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In classical times (let's say 200 BC to 400 AD), how accurately could a Greek/Roman/Babylonian/etc. astronomer determine their location on the earth by coordinates like latitude/longitude?

Could they find their position to the nearest degree? Minute? Second?

I'm guessing they could find their latitude fairly accurately, but not have a good way of determining their longitude -- but I'm really not sure.

Note: This doesn't have to be at sea, it could be determining the position of a site on land. And I'm looking for how accurately they could determine latitude/longitude, not simply whether they could.

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Well, if they couldn't determine it, then I would say the accuracy is roughly zero. –  American Luke Jul 9 '12 at 22:41
@Luke, I'd have to agree with that. –  Joe Jul 9 '12 at 22:48
@Joe, I don't have enough info to compose a full answer but I'd like to bring up the case of Pytheas who is unfortunately known to us only by second hand relations (as is Himilco). One of the most interesting references on Pytheas is Barry Cunliffe's book (0140297847) and there is a fairly detailed discussion of latitude determination - and some evaluation of its precision - in wikipedia's article here‌​. –  Alain Pannetier Jul 11 '12 at 8:52
Some information about ancient latitude/longitude navigation system is in the 1421 book by Gavin Menzies. However, information in this book is certainly to be taken with caution, and I do not have my copy offhand. –  Jean-Christophe Dubacq Jul 13 '12 at 19:17
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1 Answer

Latitude can be calculated from observations of stellar objects (typically using something like an astrolabe) and a bit of math. The Greeks could do this as early as 150BC, but only on dry land. The Mariner's Astrolabe wasn't invented until around 1300 CE.

Nobody had a good way to determinte longitude in realtime aboard a ship before the invention of the marine chronometer in the early 1700's. The closest anyone came was the Chinese, who managed to work out the longitude of various places on the Indian trade routes in 1421 by placing observers on said places to observe various lunar and stellar positions simultaniously. This information may have made their maps better, but wasn't particularly useful to a navigator out of sight of land.

Before that, the typical technique used was dead reckoning, which was incredibly inaccurate. Basically, the navigator would chuck a hunk of wood out the back of the ship, try to estimate their speed based on their relative speed to the jetsam, and try to calculate their distance from the last time they did that based on that speed. Obviously this doesn't take currents into account at all, and any errors are likely to accumulate every time you do it.

What was typically done in the Medeteranian in ancient times was that navigators just kept themselves in sight of land. Even then, bad things could happen. For instance, the Odyssey is essentially a story of an ancient Greek who got blown off course sailing home from nearby Anatolia, and spent 10 years trying to find his way home.

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I'm aware of how you calculate latitude, but what I'm looking for is the accuracy. How accurately could the Greeks determine their latitude? –  Joe Jul 9 '12 at 22:19
I don't think it was possible to get accurate readings if they believed the Earth flat. Also, did they have the concept of this system, anyway? –  slybloty Jul 10 '12 at 13:22
@slybloty - That's kind of a myth. The ancient Greeks realized the earth was spherical (and came up with a pretty good estimate of its size), and any mariner could look at the curved horizon and see it with their own eyes. –  T.E.D. Jul 10 '12 at 13:27
I didn't get into that, because without a good way to do longitude it didn't matter very much. Note that the main axis of the Mediterranian is longitudinal, and as I said ancient mariners typically just kept in sight of land. I've modified the answer a bit to make this clearer. –  T.E.D. Jul 10 '12 at 13:41
@T.E.D. You might know that the earliest known Greek traveler to the English Isles is supposed to be Pytheas (it is as you may know the etymology of Britain, which suggests that Celts were probably as tattooed as the Picts themselves). Pytheas seems to have gone much further Northwards than Britain actually, was apparently enrolled for his mathematics skills and, more to the point - that was around 330/300 BC - had developed several ways to calculate latitude. Regarding the hunk of wood: that's the etymology of our... –  Alain Pannetier Jul 10 '12 at 21:30
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