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Murray Campbell asked-

> I though i would begin by learning the noon shot, but
>i am at 52 north and the sun is so low and flat right
>now and i'm just learning to use the sextant....i will
>try to take the sites way before and after noon and
>average them as George mentioned in the recent LAN
>discussions.
>
> 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 you are at the very early stages of learning to use the sextant, I would
start off by measuring only the latitude (by the Sun's maximum altitude)
and put off measurements for longitude until later.

But now for your questions-

The usual recommendation (with which I would agree) is to keep the altitude
of a body above 10 deg if you can, and certainly above 5 deg.

The problem is that the refraction correction grows at lower altitudes.
When you get down to 10 deg, the refraction is already more than 5'. That
would be acceptable if the refraction were completely predictable, but it
isn't. You get bands in the atmosphere at low angles which sometimes show
up bt causing the disc of a low Sun (for example) to become rather squashed
and distorted. So the refraction correction can become inexact. However, at
10 deg alt, any such error is unlikely to exceed 1 minute.

Even at midwinter from your latitude of 52deg, you could take
equal-altitude sights about 2 hours before and 2 hours after noon, at which
the Sun altitude would be about 10 deg, so you shouldn't find your latitude
to be a serious limitation.

You could make deliberate equal-altitude observations as I suggested, or
simply measure Sun altitudes morning and afternoon at somewhere around
10:00 and 14:00 GMT without any need for the altitudes to be exactly equal,
and simply work out intercepts and cross two position lines (perhaps with a
third position line at noon). It comes to the same thing, more or less.

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

Correcting for the time difference between max altitude and local apparent noon.

If you are going to use the equal-altitude method discussed in that
previous mailing, remember that it gives you the moment of maximum
altitude, and not moment of local apparent noon. So you have to make a
correction for the time difference between them. In the previous example we
considered a ship doing 20 knots toward the Sun, at winter solstice. Winter
solstice was chosen because then the Sun itself wasn't moving in
declination.

But even for an observer stationary on the ground, as I presume you will
be, at times other than the solstices the Sun's declination will be
changing. At the spring equinox it will be moving toward you at its
fastest, which is about 24' per day, or very nearly 1 knot. You can work
out the rate of change from the almanac, then use the same formula I gave
for a moving vessel to obtain the time difference between max. alt and LAN.
Of course, the correction will be a lot less than it was for the fast
vessel.

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

Instead of the Sun, you could use other bodies, stars or planets: this
would be forced upon you in Winter if you lived a lot further North than
you do. But you can't use equal-altitudes, because stars and planets are
visible over only a short interval, around dawn or dusk.

Instead, at dawn or at dusk, measure altitudes of two or more stars or
planets, in different directions around the sky, work out the intercepts
from an assumed position, and cross the position lines to give your
position. This can be very accurate.

George.


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