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Thanks Greg and David for your kind explanation. Indeed, I've also findout that it's relatively eazy to achieve a good tangency if I were to shoot a limb sight with AH, and one doesent even need to rock his sextant to ascertain tangency since the direct sun image itself is a good reference to check perpendicularity. Failure to align,or 'skewer' the direct and reflected sun images on a vertical 'string' is a telltale sign that one has his instrument held sideways.

Greg, since you have mentioned index error, one thing baffles me is that why index error tends to frequently change, or put in another word 'drift',sometimes on a daily bases, even if I had left the mirror settings on the sextant untampered with for month? I've heard about the problem on a post and am expriencing it right now. Yesterday I took down an IE as 3.2' by 4 successive sun observations, yet it was only 2' under a similiar circumstance on the day before. So what would be the key factor thatcontribute to the IE drift?

David,I shall try it again to see if I can still get a consistent accuracy.