Posted by Andy Clayton on October 28, 2007 at 12:52:46 from 83.104.41.160 user cousin_jack.
In Reply to: Re: Longitude, was Time Ball, was Signal Stations posted by Mike Field on October 28, 2007 at 12:00:57:
People have mentioned taking latitude at noon and doing 'the sums'. (You subtract the horizon/sun angle from 90 to get the lat.) A safer, more accurate and easier method is to sight the pole star at night.
a) It's safer as there is no danger to eyesight, all those old seamen and pirates were wearing eye-patches for a reason!
b) It's more accurate as you are observing a small point of light which doesn't move much. It does describe a very small circle but for navigation purposes it's as good as being motionless.
c) The angle you measure to the pole is the same as your lat. At the north pole it is straight overhead ie 90 lat, at the equator - if it could be seen, it would be horizontal or 0 lat.
One of the reasons early mariners, pre C15th, were reluctant to follow the Atlantic coast of Africa southwards was the fact that their guiding star was dipping down too low to take obsevations to. Other reasons such as the increasing heat and desertification of the lands and the north-easterly trades threatening to impede the home run only added to their woes.
Navigation is a fascinating subject and modern gps takes all the satisfaction (and frustration) out of doing it. I remember years ago doing an astro-survey, as an exercise, and it took the best part of an hour just to compute the time relationship between ourselves and the recorded time position for the star in the almanac, the positional relationship took even longer but the calculating devises in those days were more primitive than what's available today.