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Basic
Idea: The sun is always over SOME where – at some point on Earth , right
now, you can look directly up - and there’s the sun. If you
had a book of data with where this point was at each moment , and you had
a good watch, and if you were lucky enough to be right under the sun –
why , you’d know where you were. This is
the geographic point of the sun.
Next
step is to consider yourself in a boat during the Equinox – that moment
(twice a year) when the sun is directly over the equator – and it’s
noon where you are , so the sun is as high as it can get. You take out
your sextant, and find the sun is, indeed, 90 degrees high. You look at
your nautical almanac , and
see that for the time on your watch the sun is exactly over the equator.
In other words it’s right over your head as it’s also right over the
equator – which means you’re on the equator also
–
Now,
imagine at
the same moment someone in another boat (or they could be on a mountain or in a plane)
who’s been sailing south from Miami for days takes
a sextant reading and finds the sun is 85 degrees over the southern
horizon. It’s noon for them too, so the sun is also as high as it will
get. This 5 degree difference says they are 5 degrees north of the point
under the sun, or 5 degrees north of the equator.
Notice they had a general (a dead reckoning) idea of where they
were –
So here
are your tools – a nautical almanac , a good watch , and a means for
taking exact readings of the height of the sun.
Things
get more complex if the sun is not as high as it will get (if it's not
noon) – but I’m
not going to try to explain that – it’s been done - Mary Blewitt wrote
my favorite book on the subject and you should go here http://www.celestialnavigation.net
to buy it through Amazon. The same site will also give you far more detail
on what I just discussed.
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