When reading a sextant, you must account for the height of your eye above sea level (known as “dip”). This is done by calculated at 0.98 x square root of (height of eye above sea level in meters x 3.28)
Sextants are sexy
An eBay search for “sextant” (a traditional tool used for nautical navigation) is apparently related to things like “sexytrash”, whatever that is. I think that tools like this are sexy, but not the kind eBay thinks.