wrote:

> "The airport elevation at Rocky Mountain House, Alberta is 3,242  feet."
>
> Which would decrease refraction values (absolute values) for all altitudes
> below 45 degrees by more than a tenth of a minute of arc --and increasing
> to 0.5  minutes of arc at 10 degrees altitude (10% of the standard sea
> level values).  Did Thompson correct for this?

I don't think so, but his journals are just too devoid of details to know
for sure. He always records temperature so I might be able to discover
whether he takes that into account by re-doing one of his calculations
where the temperature is quite extreme.

> Most accounts of lunars were
> written for observers at sea level, literally. Of course, if he measured
> barometric pressure  at all, I suppose it would necessarily have been with
> a barometer calibrated at  low altitude which would account for refraction
> at mountain altitudes automatically (assuming he had tables that covered
> such low pressures).

There are some instances where he determines barometric pressure by
measuring the boiling point of water, but only in high mountain passes. I
doubt that he accounted for pressure elsewhere on his travels. I was just
wondering if the apparent systematic error in his latitude data might be
from neglecting the altitude.

Ken Muldrew.

   

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