Can you see Brittany from a cliff in Cornwall?
December 26, 2016 5:39 AM   Subscribe

I'm reading a novel set near Polruan in which the author claims that you can see France with the naked eye from the top of a cliff on a clear day. I've tried a number of searches, including a horizon calculator, and it doesn't look feasible. Is this just a case of poetic license on the author's part?
posted by mareli to Science & Nature (13 answers total) 1 user marked this as a favorite
 
This was a few years back and so my memory is hazy, but I know I've been to a beach in Nord-Pas-de-Calais from where you could see the cliffs at Dover, so it's not out of the realm of possibility.
posted by Tamanna at 6:05 AM on December 26, 2016


Playing around with HeyWhat'sThat.com (which is generally a fun website) seems to confirm that there's no chance of actually seeing France.

I know I've been to a beach in Nord-Pas-de-Calais from where you could see the cliffs at Dover, so it's not out of the realm of possibility.

Calais & Dover are only about 30 km apart, though. From Polruan to Bretagne is at least 180 km.
posted by Johnny Assay at 6:20 AM on December 26, 2016 [6 favorites]


In exceptionally good weather, you can see the Cliffs of Dover from Calais. The opposite is not true due to the curvature of the earth; max visibility is 21 miles, but the Cliffs do stick up a bit. From Polruan in Cornwall, all you can see is Salcombe, which is... also in Cornwall, on the other side of the bay.

However, unless it is a crucial plot point, it seems more likely a character in your book is using a common expression: "On a clear day, you can see France." This is used everywhere, including Scotland, from where you absolutely can not see France.
posted by DarlingBri at 6:26 AM on December 26, 2016 [10 favorites]


From the top of a cliff, yes, depending on the height of said cliff. The horizon line is 25 miles out at sea if you are 6-foot tall standing at sea level, it's 200 miles (322km) out if you're on a 10,000 foot (3,000m) high mountain. 180km is only about 112 miles; you'd need to be on a cliff of 2400m to see that far – to sea level! Anything higher than sea level on the other side would be visible from a shorter height.

Anecdotal reference: on a clear morning you can easily see the mountains of Corsica from hilltops on the French Riviera. The northern coast of Corsica is about 200km away, and that's looking roughly the same direction (southeast) as from Polruan to Brittany.
posted by fraula at 7:00 AM on December 26, 2016 [2 favorites]


Plymouth to Roscoff is six hours on the ferry - not exactly the same vantage point but close enough that I'd be really surprised if it was anything other than poetic license.
posted by terretu at 7:12 AM on December 26, 2016


Dover is nowhere near Cornwall, folks, get out your maps. I knew you could see France from there.
Darlingbri, it is a sort of plot point, not a vague expression. Fraula, my father lives near Nice and has claimed the same about seeing Corsica.

I found a cool tool that calculates "as the crow flies" distances. Nice to Calvi in Corsica = 109 miles. Polruan to Roscoff in Brittany= 115 miles.
posted by mareli at 7:18 AM on December 26, 2016 [2 favorites]


a common expression: "On a clear day, you can see France." This is used everywhere, including Scotland, from where you absolutely can not see France.

Huh - is that a common expression? This is the first time I've ever heard it, except as a literal description of places where you actually can see France on a clear day (British, 42, lived in England and Scotland).
posted by penguin pie at 7:26 AM on December 26, 2016 [3 favorites]


The path of light is bent by the atmosphere, so straight-line geometry is not completely accurate. The amount of bending is mostly due to the differences in pressure, and pressure is determined by altitude and temperature. If there are layers of air at different temperatures, this can cause mirage effects. There is one such effect that makes distant shores appear higher than they actually are. (I don't remember if it's cold water causing a layer of cold air under warm air, or the opposite, warm water causing a warm layer under cold air.)

Anyway, my point is that there can be unusual viewing conditions. I think mostly they would effect an observer on the beach, not an observer on a bluff.
posted by SemiSalt at 7:48 AM on December 26, 2016 [3 favorites]


folks, get out your maps.

That's a bit snippy towards well-meaning pedants trying to help you.

More data points:

Blackbottle Rock above Pouan is a 119 meters high

Jobourge in Brittany is 123 meters high

Distance between the two is 138 miles

Alternatively, La Feuillée tops out at 381 meters

Distance is perhaps 140 odd miles

You do the math
posted by IndigoJones at 8:18 AM on December 26, 2016 [12 favorites]


I lived in Cornwall for 20+ years. I never managed to see France from a clifftop in all of that time.
posted by pipeski at 8:59 AM on December 26, 2016 [1 favorite]


FWIW on a good day it's possible to see Snowdon from Bow Fell (around 165km). Both are over 900m, and it's mostly sea between them. The highest point in Cornwall isn't even half the height of Bow Fell.
posted by mushhushshu at 10:52 AM on December 26, 2016 [1 favorite]


I lived in Cornwall for 20+ years. I never managed to see France from a clifftop in all of that time.

How often did you try?
(Not asked in snark: I know almost nothing about Cornwall, and definitely not how frequently the days are clear or how common it is to be on a cliff.)
posted by trig at 10:14 PM on December 26, 2016


Of interest/possible relevance here is the story of Hy-Brasil - a land which was said to lie somewhere to the west or Ireland. It was said to be cloaked in mist for all days save once every 7 years. Unlike many other phantom islands, Hy-Brasil's claim to exist seems to have been based on some kind of mirage (perhaps a fata-morgana) that can be observed when looking west into various types of weather.

I mention this to point out that a character can get a solid impression of seeing a land that is too distant to see by ordinary geometry - if they are in the right frame of mind (expeditions were mounted to search for Hi-Brasil and it appeared on maps). Sometimes the land is just in their imagination - but sometimes particular atmospheric conditions can act like a lens to give a proper sighting of land that would not normally be visible. The maximum distance at which an actual object could be seen as a fata-morgana mirage seem to be dependent on the strength of the thermal inversion that serves as an atmospheric lens.

TL/DR: An observer in Cornwall could see land in France, as part of a Fata-Morgana mirage - on days when there was a sufficiently big contrast between cold air at the sea surface and warmer air moving it above it. Whether or not a sufficiently big contrast to let this happen in real life could occur - would be an interesting question for a meteorologist - but it certainly extends the range over which one could see France. The observer could also convincingly "see" France as a mirage where the image was generated by something other than the land mass itself.
posted by rongorongo at 12:12 AM on December 27, 2016 [1 favorite]


« Older testing tap water - home chemistry   |   Does meal prep make me high-maintenance? Newer »
This thread is closed to new comments.