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From: Chuck Taylor (no email)
Date: Wed Aug 04 2004 - 11:36:20 EDT
--- Andrew Corl <> wrote:
> Is there a way to use the GHA for the sun from the
> daily page from one
> of the online almanacs to calculate noon GMT, and
> then use the table
> "Convert Arc to Time" to calculate LAN? Is that
> possible?
By definition, LAN occurs when GHA Sun equals your
longitude. If you know your longitude, you can go
into the nautical almanac to calculate the time at
which LAN occurs at your location. Figuring LAN this
way, and taking a noon sight of the sun at that time
to calculate your latitude is a common practice.
In theory, if you had an accurate timepiece and could
observe the exact instant that the sun crosses your
meridian (when it reaches its highest point), you
could then calculate your longitude by converting
time to arc. In practice, it is very difficult to
tell the exact instant when the sun reaches its
highest point, so this method is impractical. You can
get an approximation good enough for "lifeboat
navigation" by the following procedure:
1. Record the altitude of the sun a few minutes before
LAN and note the time.
2. Observe the maximum altitude of the sun (to get
latitude).
3. Set your sextant to the altitude recorded in step
1, and note the time when the sun descends to the same
altitude.
4. The time of LAN is then the average of the times
noted in steps 1 and 3. From this you compute
longitude. This assumes you didn't move very much
between the times of steps 1 and 3.
The more traditional way of determining longitude was
to use a time sight at the time the sun crosses the
Prime Vertical (i.e., the time at which the sun is due
east or due west of you). This procedure is described
in Bowditch and elsewhere. It requires that you know
your latitude, which you can get from a noon sight or
from an observation of Polaris.
Chuck Taylor
North of Seattle
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