The Economist, 5 March 2011, p.3 special property report, noites that the worlds tallest building is in Dubai, and measures 838 metres in height.  Using a rough rule of thumb (sorry, only works in imperial, not metric,units) that (distance to horizon in miles) = 1.33 x (square root of height in feet above the ground), the horizon from the top of this building should be around 58 miles away, if perfect visibility.  Or, you should be able to see this building from 58 miles away.  If you stood on a tall mountain even further away you'd still theoretically see it, only air turbulence and dust would likely stop you doing this, as then the width of the building would be significant too.  Could you see an object only a few tens of metres wide from 100 miles away?  The tallest objects are often the thinnest (e.g. broadcasting aerials), so height militates against subtended angle on the eye here.

So does anyone know what is the record distance (ground-based vantage point)  for seeing a man-made ground-based object from?  London's Docklands / City tower blocks are visible from 20 - 25 miles away, maybe more.  How far can the direct visual impact of an urban building (or a close collection of them, which would provode a wider subtending angle) reach? 

Dr Hillary Shaw
School of Business, Management and Marketing
Harper Adams University College
Newport
Shropshire
TF10 8NB