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Perhaps... By the way, could he mean longitude instead of latitude? I find it more logical to mention Volga river delta in this case.

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Longitude measurements, for obvious physical reasons, are terribly difficult and they become decent only in modern times with high quality mechanical clocks. No chance of confusion.

For people who may not know of these obvious reasons: measuring latitude can be done by looking at the sun's elevation when it is the highest in the sky or by looking at the position of the northern star at night.

Longitude requires to compare the difference between the solar time and a reference time, kept with a clock. Good clocks that stay accurate even on a moving boat took a lot more time to be invented.

You can get longitude without a clock, via lunar distances. It's just not an easy method, and didn't really become practical until the Nautical Almanac provided the requisite tables, not long before the first proper marine chronometers came into use.

But people often forget how much mathematical training was needed to navigate in the age of sail.

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