Long by chron

[neil_s]

Rumata's post just below got me thinking about the Longitude by chronometer method, where you start with a DR Latitude. Your sun sight allows you to calculate the LHA. The difference between that and GHA from the almanac  gives the Longitude. I am still bemused by the process of taking the Long by chron in the morning and then having to correct it when you take the meridian altitude at noon. Why not take it in the afternoon, when you already have the noon Latitude? I guess the master wanted a noon position and no other time would do! Anyway, you might have noticed the Endurance 22 expedition to find the wreck of Shackleton's ship in the Weddell sea. Three guys, Bergman, Mearns and Stuart have written a pertinent paper examining errors that may have been incorporated into the measurements that Frank Worsley, the ship's Master, made to fix the position when she sank. They site errors in the star maps and nautical almanac for 1915 and suggest that the ship will lie about 3 Km away from Worsley's position. To get back to Long by chron - this was the method Frank Worsley was using and in their paper, Bergman, Mearns and Stuart say "Ideally, the time sight should be made when the body being observed lies on the prime vertical, due East or West as the longitude obtained is independent of the estimated latitude and any uncertainties therein" Am I right, then in thinking that this method not only gives your position without having to do any plotting, but also is not sensitive to the latitude estimate, if you shoot the Sun when it lies due East? Bergman, Mearns and Stuart observe that Worsley did not do this, even though the Sun would have been high enough in the sky for a good sight. Long by chron might be making a comeback!