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--- Murray Campbell wrote:
>
>  My beginner question is what the practical limits
> are for measuring altitudes accurately at high
> lattitudes?
> If the sun is above the horizon is it always useable
> or is it best (and more accurate) to switch to a
> body higher in the northern sky?

If the body is lower than about 15 degrees, and the
temperature and barometric pressure differ
significantly from 50 degrees Fahrenheit (10 degrees
Celsius) and 1010 millibars, then you would do well to
apply T&B corrections.  The table is near the front of
the Nautical Almanac and its use is described in
Bowditch and Dutton's, among other places.  This
adjusts your correction for refraction for
"non-standard" conditions.

By the way, in the Northern Hemisphere between the
September and March equinoxes, the maximum altitude of
the Sun is given by

     90 - (latitude) - (declination of the Sun)

At your latitude (52 N), the Sun does not reach 15
degrees in altitude at the winter solstice.

Chuck Taylor
 47 d 55.2' N
122 d 11.2' W


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