Message text written by "Arthur Carlson" >But how can it be that hard to determine the longitude of, say, Mexico City with an accuracy of at least 10 degrees, even given only 16th century technology? I would imagine sending a couple grad students over to record the time of day (night) that various stars disappear and reappear behind the moon, sending the tables back to Spain where similar observations were made, and setting the brightest mathematical minds of the empire to work comparing the two sets of observations to come up with a fix on the longitude.<
The whole matter of longitude was a major problem. At the time of Columbus most marine navigation was done by moving North /South to a required latitude (determined by the altitude of the sun or pole star) and maintaining course on that until one arrived at one's destination. Indeed it was only in 1484 that the Portugese started to try and find a way of navigating in the Southern hemisphere where there is no pole star - the so called 'Regiment of the Sun' was the result. At that time and for centuries later too all navigational knowledge was regarded as a great military secret and therefore there was little dissemination of information in Europe on such matters. The problem of estimation of longitude (anywhere let alone at sea) wasn't really solved until nearly into the eighteenth century. It wasn't as if there had been techniques available before then other than to use an accurate clock, it was simply that such alternatives - like using knowledge of the moon's position - were only being developed at that time too. It wasn't until the seventeeth century that Galileo first tried to use the motion of Jupiter's moons as a universal clock but it came to nothing and he had religious problems with his ideas of the solar system, resulting in his house arrest. Cassini around 1666 published his first accurate tables of the Jovian moons and by 1693 the problem of finding longitude at places where a telescope could be held rigid was solved and measurements made to a few tens of minutes of arc possibly even down to 15 mins by making a whole series of observations - something that required skill and experience - not probably something that any 'graduate student' of the time might be expected to be able to do. I suppose that if the right people had been available in the right place at the right time one might have been able to make an estimate to a few degrees a bit earlier but I don't think that it would have been possible a century earlier especially since Spain wasn't in the forefront of the technology at that time. Use of the moon's motion came later because the moon's motion is so complex - indeed the main competition to Harrison's clocks in the 1700s came from the lunar tables that had by then been calculated. As if all this isn't enough I do not think that in the sixteenth century there were any accurate ways for measuring the time of occulation of a star by the moon. I also believe that in the earliest days astronomy and marine navigation were two nearly separate 'sciences' that didn't really communicate with each other. All in all I think it was a combination of lots of reasons. Hope that helps Patrick
