NavList:

George wrote:

".....knowing GMT, you look up in the almanac the predicted declination
and GHA of the body; Sun, Moon, star, or planet. (for a star that involves
adding GHA Aries and SHA of the star). Use its dec, with the observed alt,
and your known lat, to find the LHA of the body, in the equation shown
above. Subtract LHA from GHA. And the result is your (Westerly) longitude ....."

Ah! but its this bit which is the problem ... "and your KNOWN lat".

The PZX triangle explains all. Essentially the problem always has been that if you know your Latitude with certainty you can find (calculate) the longitude from a single observation with certainty.
And conversely, if you know your Longitude with certainty you can calculate your Latitude with certainty. Neither is possible with a single observation.
It required two observations.

This is why the traditional noon meridian passage of the sun was almost a religious requirement with sea navigators, as it is a relatively easy way of finding aquite accurate (but technically not exact) Latitude. This Latitude can then be used with other observations later for finding Longitude to sufficient accuracy for sea navigation use.

The various methods of finding Latitude or Longitude separately became obsolete with the discovery of position line navigation by Marc St Hilaire, where a single sight gives a navigationally useful position line; ...and two sights a fix - i.e. Latitude and Longitude simultaneously.

Douglas Denny.
Chichester. England.

----------------------------------------------------------------
NavList message boards and member settings: www.fer3.com/NavList
Members may optionally receive posts by email.
To cancel email delivery, send a message to NoMail[at]fer3.com
----------------------------------------------------------------