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Gary LaPook writes:

You just rediscovered the "time sight." However, as Sumner discovered,
the calculated longitude is only accurate if you have an accurate
latitude from which to do the computation, error in assumed latitude,
error in computed longitude.( Except on two days of the year.)

Gatty's "The Raft Book" has tables for calculating your latitude from
the duration of time between sunrise and sunset and, I suppose, you
can do the same calculation with the sunrise-sunset tables in the
Nautical Almanac. The Air Force survival manual, AFM 64-5, has a
nomogram that does the same calculation. Of course, neither of these
methods work near the equinox.

gl
On Mar 7, 10:43 pm, "Peter Fogg"  wrote:
> I was just sitting here using my calculator to do this as Paul's
> suggestion arrived.
>
> It seems fitting to assume that 40 South is at that latitude, and have
> assumed E152d of longitude, in a small boat from which the sun is
> observed from an elevation of 3 metres.
>
> Tomorrow 9 Mar 2008 sunrise is observed at 05h 43m, and sunset at 18h
> 21m (local time).
>
> If the position was S40d 00'  E153d 00', then sunrise at 05h 38m and
> sunset at 18h 17m (your mileage should NOT vary).
>
> So the answer is yes; with the accurate time longitude can be
> calculated via observation of sunrise and sunset.
>
> On Sat, Mar 8, 2008 at 5:20 PM, Paul Hirose  wrote:
>
> > 40 South wrote:
> > > Supposing one was in a small boat with an accurate timepiece and the
> > > necessary tables, how accurate could you determine your longitude by
> > > observing the rising or setting of the sun or any other celestial
> > > body?
>
> > You could run an experiment using one of the online calculators at the
> > U.S. Naval Observatory site. Enter a latitude and longitude (some
> > convenient integer number of degrees) and note the calculated time and
> > azimuth of sunrise. Then try changing the latitude or longitude by one
> > degree and see how much difference it makes.
>
> >http://aa.usno.navy.mil/data/
>
> > The third link down is probably the best for this.
>
> > --
> > I block messages that contain attachments or HTML.
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