The first reference to western techniques of navigation in Japan is in the Genna-Kohkaisho or The Navigation Arts in the Age of Genna by Koh-un-Ikeda, written in 1618 just sixty-three years after the first Portuguese visit to Japan. During these sixty-three years the Japanese had built some western-style sailing vessels and actually used western techniques for navigation to south-east Asia. The author shows a good grasp of western navigation. For instance Ikeda had asked his teacher, the Portuguese captain Gonsaro (Japanese pronunciation) ‘Is there any method of obtaining the latitude when the sky is clouded for a short time just as the Sun or the star transits the meridian?’ Captain Gonsaro answers ‘It is of course impossible, no one in the world can ever do it.’ Ikeda suggests the possibility of estimating the altitude of the Sun or star on the meridian and one can see now that this foreshadows the famous ex-meridian method that was developed much later.