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Pushing back the Horizon

So lately I’ve been thinking about a ‘fact’ I was told as a child: on a clear day you can see as far as Wales from the top of Mt. Lienster.


Being young and fascinated I was always a little awestruck at the notion I could see all the way across the sea and into another country, but it was never clear enough when I was up there.

So fast forward to last Friday when I went to Inis Mór with the family. We were on the boat trying to spot the coast through the overcast and misty day. My younger brother was reading the leaflets handed out by the tourist information guys and informed me that Dún Aengus is a fort built on the side of a 100 meter high cliff face. My number senses began tingling. How far could you see from there? Could you see the mainland, the other islands, boats how far out at see?

I had 40 mins to kill and so the 3G went on and I began googling. Wikipedia politely informed me you can use the Pythagoras theorem to estimate, based on assumption of a spherical earth and no atmospheric refraction or occlusion. Distance to the Horizon in km = 3.57 by square root of the vision height.

I whipped out the mobile office excel app and banged in the numbers for 1 to 100 meters and bobs your mother’s brother. 35 km.



Distance from Dún Aengus to mainland is 12 km, so on a clear day you’d be likely to see exactly what your enemy clan on the mainland were plotting.



View from Dún Aengus on a clear day looking northeast towards the mainland of galway, Ireland. Clearly visible are the coast and other landmasses in the ocean.

So back to Co Wexford and normality today and sitting out having a beer and a barbecue I remembered my question. Could you actually see to Wales?

Using the same formula as before and data from wikipedia we get a best case distance of 128.93 km.

Daft Logic says the closest point of land in the mainland UK from Mount lienster is indeed Wales, a little island called Ramsay island near the town of St. David’s off the coast of Pembrokeshire Coast national park a distance of 123.75 km.


The closest point of land in Wales from Mount Lienster is off the coast of Pembrokeshire.

That’s just inside our calculated visible distance. Amazingly enough, it turns out to just be on the limit of plausibility.
Score one for science and maths I guess!

Edit: since posting this I did a little more digging around and found two very interesting images.


This map shows signal strength from the transmitter atop Mount Lienster. We can see it is strongest in areas closest to the transmitter, with “shadows” cast behind the hills, and weakening as the distance becomes greater from the source. Notably, the signal reaches well into Wales.



Mt. Snowden in Wales is at a distance of 189 km and an elevation of 1,085 m, making it an excellent point on the horizon to search for, right about 72.5 degrees East of North.



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