Last year around April, residents in the state of Punjab in northern India were astonished to see the Himalayas from the rooftop of their homes for the first time in decades. The country was in the middle of a lockdown to stop the spread of Covid-19. The shutting down of factories and clearing of traffic from the streets had a significant impact on the quality of the air. The improvement in air quality became evident when the Dhauladhar mountain range, a part of the lesser Himalaya, located some 200 km away, drifted into view one morning. For many residents, it was the first time they had seen the peaks. This brings us to the question: how far can we see on a clear day?
The Himalayas. Photo: Sitoo/Flickr
There is an old joke that people living out on the prairies can see their dogs run away for days. But being in a flat and treeless plain doesn’t warrant limitless view into the horizon, for the obvious reason that the earth is not flat. The curvature of the earth plays the most important factor of how far into the distance you can see on any given day. If you stand about 6 feet tall, and the ground before you is absolutely flat, the horizon will be about 5 km away, which isn’t much. But climb to the top of a ten-story house and your horizon will expand to 22 kilometers. The higher you go, the further you can see. From the top of Mount Everest, a person can see 336 kilometers away.
How far you can see is also determined by the height or size of the object you are looking at. Tall objects can be located beyond the curvature of the earth, and still be visible. With a pair of powerful binoculars, it might be possible to see the top of the head of a 6 feet person standing nearly 10 km away.
Another factor that can allow for longer sightlines is the temperature of the air. When the air near the earth’s surface is cooler than that above it, a condition known as temperature inversion, the layers of air can bend light in such a manner that it follows the curvature of the earth increasing our line of sight.
The longest line of sight to exist on earth is the one that runs from Mt Dankova in Kyrgyzstan to Hindu Tagh in China, a whopping 558 kilometers. Unfortunately, nobody has ever see Mt Dankova from Hindu Tagh or Hindu Tagh from Mt Dankova. The distance is far too great. Light scattering by air molecules and dust particles reduce visibility to only about 250 kilometers. However, if the atmospheric conditions are just right, it is possible to see longer distances.
The furthest object ever photographed was Pic Gaspard in the French Alps, from Pic de Finestrelles in the Spanish Pyrenees—a distance of 443 kilometers. The image was captured by Mark Bret using a high-zoom bridge camera on 13 July 2016. It is recognized by the Guinness Records as the longest line of sight on earth photographed.
The view of Pic Gaspard, 443 Km away. Photo: Mark Bret
Map of sight illustrating the overwhelming distances.
Mark Bret maintains a website called Beyond Horizon where he lists other possible lines of sight.
It would be absolutely wonderful if the online map sites (Google Maps, Mapquest, Bing Maps, etc.) had a "sightlines" option. Selecting that would draw a 360 degree ring around your location, based on your location altitude, showing how far you should be able to see in a direction, taking in to account any mountains, terrain, earth curvature, etc. that may be blocking or limiting your view.
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