Dear Frank and all,
For measuring the sun's altitude at sea that well-tried instrument, a
sextant, works extremely well. When the sun is almost overhead one is
surely in the calm of the tropics and away from a heaving deck. But you
can use a sextant effectively even in a small yacht in bad weather, anyway.
I have a little book ("A Manual of Modern Navigation" by S. M. Burton,
1941) with a chapter on the particular case of very high altitudes. It
declares: "It is a graphical illustration of the principle underlying
all ordinary astronomical methods".
It's over sixty years since I used this method but recall that you take
and time the sun's altitude as it approaches noon while facing eastward,
then repeat at about the same short interval after noon facing westward.
Burton gives a table of the sun's actual bearing with zenith distances
from 0 ° 10' to 1 ° 40' at intervals from 2 mins. to 10 mins. from noon.
But sweeping your sextant along the horizon will sort this out, anyway.
On land I believe a theodolite can be used to give very accurate sun
altitudes. But a sextant can also be used on land with the aid of an
artificial horizon. Treacle is very effective for this purpose and gives
a good reflection of the sun.
Frank 55N 1W
On 28/07/2011 14:54, Frank King wrote:
Dear Frank,
I enjoyed your message about determining your
position when the sun is close to overhead.
Your theory is sound and I certainly cannot
compete with your practical experience but
something bothers me...
You measure the sun's altitude assuming
it is within a few minutes of arc from
overhead...
How do you measure the altitude of the sun?
On land I could measure the altitude with
a vertical stick on flat ground but it is
hard to be sure that the stick is vertical
to within an arc minute or two.
On the deck of a heaving ship, when you
don't even know which part of the horizon
to face, I would guess a sextant would not
be too useful.
What's the trick? How did you do it?
All the best
Frank
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