The maps were very accurate in terms of latitude (north/south position) which meant ships could find a particular port or harbor even if they had only a rough idea of their longitude (east/west position) by sailing along the latitude lines until they hit land. How big it made a particular piece of land look was irrelevant.
It’s not well known, but maps started to become super-accurate in terms of longitude as well in the 1670s or thereabouts. Astronomers had observed that Jupiter had eclipses of its moons about a thousand times a year, and thanks to Isaac Newton and others they were able to predict the exact times (in Greenwich England) of these eclipses a few years in advance and publish them. Observers around the world with good telescopes could view these eclipses and note the time differences between their observations and the published Greenwich times and use this to precisely calculate their longitude. It was not really possible to do this reliably on board a sailing ship because of the difficulty of keeping a high-powered telescope trained on Jupiter on a pitching and rolling deck (although many attempts were made) so this led to accurate maps but still not fully accurate navigation until the perfection of ship’s clocks a century later.
The other thing is on the Mercator, if you measure from point A to point B, and it gives you a compass heading, this is the correct compass heading to use to navigate to point B from point A. This is not true unless a map is designed for this purpose.
It won’t necessarily be the shortest distance – to plot that out, it is best to use a globe and chart the arc of a great circle path, which is a straight line on the curved surface of the earth. But that path will have the compass changing all the time.
If you are navigating by compass, which ships and airplanes had to do more often way back when, the mercator was great.
Posting for the benefit of @AAA@feddit.org and anyone else who didn’t know.
The maps were very accurate in terms of latitude (north/south position) which meant ships could find a particular port or harbor even if they had only a rough idea of their longitude (east/west position) by sailing along the latitude lines until they hit land. How big it made a particular piece of land look was irrelevant.
It’s not well known, but maps started to become super-accurate in terms of longitude as well in the 1670s or thereabouts. Astronomers had observed that Jupiter had eclipses of its moons about a thousand times a year, and thanks to Isaac Newton and others they were able to predict the exact times (in Greenwich England) of these eclipses a few years in advance and publish them. Observers around the world with good telescopes could view these eclipses and note the time differences between their observations and the published Greenwich times and use this to precisely calculate their longitude. It was not really possible to do this reliably on board a sailing ship because of the difficulty of keeping a high-powered telescope trained on Jupiter on a pitching and rolling deck (although many attempts were made) so this led to accurate maps but still not fully accurate navigation until the perfection of ship’s clocks a century later.
The other thing is on the Mercator, if you measure from point A to point B, and it gives you a compass heading, this is the correct compass heading to use to navigate to point B from point A. This is not true unless a map is designed for this purpose.
It won’t necessarily be the shortest distance – to plot that out, it is best to use a globe and chart the arc of a great circle path, which is a straight line on the curved surface of the earth. But that path will have the compass changing all the time.
If you are navigating by compass, which ships and airplanes had to do more often way back when, the mercator was great.
Posting for the benefit of @AAA@feddit.org and anyone else who didn’t know.
Thanks a lot for the explanation. I had no idea, but now with that knowledge it’s quite obvious why and how it works with that kind of map.