The altitude of the sun should be measured from its centre, but it is difficult to decide exactly where its centre lies. Measure either from the bottom (lower limb) or top (upper limb) of the sun and apply a small correction. The sun's semi-diameter varies from a minimum of 15' 46" to a maximum of 16' 18" with a mean of 15' 59".6. So add 16' when you measure to the lower limb and subtract 16' if you measure to the upper limb. Semi-diameter applies only to sun sights but not sun sights taken by a shadow astrolabe.
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