Seventeenth Century French Jesuit Longitude Determinations in Asia: The art of Rectifying Clocks

Two methods of setting clocks to show correct time during the 17th century by French Jesuits in
China are investigated and their computations and accuracy are analysed. These methods were used to
rectify the clocks used for timings of eclipses of the Galilean moons or for lunar eclipses that in turn were used
to determine geographical longitudes by Jesuit missionaries in Siam and China. It is found that the calculations
needed, in spite of being quite complicated, were well done and very accurate.