NavList:

Frank Reed you recommended using meridian passage for calibrating sextants.  Yes, that would be good.  I’ve noticed shooting the Sun around ‘mer-pass’ time with a pendulous reference sextant suitably mounted in a static situation and the Sun’s lower limb placed on the horizontal line of the graticule, the Sun barely seems to move vertically for several minutes.  That’s great for finding latitude but hopeless for finding longitude unless you record for 30 minutes either side of ‘mer-pass’, plot on graph paper and fold the paper in two to find the best fit as you advise.  There must be great scope here for a lunar and a ‘mer-pas’ sextant calibration app along the lines of GPS Anti-spoof although probably not much money to be made from it.  DaveP