Celestial Navigation · Sun — Lower vs Upper Limb
When can an upper-limb sun sight be preferable to a lower-limb sight?
- AThe upper limb is always more accurate
- BUpper-limb sights need no semidiameter correction
- CWhen the lower limb is obscured (e.g., by cloud or haze low on the horizon), the upper limb is observed and the upper-limb correction applied✓ Correct
- DOnly at meridian passage
Explanation
Navigators normally bring the sun's lower limb to the horizon, but if it is obscured the upper limb is used. The almanac provides a separate upper-limb correction (semidiameter subtracted rather than added) to reduce the sight to the sun's center.
Authority: Nautical Almanac, sun altitude corrections; Bowditch (Pub. No. 9)
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