Parallax is the difference between what you would see if you were standing at the middle of the earth - where angles should be measured - and between measuring the angle for real on the surface.
Parallax is included in the combined correction in the Nautical Almanac but there is no easy way for the Crash Bag Navigator to calculate it. For the sun and stars it is small enough to be ignored. By comparison the moon is so close to the earth its parallax can be significant. When overhead, its parallax is zero. When it is on the horizon its parallax is about a degree.
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