How far can you see
How far can you see
How far can the human eye see?
By Natalie Wolchover published 7 May 12
The human eye can see far beyond Earth’s horizon.
Earth’s surface curves out of sight at a distance of 3.1 miles (5 kilometers). But our visual acuity extends far beyond the horizon. If Earth were flat, or if you were standing atop a mountain surveying a larger-than-usual patch of the planet, you could perceive bright lights hundreds of miles distant. On a dark night, you could even see a candle flame flickering up to 30 miles (48 km) away.
How far the human eye can see depends on how many particles of light, or photons, a distant object emits. The farthest object visible with the naked eye is the Andromeda galaxy, located an astonishing 2.6 million light-years from Earth. The galaxy’s 1 trillion stars collectively emit enough light for a few thousand photons to hit each square centimeter of Earth every second; on a dark night, that’s plenty to excite our retinas.
Back in 1941, the vision scientist Selig Hecht and his colleagues at Columbia University made what is still considered a reliable measurement of the «absolute threshold» of vision — the minimum number of photons that must strike our retinas in order to elicit an awareness of visual perception. The experiment probed the threshold under ideal conditions: study participants’ eyes were given time to adapt to total darkness, the flash of light acting as a stimulus had a (blue-green) wavelength of 510 nanometers, to which our eyes are most sensitive, and this light was aimed at the periphery of the retina, which is richest in light-detecting rod cells.
The scientists found that for study participants to perceive such a flash of light more than half the time, the subjects required between 54 and 148 photons to hit their eyeballs. Based on measurements of retinal absorption, the scientists calculated that a factor of 10 fewer photons were actually being absorbed by the participant’s rod cells. Thus, the absorption of 5 to 14 photons, or, equivalently, the activation of just 5 to 14 rod cells, tells your brain you’re seeing something.
«This is indeed a small number of chemical events,» Hecht and his colleagues concluded in their seminal paper on the subject.
Considering the absolute threshold, the brightness of a candle flame, and the way a glowing object dims according to the square of the distance away from it, vision scientists conclude that one could make out the faint glimmer of a candle flame up to 30 miles away.
But how far away can we perceive that an object is more than just a twinkle of light? For something to appear spatially extended rather than point-like, light from it must stimulate at least two adjacent cone cells — the elements in our eyes that produce color vision. Under ideal conditions, an object must subtend an angle of at least 1 arcminute, or one sixtieth of a degree, in order to excite adjacent cones. (This angular measure stays the same regardless of whether an object is nearby or far away; distant objects must be much larger to subtend the same angle as near objects). The full moon is 30 arcminutes across, whereas Venus is barely resolvable as an extended object at around 1 arcminute across.
Human-scale objects are resolvable as extended objects from a distance of just under 2 miles (3 km). For example, at that distance, we would just be able to make out two distinct headlights on a car.
Originally published on Live Science.
Natalie Wolchover was a staff writer for Live Science from 2010 to 2012 and is currently a senior physics writer and editor for Quanta Magazine. She holds a bachelor’s degree in physics from Tufts University and has studied physics at the University of California, Berkeley. Along with the staff of Quanta, Wolchover won the 2022 Pulitzer Prize for explanatory writing for her work on the building of the James Webb Space Telescope. Her work has also appeared in the The Best American Science and Nature Writing and The Best Writing on Mathematics, Nature, The New Yorker and Popular Science. She was the 2016 winner of the Evert Clark/Seth Payne Award, an annual prize for young science journalists, as well as the winner of the 2017 Science Communication Award for the American Institute of Physics.
Curious Questions: How far can you see on a clear day?
A walk to a local hilltop prompts Martin Fone to wonder just how far you can see, assuming perfect atmospheric conditions.
It is worth all the effort, clambering laboriously to the top of a hill or viewing platform to gaze in awe on the vista around me. I often wonder just how far I can see if I am blessed with a clear day. I recognise that Alan Jay Lerner’s song, On a Clear Day (You Can See Forever) from the eponymous 1965 Broadway musical, is a tad hyperbolic, but were The Who with their I Can See For Miles from their 1967 The Who Sell Out album, nearer the mark?
Does the old midwestern joke that people living out on the prairies can see their dog run away for days contain a scintilla of truth? Inevitably, the answer is not as simple as it may seem.
The first complicating factor is the fact that the Earth is spherical, something that had been recognised, at least by scientists, for over two millennia. Eratosthenes, a Greek from Alexandria, who died in 194 BCE, realised the planet was a globe and even calculated the circumference of the 36 th parallel running through Greece and Crete. Strabo, the first century CE geographer, pre-empted Columbus by some 1,500 years in surmising that by sailing west from Spain you would end up in India in the east. By the time Pliny the Elder wrote his Naturalis Historia in 77AD, the Earth’s spherical shape was common knowledge; ‘we all agree on the earth’s shape. For surely we always speak of the round ball of the Earth’ (II.64).
While you are standing on terra firma you might be forgiven for thinking that the planet is flat. However, a little bit of observation will demonstrate the errors of your ways. Look out of an upper storey window of a building and then compare what you see with your view from the ground. Assuming that there are no annoying obstacles in the way, if the Earth was flat, your view should be identical. However, that distant object on the horizon you could see from the window would have disappeared when you looked at it at ground level. The Earth’s natural curvature, around 8 degrees per mile, has taken the object out of the line of your sight.
If you are lying on the ground with your eyes about a foot off the ground, the maximum distance you would see before the Earth’s curvature intervened is around a mile. Stand up, and assuming your eyes are around five feet off the ground, the distance is extended to around 3 miles.
Let’s put it another way: if you were planning to run a 5-kilometre race on a perfectly flat track, you would not be able to see the finishing tape from the starting blocks. The higher you are, though, the further the horizon line will be. Standing 1,000 feet above sea level, it would be 38.7 miles away and 208.8 miles away from the top of Everest.
“Buildings like London’s Shard are tall enough to counter the effect of the Earth’s curvature and so can be seen from points between the South Downs to the Thames Estuary, over 40 miles away.”
These distances can be calculated with help from Pythagoras’ theorem; I always wondered at school what I was going to use it for. If we assume the Earth is perfectly round with a radius of 6,378,137 metres, we can construct a theoretical triangle with the centre of the earth forming one point, the horizon a second point which we assume to be at right angles to it and the observer’s height above sea level the third. Fortunately, these days there are computer programs that can do all the hard work for you. Of course, the planet is not perfectly spherical and so some inaccuracy is introduced into the results, but they are good enough to satisfy the idlest of curiosities.
However, there are other factors to consider, not least the quality of the air. The molecules in the air attenuate the light, so that even in the most perfect conditions, the maximum distance you would see is reduced to around 150 miles. Such perfect conditions are extremely rare as tiny particles suspended in the air form a light haze and reduce visibility further. Ironically, fine settled weather, which would normally encourage you to climb up to a viewing point, results in little in the way of air movement and so more particles hang around. The result is visibility can be disappointingly poor and those distant hills are often obscured by a haze.
How far you can see is also determined by the size of the object you are looking at and that is down to your visual acuity. If you are blessed with 20/20 vision, (by which ophthalmologists mean you can see from 20 feet something you should see from that distance), when you are standing at sea level, an object needs to be over 4.4 feet square to be seen over the horizon. Buildings like London’s Shard are tall enough to counter the effect of the Earth’s curvature and so can be seen from points between the South Downs to the Thames Estuary, over 40 miles away.
You also need a clear sightline, if you want to see as far as you can. This is often harder to achieve than you would imagine as obstacles — some natural, other man-made — interpose themselves between you and the horizon.
Assuming you have all the ingredients in place, a high vantage point, a tall object to view, clear sightlines and excellent air conditions, then computer models suggest that the longest sightline in the world is the one that runs from Mt Dankova in Kyrgyzstan to Hindu Tagh in China, some 558 kilometres.
No one has ever photographed this sightline to prove the point. Marc Bret, though, took a picture at dawn on July 16, 2016 of the longest sightline to be captured on film, from Pic de Finestrelles in the Spanish Pyrenees. The furthest object visible was Pic Gaspard in the French Alps, 443 kilometres away. Marc’s website, Beyond Horizons, has this and several other astonishing pictures.
The Alps, contemplated from another new summit of the Eastern Pyrenees, last Sunday 23/09.Clean skies recently beaten…
Look upwards and there are no annoying obstacles to disrupt your vision. Distances are transformed dramatically. The moon, large, bright and often clear enough for us to see some of the details of its surface, is 239,000 miles away, while the sun, whose rays we chase with an obsession verging on mania, is around 93 million miles from our planet. Of the planets orbiting with us around the Sun, Saturn at 746 million miles is the furthest we can see with the naked eye. Squint hard enough and you might even see its rings.
Distances are even more astronomical when you consider the constellations. The Andromeda Galaxy is a rotating cluster formed of around one trillion stars, twice the number of the Milky Way. Even though it is 2.5 million light-years away, the light emanating from its burning stars is so bright that it is the furthest object from Earth seen with the naked eye.
I cannot help wondering, though, when I gaze into the impenetrable blackness of the night sky, just how far I am seeing. There has to be something there for you to see before you can measure it, a deeply philosophical thought to ponder on. You never know, Alan Jay Lerner may just be right.
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How Far Can You See in the Universe?
When you look into the night sky, you’re seeing tremendous distances away, even with your bare eyeball. But what’s the most distant object you can see with the unaided eye? And what if you get help with a pair of binoculars, a telescope, or even with the Hubble Space Telescope.
Standing at sea level, your head is at an altitude of 2 meters, and the horizon appears to be about 3 miles, or 5 km away. We’re able to see more distant objects if they’re taller, like buildings or mountains, or when we’re higher up in the air. If you get to an altitude of 20 meters, the horizon stretches out to about 11 km. But we can see objects in space which are even more distant with the naked eye. The Moon is 385,000 km away and the Sun is a whopping 150 million km. Visible all the way down here on Earth, the most distant object in the solar system we can see, without a telescope, is Saturn at 1.5 billion km away.
In the very darkest conditions, the human eye can see stars at magnitude 6.5 or greater. Which works about to about 9,000 individual stars. Sirius, the brightest star in the sky, is 8.6 light years. The most distant bright star, Deneb, is about 1500 light years away from Earth. If someone was looking back at us, right now, they could be seeing the election of the 52nd pope, St. Hormidas, in the 6th Century.
There are even a couple of really bright stars in the 8000 light year range, that we might just barely be able to see without a telescope. If a star detonates, we can see it much further away. The famous 1006 supernova was the brightest in history, recorded in China, Japan and the Middle East.
It was a total of 7,200 light years away and was visible in the daytime. There’s even large structures we can see. Outside the galaxy, the Large Magellanic Cloud is 160,000 light years and the Small Magellanic Cloud is almost 200,000 light years away. Unfortunately for us up North, these are only visible from Southern Hemisphere.The most distant thing we can see with our bare eyeballs is Andromeda at 2.6 million light years, which in dark skies looks like a fuzzy blob.
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If we cheat and get a little help, say with binoculars – you can see magnitude 10 – fainter stars and galaxies at more than 10 million light-years away. With a telescope you can see much, much further. A regular 8-inch telescope would let you see the brightest quasars, more than 2 billion light years away. Using gravitational lensing the amazing Hubble space telescope can see galaxies, incredibly far out, where the light had left them just hundreds of millions of years after the Big Bang.
If you could see in other wavelengths, you could see different distances. Fortunately for our precious radiation sensitive organs, Gamma and X rays are blocked by our atmosphere. But if you could see in that spectrum, you could see objects exploding billions of light years away. And if you could see in the radio spectrum, you’d be able to see the cosmic microwave background radiation, surrounding us in all directions and marking the edge of the observable universe.
Wouldn’t that be cool? Well, maybe we can… just a little. Turn on your television, some of the static on the screen is this very background radiation, the afterglow of the Big Bang.
What do you think? If you could see far out in the Universe what would you like a close up view of? Tell us in the comments below.
Podcast (audio): Download (Duration: 4:06 — 3.8MB)
Dungeon Master Assistance
A place to share thoughts and ideas about Dungeons and Dragons
D&D 5E – How far can you see
So What Can I see From Here?
Usually, the limit to how far characters can see will be some obstruction, such as a building, a forest, or some hills. Mist and darkness also limit vision. Sometimes, however, the characters will be on flat plains on a clear day and the only limit to their vision will be their perception and the horizon. Once something goes below the horizon, it can’t be seen. But where is the horizon?
| Height in feet | Miles away |
| 3 | 2 |
| 6 | 3 |
| 10 | 4 |
| 20 | 5 |
| 30 | 6 |
| 40 | 7 |
| 50 | 8 |
| 60 | 9 |
| 70 | 10 |
| 80 | 11 |
| 90 | 12 |
| 100 | 13 |
| 300 | 20 |
| 500 | 25 |
| 1,000 | 40 |
| 5,000 | 80 |
| 10,000 | 100 |
| 30,000 | 200 |
| 100,000 | 400 |
| 500,000 | 800 |
This table has been simplified for gaming use. On an earth-sized planet, the horizon for a six-foot tall person standing at sea level or on flat plains will be about 3 miles. This means that they can see features that are at ground level for up to three miles (depending, of course, on the quality of their vision and the size of the object). Features that are higher than ground level can be seen further.
To determine how far away you can see something, just add together all of the heights. For example, if a 6 foot man is on a 4 foot horse standing on a 30 foot hill, how close would you have to be to a 60 foot tall tower to see it? First add all the heights together 6 + 4 + 30 + 60 = 100 feet. Look at the table under “height in feet” and find 100 feet. Then look across under “miles away” to find 13 miles. So the tower could be spotted if it was no farther away than 13 miles.
You could see a 14,000 foot mountain a little more than 100 miles away.
This is good for seeing features on a map, such as lakes, forests, mountains, towns, etc. but knowing how far you can see is often not what your Player Characters need to know. Just because you can see 3 miles doesn’t mean that you can see a monster on the horizon. For that we need another table.
Perception Distance Table
In this table “perceive” means that you can see it and may notice it with a perception check. If you do notice it you will recognize the creature type if you have seen one before. If you don’t know what type of creature it is you will be able to tell the creature’s coloration, size, shape, number of limbs, wings, etc. If the creature is moving, you will also be able to tell which direction it is traveling and about how fast.
“Identify” means that you can see details and may recognize an individual that you have met before.
Here is a simple rule of thumb that is accurate enough for gaming use:
Distance away (in feet) that you can perceive an item is its size (in feet) times 60.
Distance away (in feet) that you can identify an item is its size (in feet) times 12.
Round fractions down to the nearest 5 ft.
The item’s size is its longest dimension (height or width).
One more thing.
You can perceive a burning candle 1 1/2 mile away.
How Far Can You See from a Plane? : Top Reasons Affecting the Distance You Can See From an Airplane!
At first glance, this seems like a simple, straightforward question. Straightforward and simple, usually meaning there is much more to the question than meets the eye. Seriously, this is an excellent question and one that deserves a serious answer. From scientific to philosophic, it’s all how you look at the question.
So, how far can you see from a plane?
The technical answer is all well and good, but there are other things involved that could change that answer. That actual distance you can see can be affected by weather conditions, time of day, even the direction you look. Taking these things into consideration makes this more of a bar bet question than a scientific exercise.
How to Calculate How Far You Can See From a Plane
Well, the answer is quite short. In order to calculate how far you can see at any given height, you use any of the following formulas, depending on if you’re using the metric or imperial system.
Metric System
Imperial System
Factors Determining How Far You Can See From a Plane
The straightforward mathematical answer uses trigonometry. The equation calculates the distance from the observer to the point on the circumference of the earth the furthest away that can be seen, the horizon.
To get a slightly more elaborate answer that takes more aspects into account, we must describe the different conditions.
There are other ways to look at the question and other considerations to be taken. Before we get deeper into the discussion, let’s look at some of the more obvious physical factors that can be involved.
Let’s now look closer at these factors.
1. Is the Earth Really a True Sphere?
The formula used above to calculate the distance you can see the horizon from some height above sea level assumes a spherical earth. The truth is the earth is not a true sphere. As the earth spins, it bulges around the equator, but it doesn’t even bulge evenly. The masses of the continents are spread unevenly around the surface, causing even more distortions in the shape of the earth as it rotates.
Anything tall can affect the distance you can see when flying at altitude in a plane. For example, a 10,000-foot-tall mountain many miles over the horizon may still be visible above the edge of the horizon when viewed from 30,000 feet.
This can effectively add hundreds of miles to the distance you can see.
2. Light Can Bend (Refraction)!
Light doesn’t always travel in a straight line for one point to another, especially at great distances. Atmospheric conditions can reduce or enhance the distance one can see. A curious phenomenon that occurs in the atmosphere that surrounds the planet is called refraction.
This bending of light can cause light to seem to bend or curve over the horizon line, making it possible to see slightly further than the horizon, especially if looking at tall objects that appear over the horizon line. The overall effect is to make the taller object appear just a bit further away because the light is traveling further to get to your eye.
3. Atmospheric Conditions
Other atmospheric effects can certainly prevent you from seeing very far. If the plane happens to be flying through or over clouds, the distance you can see maybe nearly zero. Flying above clouds makes calculating the distance you can see more difficult.
In remembering our earlier calculation, there appeared a constant, 1.22 (3,6 in the metric system). That constant is mathematically derived using an approximation of the radius of the earth. You must rethink the entire equation to compensate for the altitude of the clouds.
Without going into a lot of in-depth trigonometry, let me say that the distance you can see is shortened because you are effectively closer to the top layer of clouds.
These are all capable of reducing over distance and must be considered when judging the distance you can see from the airplane.
3. Impact of Darkness and Light ( A little off-topic…)
Flying at night offers an entirely different view of the world below. I think most people would argue that after dark, the distance you can see from an airplane is considerably reduced. The answer for me comes from looking at things from a different perspective.
At night, a completely different opportunity presents itself. Consider not looking down but looking up. Sitting in that airplane cruising through the atmosphere at 30,000 feet, you have a unique opportunity that most people never think about.
The greatest hindrance to observing space from the earth is the thick blanket of atmosphere that surrounds us. Astronomers go to great lengths to mitigate the effects of the atmosphere on their telescopes by locating themselves at extreme altitudes on mountaintops.
In that airplane, you are 30,000 feet high. Much higher than the most remote observatory built on the ground.
Look up!
The atmosphere is much thinner and you will be surprised at what you can see.
That’s a long way. I guess, in a way, you can say that you are able to see that far at night flying in that airplane.
Final Notes
So, the answer to the question, “How far can you see from an airplane.” is not always what meets the eye. Scientifically and mathematically, that distance can vary a lot because of many different factors.
Environmental conditions, geological factors, and the time of the observation all make a difference in how far you can see from that plane in which you are riding. Your philosophical perspective can also provide an entirely different sort of answer by just looking away from earth and toward the heavens.
Источники информации:
- http://www.countrylife.co.uk/out-and-about/curious-questions-how-far-can-you-see-on-a-clear-day-219668
- http://www.universetoday.com/110858/how-far-can-you-see-in-the-universe/
- http://olddungeonmaster.com/2017/02/20/dd-5e-how-far-can-you-see/
- http://www.highskyflying.com/how-far-can-you-see-from-a-plane/