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On 6/26/2013 5:30 AM, Sam Willis wrote:
> I understand that this might be done by shooting the sun when it bears
> directly east or west but I am uncertain as to how.

The first step is finding exactly when the sun will be 90° or 270°
from your assumed position. As you want a right triangle:

cos LHA = tan declination/tan latitude.

Make your best guess of when the sun will be 90 or 270 and use that
declination. You can fine tune it later by successive approximation.
(In use this method is relatively insensitive to latitude being off by
10', but relies on declination being correct for the time of the event.)

Sanity check
Tan Z = sin LHA /((cos LHA * sin Lat) - (cos Lat * tan dec))
If tan Z negative, add 180° to Z
If hour angle is less than 180°, add another 180° to Z

Use LHA and GHA to determine the time of the event from your assumed
longitude.

Then, as mentioned in previous posts, do a standard reduction. The
length of the intercept will give you the information to determine your
longitude assuming your latitude is in the ballpark.

Bill B

   

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