NavList:

Thanks Reed, much appreciated for your detailed explanation on the issue. I think I have pretty much understood your point,treating the problem of flight altitude or earth oblateness is just like treating a standard parallax correction.  I was following your idea and doing a quick calculation to get a good sense on how much the parallax value would be affected when an additional small height is been added or subtracted to observer's distance to earth's center, representing one's location in different altitudes. Assuming the moon being observed is at the celestial horizon,also the lunar-earth distance is at its shortest:about 363000km. These two factors would result in the parallax effect being the most noticeable, then P=asin((6371+alt)/363000)) ,alt ranges from( -20km to +20km),the range should cover ground levels in both Poles as well as to exceed any aircraft's max ceiling when measured directly above the equator. With a little calculation,a variation range of only 00′22.74" is found, so one's practical altitude difference will only offsets the parallax value of about 0.6%(at max), should the moon be observed at higher angles and the observer not in geographically extreme regions we would expect to see an even lesser offset. So in general,when one is doing his corrections to sextant heights,parallax,refractions or other corrections should outweights altitude correction,since the latter two would only introduce a maximum error of 0.23 nm, almost negligible.