20 miles isn’t bad; I was pretty happy to be that close on my first real attempt. That was done at a time when the horizon was visible, though. The second mate, who was pretty good at navigation, would occasionally get her sextent out at twilight, when the first bright stars were out and we could still see the horizon. (I don’t know which stars they were, or what could be determined by their elevations.) I never saw her take a sighting after dark.
Is 20 miles close enough? If you’re trying to find an island with a mountain on it, that should work. I can understand why navigators wanted to be a lot more precise, too.
Well, yes, if you want to measure the altitude of a star above the horizon, you need to do it during twilight in order to see both the star and the horizon, or else try to use an artificial horizon.
NB a marine sextant was first implemented around 1731, according to Wikipedia; in any case, such accurate instruments are not ancient.
Also, for those half-mile positions in both latitude and longitude, you will need a pretty good almanac and/or chronometer, not merely knowledge that the earth is roughly spherical and a so-so estimate of its radius.
The sextant isn’t ancient, but it’s the culmination of a long technological evolution of simpler astrolabes. And if you have the choice of a high-precision tool or a low-precision tool, of course you’re going to choose the high-precision one, but even the low-precision one might be good enough to get you where you’re going (with maybe a day or three added for a less-direct route and the need for more course corrections as you get close).
Still not ancient, but I found mention of an astrolabe from 771 by Al Fazari, and al Khwarizmi designed his staff and his quadrant in the 9th century. However, neither of these were considered quite adequate for work at sea by Arab pilots, who developed the wooden rectangle + knotted string already mentioned for measuring altitude (for one thing, it is easier to feel knots in the dark than to try reading something). The polar star was about 3½° off at the times we are talking about, but (like today) they made appropriate corrections, and it is known that differences in altitude of the polar star at the beginning and end of a navigational cycle were converted into distance (today we would say 1 degree = 60 nautical miles). These people did sail out into the Atlantic ocean sometimes. And so on, plus the astronomical tables coming out of Baghdad, Samarkand, and Toledo—back to the OP, it is not even a question whether anyone relied on the presumption that the Earth is round.
Columbus was just a bad navigator, among bad other things.
Was he? The basic problem is he had no clue where he was going (even if he had gone to the intended destination). I’m not aware if Marco Polo had mentioned the latitude of China, nor whether the “spice islands” of Indonesia had a known latitude in Spanish lore. Perhaps description of the climate was a rough clue. If we accept the theory that he knew of western (“eastern”) lands from the stories of Basque fishermen reaching the Grand Banks and the Gulf of St. Lawrence, he knew that was too far north and the land was beyond any grand civilization.
I hope this isn’t a drift too far, but where did Erastosthenes get the idea that the earth was round? To know that back then must have presumed some kind of gravity/force theory to explain why people on the “bottom” of the earth didn’t fall off, right?
It seems to have been the common assumption of astronomers at the time, since it explains how you can see more stars when you travel south. In the UK we never see the ‘sting’ of Scorpio, but in Greece it is gloriously visible; the stars of Argo were known to Greek astronomers, but they are mostly invisible north of the Alps.
Aristotle had a theory of gravity, but it was wrong; he said that heavier objects fall faster than light ones, but some of his contemporaries disagreed. Not until Newton did anyone formalise the fact that the mass of both objects (the falling object and the Earth) must be considered.
Wikipedia says we don’t know for sure how the ancient Greeks got the idea that the earth was spherical. It may have come from observation of the altitude of the circumpolar stars on long voyages to the north and south. By Eratosthenes’s time Aristotle had written on the subject and presented several pieces of evidence, including the altitude of stars at different locations, and the fact that the shadow of the Earth on the Moon during a lunar eclipse is always circular. Aristotle did believe that terrestrial objects were drawn toward the center of the earth, although not because of a force of gravity, but because that was their “natural place”. Stars and planets were not attracted to the Earth because their “natural place” was in the heavens.
If I am wrong, I will admit it, but is there a summary/estimate of where he actually was (to the extent anything can be determined about it) versus where he thought he was? His journal from 1492–3 has stuff like “Here the pilots revealed their estimates of position: the pilot of the Niña found himself 440 leagues from the Canaries, the pilot of the Pinta, 420, and the pilot of the ship in which the Admiral sailed, an even 400” but I can’t make out what his true estimate of his position was or how far off it was by landfall (100 miles? More?)
As an aside, we sometimes talk about “unification of forces” as something sought by modern physicists, but the first force unification theory was Archimedes’ work on buoyancy, which unified the forces of gravity (which makes heavy things go down) and levity (which makes light things go up).
Though of course, Archimedes was long after Eratosthenes.
There are people yet today who think the earth is flat.
It wasn’t universal then either, thousands of years ago. The zodiac depiction of the stars and heavens found at Dendara is probably what, 5000 years old?
Most people today wouldn’t know where they are or even what day it is after about 6 months or so without a calendar. To realize people thousands of years ago had mapped out planetary procession and all that stuff tells us they knew the earth was a globe.
Before chronometers, that is actually not bad at all over those distances. Mind you, I have heard both sides on this, and I am not sure which side is correct.
His navigational genius was evident from the beginning as he headed southward to the Canary Islands rather than due west, where other ships had stalled. He then crossed the Atlantic and on October 12 made landfall in what is now the Bahama Islands.
The first real navigation charts were Portolan charts. They did not consider the roundness of the earth, but over such short distances, that wasnt such a big deal,
At this time, mariners began to realize that maps would be helpful and began keeping detailed records of their voyages that land-based mapmakers used to create the first nautical charts called Portolan Charts (c. 13th Century). The charts, created on sheepskin or goatskin, were rare and very expensive, often kept secret so that competing mariners would not have access to this knowledge. What they lacked in accuracy they made up for in beauty. Lands and ports on the chart were highly decorated with depictions of buildings and flags.
The size of the lands on the chart was more a reflection of their importance to trade routes than their actual geographical size. The charts did not have latitude or longitude lines but did have compass roses indicating bearings between major ports. They were, of course, not very accurate because the ability to measure distances at sea had not yet developed, nor was there an accurate method to portray the spherical surface of the earth on a flat piece of material. At this time, mariners began to realize that maps would be helpful and began keeping detailed records of their voyages that land-based mapmakers used to create the first nautical charts called Portolan Charts (c. 13th Century). The charts, created on sheepskin or goatskin, were rare and very expensive, often kept secret so that competing mariners would not have access to this knowledge. What they lacked in accuracy they made up for in beauty. Lands and ports on the chart were highly decorated with depictions of buildings and flags.
The size of the lands on the chart was more a reflection of their importance to trade routes than their actual geographical size. The charts did not have latitude or longitude lines but did have compass roses indicating bearings between major ports. They were, of course, not very accurate because the ability to measure distances at sea had not yet developed, nor was there an accurate method to portray the spherical surface of the earth on a flat piece of material…The first accurate representation of the spherical earth surface was the Mercator Projection (Gerardus Mercator 1569). Of great value to navigators because a compass bearing could be shown as a straight line (and they could, therefore, sail the shortest distance between two points), but the problem of determining longitude delayed the use of these charts for some seventy years after they were introduced.
This is an area I pretty much have to rely on the experts to answer. I’m not a specialist in early modern Europe or sailors in general. Christopher Columbus was a garbage person, but sometimes garbage people are good at things. I can’t recall any scholarly work I’ve read suggesting Columbus was a bad navigator in general. They all agree he underestimated the circumference of the Earth, most agree he was a jerk, but he knew his way around a ship.
I have read that they realized that a lunar eclipse is just an earth shadow and that shadow was of a round object. Also, it is an easily demonstrated fact that on a given day, the further south you go the higher the sun gets in the sky. Only a round earth is consistent with that. I believe that what Erastothenes did was observe that the sun at Aswan was directly overhead on midsummer noon, while at Alexandria, due north, it was 7 degrees from overhead. Now you have to measure the distance between them. One possibility is that drive a chariot between them and count the number of turns of the wheel. Since you can measure the circumference of the wheel, that should do it. So long as you don’t lose count. One uncertainty is that he reported it in stadia and we don’t know the exact measure of the Olympic stadium.
There’s a tool for that. At least, we know they had mechanical odometer wagons by Hero’s time, and the technique is fairly simple, and similar to parts of the contemporaneous Antikythera Mechanism.
Plus, don’t forget the Greeks in their obsession with geometry also considered the sphere something approaching perfection - so not unusual they would attribute the shape of the earth to a sphere. Also, observation of the stars showed they were sort of like an enveloping sphere enclosing the earth. As others mention, as you move north and south the view changes exactly as would be expected from moving on a sphere and looking up at another one.
yes, we’d be lost outdoors today - but we have exTwitter abd TikTok and TV and spend much less time just staring at the sky like the ancients. Back in the day, thinkers had plenty of time to stare and the sky and notice patterns,
Columbus, too, was asking his navigators how far they’d come longitude-wise, which was the part impossible to calculate in those days. All they could do was make occasional measures of the speed through the ocean (knots) and guess at the current they were travelling in. 10% difference is not bad, it shows they were actually keeping track.