Can You See 100 Miles Away?
Can you see 100 miles? Sure you can! In fact, you can see across millions and millions of miles—looking in the right direction and under the right conditions, of course.
Before we dig into the long-distance sightlines the human eye’s capable of perceiving, we ought to quickly sketch out the basics of vision. We see objects because they reflect light particles, or photons, which are then absorbed by photoreceptor cells in the retinas lying in the back of our eyeballs. There are actually two varieties of those cells: cone cells, which register color, and rod cells, which allow us to perceive grayscale tones in dim light. Studies have suggested that as few as five of those more receptive rods may need to activate for our brain—alerted by the optic nerve that transmits the electrical signal produced by the photon-triggered photoreceptor cells—to register an image of something.
As long as our retinal cells receive a photon from an object, we can theoretically see it. It’s often stated that, on a dark night, the human eye should be able to pick out the glow of a candle from upwards of 30 miles away. That would be a mere point of light; as Natalie Wolchover notes in this LiveScience article, models suggest we can perceive an actual object with spatial dimension from just shy of two miles off.
You can enjoy your formidable long-distance vision most spectacularly on a clear night, when none of those twinkling stars you see are closer than 24 trillion miles away. The generally accepted farthest object the naked human eye can see is the blurry luminosity of the Andromeda Galaxy, a spiral galaxy (like our own Milky Way) that’s a whopping 2.5 million light years—some 23 quintillion miles—away from Earth. (As Adam Hadhazy acknowledges in this fascinating BBC writeup—source of much of the above info—some people claim to be able to see farther yet, to the Triangulum Galaxy about three million light years away, but that’s disputed.)
The most distant individual star visible with the naked eye from Earth is Rho Cassiopeiae, which appears as a dim point on the outskirts of the constellation Cassiopeia, the Queen. That dimness is only a function of the fact that Rho Cassiopeiae, a rare yellow hypergiant that may be nearing explosion as a supernova, lies more than 8,000 light years from Earth; it’s actually an incredibly bright star, radiating hundreds of thousands times more light than our Sun and outsizing it by 400 or 500 times.
The Moon, incidentally—our closest celestial companion—is about 240,000 miles from Earth. The planet Venus, next-closest to the Sun after Earth, is the third-brightest object in our skies after Sun and Moon, but, as Wolchover notes in the aforementioned LiveScience piece, it’s right at the lower limit of what our naked eye can perceive as a spatially extended object as opposed to a smudge of light.
During the day, of course, the dazzling light of our very own home star precludes most head-spinning astronomical sightlines (though naturally the Moon, and sometimes Venus, can be seen in daylight).
Staring into the depths of space is one thing, but when we’re looking to the horizon and wondering at what maximum distance we can spot terrestrial objects (or low clouds), we’re faced with more limitations. First and foremost, there’s the curvature of the Earth, which sees the surface of the planet arcing by some eight inches per mile. Given that geometry, a 6-foot-tall person standing in flat terrain with no obstructions—or, more likely, looking out to sea from the water’s edge—sees a horizon line roughly three miles away.
But there are a number of other variables that can extend that sightline. If you raise your height or elevation as an observer, you’ll naturally be able to see farther (all else being equal). A mountaineer standing atop Earth’s tallest point above sea level—29,029-foot Mount Everest in the Himalaya—could conceivably see a skyline roughly 230 miles distant. If you also elevate the object being viewed, you’ll increase the distance at which its top can be seen protruding above the curving horizon.
Another factor coming into play here is the atmosphere and its influence on light refraction. Refraction describes the bending of light as it passes through a substance or medium of different densities. Air density varies with temperature. Colder air overlain by warmer air—an example of a temperature inversion—can cause light to bend around the planet’s curve, which can allow an object that otherwise would be hidden by that curvature to appear above the horizon as a so-called superior mirage. This effectively expands your sightlines, even if the refracted image of the object is an optical illusion of sorts.
Even with a good elevated vantage, our horizon-ward sightlines are often limited by haze and light scattering to a dozen or so miles. Very clear, unpolluted, and stable air allows for longer lines-of-sight.
A maximum sightline from point to point on the Earth’s surface, therefore, would come with an elevated observer position, an elevated target object, a lack of taller obstructions between them, proper light refraction, and crystalline atmospheric conditions. The longest such sightline confirmed by photograph is the roughly 275-mile one between 9,272-foot Pic de Finestrelles in the Pyrenees and 12,736-foot Pic Gaspard of the Massif des Écrins in the French Alps.
Even greater records are surely possible. Computer modeling suggests a roughly 20,000-foot summit in Kyrgyzstan, Mount Dankova, serves up sightlines beyond 300 miles to peaks in the Kunlun Mountains of China.
One intriguing record noted by Andrew T. Young—an adjunct professor emeritus of the San Diego State University astronomy department—is of a 1923 expedition in the deserts of what is today southeastern Kazakhstan that reported seeing high snow peaks some 466 miles distant. If accurate, Young notes, this remarkable long-distance observation may have been the result of a perfect medley of geographic and optical conditions, including the notable height of the mountain range in question (above 15,000 feet), the decent elevation of the observing party, clear and dry air, a lack of intervening topographic obstructions, and a superior-mirage effect as well.
Incidentally, it’s interesting to consider the potentially great distances at which one could view the top of a faroff thunderhead. Such a storm cloud—a cumulonimbus—can extend to altitudes of 60,000 feet or more, obviously well above the tops of our planet’s tallest mountains. Clear conditions would theoretically allow a big thunderhead to be visible from several hundred miles away, not least if you had an elevated vantage. Such a distant weather feature can be especially visible at night, given the high visibility of lightning bolts or, at least, cloud structures illuminated by those flashes.