Highcliff is a prominent crag on the northern escarpment of the North York Moors, overlooking Guisborough, the town I grew up in. When I was a child I can remember noticing that, from its top, you can see quite a long way up the North-East coast, and over the years I have often wondered how far, but until recently I had never got round to actually answering this question. Then, last week I went up Highcliff and took the above photo.
Identifying the distant features in the photo proved surprisingly difficult, mainly because I was initially trying to do it across three 1:50000 scale Ordnance Survey maps laid out on the floor. However, I think I now have it sorted. The long faint bump on the horizon, just to the right of centre, is, I think, Cleadon Hill between Whitburn and South Shields. That is about 55km distant. And the white blobs on the horizon just to to the left of centre are probably large buildings in Sunderland at about 49km. That's the hard part. The rest is easy. In the middle distance, jutting in from the left is the Hartlepool Headland, and in the foreground are the Wilton chemical works (mainly to the left) and the Redcar steel works (mainly to the right).
It may be that the the outline of Cleadon Hill is too small and faint to be visible without optical aid, I will need to check that next time I am up in Highcliff. However, the white blobs in Sunderland are definitely visible to the unaided eye so the conservative answer to my question would be about 49km.