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    Re: Obtain a fix when you don't have an clear AP.
    From: John Brown
    Date: 2015 Jan 22, 22:52 +1030
     
     Sean
     
    Another day another rule. This time you have to ask yourself if Rigel was east or west of the meridian when you took the sight. If the body has a GHA that is greater than your longitude then it must be to the west of you and so you subtract the LHA to get back to your longitude. If the GHA is exactly the same as your longitude then the body is on the meridian and if the GHA is less than your longitude it is to the east and LHA must be added to get to your longitude.
     
    As others have pointed out the line AB on the plot represents a very small fraction of the circumference of a circle and appears straight because in global terms it is so short.
    Usually the concept of equal altitudes is explained with the flagpole. If you were to draw a circle of 30 feet or so around the flagpole and you were to measure the angle to the top of the flagpole from any number of points around the circumference then each measurement would be the same. In the same way anyone on the line AB or  on the extension of that line would get exactly the same altitude as you did provided the sights were taken simultaneously. A person on a ship at [8] 11.4 S [138] 59.3 W would see Rigel right over his head. He is at the star's geographical position on an imaginary line joining the center of the earth to the star. He is at the base of the flagpole.
     
     
    Regards,
     
    (the other)
     
    John Brown.
     
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