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In Sobel's famous book 'Longitude', she described the story of Harrison. I still have one question. What if it is cloudy on the sea? In this case, the sailors have nothing to compare with, right? This still means in many many cases, they have no means to determine their position, neither latitude nor longitude, right?

It seems that the problem is not so satisfactorily solved.

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  • $\begingroup$ Can you clarify? Are you asking how to determine location in bad weather now, or how an 18th-century navigator would? $\endgroup$
    – James K
    Jan 13, 2017 at 20:00
  • $\begingroup$ It's worth pointing out that an overcast sky is much rarer at sea than on land, especially once on gets reasonably far from shore. Not being able to calculate one's position for a day or two on a long voyage isn't such a great problem if you can work it out the next day. $\endgroup$
    – dbmag9
    Jan 13, 2017 at 22:00
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    $\begingroup$ Today: Use GPS. :-) $\endgroup$
    – chirlu
    Jan 14, 2017 at 10:59

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Sunstones are believed to have been used by vikings to determine the direction to the Sun on cloudy days. That helps you point the ship in the right direction, and by experience you can estimate the side ways drift in the water. But the longitude will still be very uncertain without precision time keeping. The news paper article I linked to says that a Sunstone was find at a wreckage in the English Channel from as late as 1592.

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  • $\begingroup$ I don't see relevance to the determination of longitude in the 18th century. $\endgroup$
    – James K
    Jan 14, 2017 at 10:35
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When it's cloudy at sea, the one measurement one still has is approximate sunrise or sunset, which can be used to determine longitude, but the uncertainty is very large, in particular at higher latitudes where sunrise and -set are slow. Having both (length of day) one can estimate latitude as well. Although one cannot determine those to the same precision and accuracy as in clear conditions, an experienced person should still be able to do a pretty good job. If they are 10 minutes off in their estimate, that translates to around 2.5° of longitude. And of course it means they can only make 2 measurements per 24 hours. Between those, they'd have to extrapolate.

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  • $\begingroup$ Where I live it is mostly so cloudy that one cannot locate the Sun on the sky. Not even by 180 degrees. The sky is equally gray in all directions. $\endgroup$
    – LocalFluff
    Jan 13, 2017 at 12:02
  • $\begingroup$ @LocalFluff For the longitude you need to know the time of sunrise or sunset. If you have both you can estimate latitude as well. It gets problematic during polar day or astronomical polar night. No need to determine the position of the Sun in the sky. $\endgroup$
    – gerrit
    Jan 13, 2017 at 15:57
  • $\begingroup$ But one doesn't see where OR WHEN the Sun rises or sets in the North Atlantic region. The whole gray sky just very slowly gets brighter and darker from day to night. There's no hint of any direction to anything in it. I suppose that's why vikings, although audacious navigators, didn't leave much traces of astronomical observations, but instead mostly prefered rivers and coasts. The sky was simply not available to them. $\endgroup$
    – LocalFluff
    Jan 13, 2017 at 16:39
  • $\begingroup$ @LocalFluff The whole sky gets brighter. Somewhere in this period, we have sunrise. That means that we can determine sunrise with an uncertainty of perhaps 10–30 minutes depending on latitude (higher latitudes means larger errors in longitude, but not in location). And an experienced person might be able to guess rather accurately at what brightness the sunrise or -set approximately is. The Vikings did not have Harrison's clock, so there was no point in estimating the time of sunrise/-set. $\endgroup$
    – gerrit
    Jan 13, 2017 at 16:45
  • $\begingroup$ No you can't guess it because the thickness of the clouds varies, and the brightness of the Sun somewhere behind it varies with such imperceptible weather changes. And the dawn is very slow. Half an hour error corresponds to half the distance between Norway and Iceland! -"Captain! We are somewhere on the Atlantic. Or on the North Sea. Since we don't see any seals I guess we can exclude the Arctic Sea for now." $\endgroup$
    – LocalFluff
    Jan 13, 2017 at 16:54
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First off, there is evidence to suggest that sunstones were used to find the direction of the sun on cloudy days. However the compass has completely eliminated the need for a sunstone as it works in cloudy weather. You are correct that taking star, planet and moon sights with a sextant in cloudy conditions is problematic. However the sky is not always completely occluded. If even a single navigational star appears for long enough than a sight can be taken. Without seeing other stars and constellations one can make educated guesses as to which navigational star this is. For example with a star finder and the most general idea of where one is on earth the name of the star could be deducted. I believe that just such a sight during Ernest Shackleton's historic voyage in Antarctica enabled him to navigate to a whaling station to get assistance.

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