Here is an article from Discover on old ‘portolan’ maps. I’ve been interested in them since childhood because of this book, which I pulled off the shelf at the Whittier Public Library sometime during elementary school period.
John Hessler, the cartographer upon whose work the article in Discovery is based, believes that ancient cartographers could have obtained the remarkable accuracy seen in portolan maps by very carefully compiling the notes and compass directions used by the pilots of the day, and iterating. Charles Hapgood, the author of the book from my childhood, took the position that, in order to account for the amazing accuracy of the maps, the cartographers would have had to have had access to accurate longitudinal reading on the various locations.
Digression: if I have a sextant, and can see the North Star, I can get a pretty darn good reading on my latitude – how far north or south I am. (at least, in the Northern Hemisphere). Finding latitude is a simple, well-understood process. But to find my longitude – how far west or east I am – I need a very accurate clock in addition to a sextant. Say I take a reading on a star above the eastern horizon. Unlike the North Star, which appears to stay put at almost exactly true north, this eastern star appears to move as the earth rotates. In order to determine my position east or west, I need to know at exactly what time the star appeared to be, say, 30 degrees off the horizon.
The way that works: my clock – a marine chronometer – is set to the time at a place at which the apparent distance above the horizon of many star has been carefully measured at precise times. Say, Greenwich, England, where, not coincidentally, the Royal Observatory is located, and through which runs the Prime Meridian. So, I know that, at Greenwich, the star I’m looking at at precisely 10:00 p.m. Greenwich Mean time is, say, 20 degrees above the horizon due east. But it’s 30 degrees above the horizon where I’m looking at it through my sextant. Using the difference between where the star appears in Greenwich and where it appears to me at the exact same time, with a little math, I can figure out where I am east and west relative to the Prime Meridian – assuming my clock is very accurate. A few seconds off can mean miles, or safely out at sea or on the rocks.
So, Hapgood assumed that there was no way to get the degree of longitudinal accuracy without very good timepieces. Therefore, he asked: who had such great clocks? and then speculated on some lost ancient race of navigators. All very exciting – and far-fetched.
Now Hessler proposes a more mundane explanation, which merely requires a vast catalog of navigator’s notes and great patience and attention to detail. Not sure we should declare the mystery solved quite yet. I suppose some place like Genoa or Venice might have the navigational notes, and certainly Italy has the geniuses, and they did lots of stuff – cathedrals, mosaics – that took insane perseverance and attention to detail. So, maybe it would work. Must think about it more.
Anyway, fun stuff!