<p><a href="http://www.livescience.com/27696-viking-sunstone-shipwreck.html" rel="noreferrer">Sunstones</a> 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.</p>

CC BY-SA 3.0
2017-01-14T10:35:56.753Z
I don't see relevance to the determination of longitude in the 18th century.
James K
CC BY-SA 3.0
2017-01-13T12:08:20.947Z
2017-01-13T12:08:20.947Z
LocalFluff
<p>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.</p>

CC BY-SA 3.0
2017-01-13T12:02:04.877Z
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.
LocalFluff
CC BY-SA 3.0
2017-01-13T15:57:41.193Z
@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.
gerrit
CC BY-SA 3.0
2017-01-13T16:39:21.513Z
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.
LocalFluff
CC BY-SA 3.0
2017-01-13T16:45:09.653Z
@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.
gerrit
CC BY-SA 3.0
2017-01-13T16:54:30.113Z
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."*
LocalFluff
CC BY-SA 3.0
2017-01-13T16:58:21.490Z
@LocalFluff Yes, the uncertainty is large (I miscalculated it earlier).  I think the Norway-Iceland distance is 30°-ish so half an hour error (7.5°) would correspond to ¼ of the distance.  Still an improvement to pre-Harrison.
gerrit
CC BY-SA 3.0
2017-01-13T17:04:56.963Z
@LocalFluff Either way, I added a point that the uncertainty is very large.
gerrit
CC BY-SA 3.0
2017-01-14T01:37:48.703Z
There is an excellent "landmark" for North Atlantic navigation, namely Greenland. With a bit of practical experience, spotting a 9,000ft mountain range, usually with a lot of cloud on top of it, is possible from huge distances away. Also, you don't need "accurate" navigation so long as you are aiming at a long coastline. If you make landfall a hundred miles from your destination, just sail along the coast. That's why early navigators carried *very* detailed "strip charts" showing the coast as seen from the sea, long before they had accurate "charts" in the modern sense of the word.
alephzero
CC BY-SA 3.0
2017-01-13T10:58:05.800Z
2017-01-13T17:03:20.573Z
2017-01-13T17:03:20.573Z
gerrit
<p>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.</p>

CC BY-SA 3.0
2017-01-13T18:09:56.800Z
2017-01-13T18:09:56.800Z
thinkerinthedark
<p>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? </p>

<p>It seems that the problem is not so satisfactorily solved. </p>

CC BY-SA 3.0
2017-01-13T20:00:01.293Z
Can you clarify? Are you asking how to determine location in bad weather now, or how an 18th-century navigator would?
James K
CC BY-SA 3.0
2017-01-13T22:00:20.573Z
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.
dbmag9
CC BY-SA 3.0
2017-01-14T10:59:01.353Z
Today: Use GPS. :-)
chirlu
<earth><coordinate><navigation>
Determining latitude and longitude in bad weather
S. Kohn
# Determining latitude and longitude in bad weather

(Asked by: username_0 on 2017-01-13)

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.

**Tags:** earth, coordinate, navigation

> **Comments:**
> **username_1**: Can you clarify? Are you asking how to determine location in bad weather now, or how an 18th-century navigator would?  
> **username_2**: 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.  
> **username_3**: Today: Use GPS. :-)  

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## 3 answers

### Answer by username_4 on 2017-01-13. Score: 8

[Sunstones](http://www.livescience.com/27696-viking-sunstone-shipwreck.html) 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.

> **Comments:**
> **username_1**: I don't see relevance to the determination of longitude in the 18th century.  

---

### Answer by username_5 on 2017-01-13. Score: 7

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.

> **Comments:**
> **username_4**: 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.  
> **username_5**: @username_4 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.  
> **username_4**: 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.  
> **username_5**: @username_4 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.  
> **username_4**: 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."*  
> **username_5**: @username_4 Yes, the uncertainty is large (I miscalculated it earlier).  I think the Norway-Iceland distance is 30°-ish so half an hour error (7.5°) would correspond to ¼ of the distance.  Still an improvement to pre-Harrison.  
> **username_5**: @username_4 Either way, I added a point that the uncertainty is very large.  
> **username_6**: There is an excellent "landmark" for North Atlantic navigation, namely Greenland. With a bit of practical experience, spotting a 9,000ft mountain range, usually with a lot of cloud on top of it, is possible from huge distances away. Also, you don't need "accurate" navigation so long as you are aiming at a long coastline. If you make landfall a hundred miles from your destination, just sail along the coast. That's why early navigators carried *very* detailed "strip charts" showing the coast as seen from the sea, long before they had accurate "charts" in the modern sense of the word.  

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### Answer by username_7 on 2017-01-13. Score: 2

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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