16

On a clear day is it possible to see the Isles of Scilly from Cornwall with the aid of binoculars? A distance of around 40km.

I know on a very clear day you can only see maximum 20km from sea level but the coast of Cornwall is higher than sea level. So does the height advantage of the cliffs enable you to see further across the horizon?

There are a couple of online forums that suggest you can see the Isle of Scilly from Cornwall but no photographic evidence.

Google map as the crow flies

33

The site HeyWhatsThat.com is a pretty neat tool for answering this sort of question. I've taken the liberty of creating a map showing the view from Lands End, which should be available to everyone at that link. Here's the computer-generated panorama:

enter image description here

The Isles of Scilly are barely visible above the horizon (the horizontal magenta line) to the west-southwest, in the direction of the brown line. The site also calculates the entire "viewshed" from Lands End and displays it on a map:

enter image description here

As can be seen from the pink shading on the Isles, their peaks can be seen from Lands End. However, it is not possible to see from the cliffs at Lands End to the beaches of the Isles.

  • Excellent tool. Thank you very much for the link! – Eric Duminil Oct 21 '17 at 15:23
29

They can be seen from Cornwall, and photographic evidence isn't hard to find.

enter image description here Isles of Scilly from Gwennap Head. Source: Bob Jones via Wikimedia Commons.

  • 7
    It looks like there's a temperature inversion, causing a superior image. The line of sight is bent and helps to see more than usual. Just like in this trick. Geometry alone (even considering Earth's curvature) isn't enough to answer this question, so it's great to have a picture as proof. – Eric Duminil Oct 21 '17 at 15:29
  • 4
    @EricDuminil As the atmosphere is not homogeneous, line of sight is not straight. It is a common phenomenon. Surveyors know it as curvature error surveying2012.blogspot.com.es/2014/06/… . Interestingly, it's sign is opposite to curvature error, effectively bending the line of sight downwards. – Pere Oct 21 '17 at 20:09
  • 2
    Sorry, there is a typo in my previous comment: this is refraction error, which is opposed to curvature error. – Pere Oct 21 '17 at 22:27
  • 1
    Note that it also often happens in the opposite direction (inferior image). In this case, the islands wouldn't be seen from the coast, even on a perfectly clear day. The line of sight isn't always bent downwards. – Eric Duminil Oct 22 '17 at 19:31
5

I saw the islands from near St Just last week, a distance of almost 30 miles. Very faint, but definitely visible. I had no trouble with binoculars separating the islands and the hill on Samson.

3

I saw them using binoculars aboard the Brittany Ferry service from Plymouth, Devon to Roscoff, Brittany on 25th August 2018. Halangy Down Transmitter on St Marys was visible. The visibility was excellent, but perhaps there was a temperature inversion happening, or something like it, since I wouldn't have expected to be able to see over such a great distance (approx 90 - 95 miles). I have seen the Lizard peninsula from Rame Head before with the naked eye, which is about 45 - 50 miles, and that was unusual, so I was really excited to see the Isles of Scilly.

2

Based on Mark Mayos excellent answer to this question and calculating the height 80 metres close to the Cornwall coast line you can only see a maximum of 32km with the naked eye.

This is based on the calculation:

calculation

where d = distance in km and h = height in metres.

  • 11
    Just a small note: the problem is about earth curvature, and that formula relates how far you can see from a certain height before the curvature of Earth act like a wall. So, about your answer; it's "a maximum of 32 km" anyway, even if you are using the most powerful zoom that you can find; it has nothing to do with "the naked eye" – motoDrizzt Oct 20 '17 at 10:19
  • 12
    But the highest point on the isles is 51 meters above sea level. – phoog Oct 20 '17 at 10:37
  • ooo interesting. I never thought of that @phoog – davidb Oct 20 '17 at 11:08
  • 9
    You can handle this by summing the horizon distances for the observer and target; so if the observer is at elevation h1 and the target at h2, they can see each other if the distance between them is less than 3.57(sqrt(h1)+sqrt(h2)). Using 80 and 51 that gets us to 57.4 km which suggests that they could in fact be visible. (Note that the answer you linked suggests that one should add the heights, but this is incorrect, as was pointed out in a comment on that answer.) – Nate Eldredge Oct 20 '17 at 12:53

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.