NavList:

   

Reply

I might be able to shed a little light on this subject; there seems to be a
fair degree of confusion.

It is well known that the altitude, alone, of a noon sight is sufficient to
yield the latitude of the observer.  However, if the exact time of local noon
can be determined, then the longitude is easily computed also, by simply
applying the equation of time, and then converting time to arc.
The problem is that the time of local noon is not easily observed, since the
sinusoidal curve of sun's altitude against time is at or near maximum, and
altitude changes very little for a period either side of the actual moment of
local noon, making it very difficult to say exactly when the sun is at maximum
altitude, and record the time, right "now".
However, the following technique can yield an acceptable estimate:-

1.  Compute the approximate time of local noon using the DR longitude.
2.  About twenty minutes before estimated local noon take a sight and record
both altitude and time.
3.  Continue following the sun with the sextant, as it rises, until it rises
no more.  As soon as the sun is observed to begin to decline, record the
maximum altitude reached.
4.  Now set the sextant exactly to the altitude recorded in step 2 above, and
then continue to follow the sun's decline, until the lower limb touches the
horizon.  Record the time at which this occurs.
5.  By symmetry, the actual time of local noon is now simply the mean of the
times recorded in steps 2 and 4.  As long as any displacement in longitude of
the observer takes place at a constant rate, it should not influence the
result.

with respect to accuracy of the above method, it can be noted that each 4
seconds of inaccuracy in estimating the time of local noon affects the
resulting computed longitude by one minute of arc.  I would guess most people
could at least do better than 2 or 3 minutes of error under favorable
conditions, and with an accurate chronometer.

Bob Young

   

Reply