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    Re: Backsight of the Sun
    From: Sean C
    Date: 2023 Dec 4, 18:16 -0800

    Frank Maxwell, you wrote: "I have done every problem in [Leonard Gray's book 100 Problems in Celestial Navigation] 100 times except problem 4-8 on page 28 concerning backsights."

    If you turn to page 95 of 100 Problems, you will find the following passage:

    The Backsight

    The backsight is rarely needed (and can't always be used), but can be handy if a body is visible, but the horizon under it is obscured. It's described in Bowditch and other texts. The way you do it is to turn your back to the sun (or moon, planet, or star) and measure the angle from the opposite horizon up and over to the body.

    There are two things to note. First, the result of 180° minus the actual altitude must not exceed the range of your sextant (probably close to 120°). Second, if the sight looks like an upper limb one, it's lower limb, and vice-versa. A rough diagram should make both of these things clear.

    Working the sight is not complicated, but it has some traps. There are two ways to find Ho. The one the author likes better is:

    (a) Apply the normal I.C. and dip corrections to Hs.

    (b) Subtract the result from 180°

    (c) If it looked like a normal sun or moon LL sight, apply the UL corrections, and vice versa. For a star or planet, apply the normal refraction correction for the actual altitude.

    Mr. Gray does not elaborate on the "second" method.


    Sean C.

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