Have you ever gazed at a seemingly endless horizon — or up at a star-filled sky — and wondered just how far the human eye can see? That horizon is about 3 miles away. And the closest star (besides the Sun) is over 24.9 trillion miles from Earth.
Despite more than 70 years of research, science remains unclear about the absolute threshold of human vision.1 Scientists do know that, with enough light, human vision is limitless.
What’s the Farthest We Can See?
The farthest object visible to the naked eye is the Andromeda Galaxy, located 2.5 million light years from Earth. Starlight from the Andromeda Galaxy travels 23 quintillion miles before it reaches your eyes.
With modern technology, it would take 94.5 billion years for humans to travel to the edge of the Andromeda Galaxy. So, the human eye can see pretty far.
What Determines How Far We Can See?
How far you can see depends on whether the particles of light (photons) that a distant object emits reach your eyes.1-4 For example, the stars in the night sky collectively give off a few thousand photons.
Vision scientist Selig Hecht and colleagues determined that it only takes five photons to excite the light-detecting rod cells in the human eye.3
Distant Objects We Can See
If you look up at the sky on a dark night, your eyes might detect light coming from over a million light years away. At 2.5 million light-years away, the Andromeda Galaxy is the farthest object the human eye can see.
Other distant objects the human eye can see on a dark night can be found at:
- 22,000 miles. The distance that satellites orbit the Earth.
- 238,855 miles. The distance at which the Moon orbits the Earth.
- 38 million miles. The closest distance of Venus, the brightest object in the night sky besides the Moon.
- 746 million miles. The distance of Saturn, the furthest planet that’s visible to the naked eye.
Stars and planets appear as just a twinkle of light because they’re so far away. Your eyes can also detect human-scale objects a great distances here on Earth.
Nearby Objects We Can See
Near objects visible to the human eye are at the following distances:
- 1 mile. The approximate distance from the bottom of the Grand Canyon to the upper rim.
- 2 miles. The distance your eyes could make out two distinct headlights.
- 6.6 miles. The height of the average passenger airplane.
- 50 miles. The distance from which city skyscrapers can be seen from the ground on a clear day.
Factors That Affect How Far Someone Can See
Many factors affect how far the human eye can see, including:
- Visual acuity
- Eye health
- Size and brightness of the object
- Line of sight
- The curvature of the Earth
Visual Acuity
Visual acuity is the clarity of your vision. People with 20/20 vision have normal visual acuity. This means you can stand 20 feet from an object and see what someone with normal vision should see at that distance.
If you have 20/25 vision, you can see something at 20 feet that someone with normal vision can see at 25 feet.
Eye Health
Your eye health is a major factor in how well your eyes function. Good eye health leads to better eyesight. That’s why it’s important to get regular eye exams and use protective eyewear when needed.
Size and Brightness
Larger objects are easier to see because more light reflects off them. Brightness also plays a huge role when viewing objects at a distance.
Brighter objects produce more photons. The more photons, the easier to see. The Sun is 93 million miles away. Yet it’s easily visible because it’s a massive, brightly glowing object.
Line of Sight
Your line of sight is an uninterrupted visual angle between your eyes and what you’re trying to see. Many things can obstruct your line of sight and affect how far you can see, including:
- Buildings
- Clouds
- Trees
- Dust
- Fog
- Smoke
Earth’s Curvature
The curvature of the Earth creates another visual obstruction. From a vantage point of 5 feet (standing on the ground), you can see the horizon about 3 miles in the distance. After 3 miles, the Earth’s curvature interrupts your line of sight.
From a higher vantage point, the horizon could be much farther away. For example, if you were standing atop the world’s tallest building (Burj Khalifa in Dubai), you might see 50 miles on a clear day.
How the Eye Processes Images
Visual perception results from rapid communication between your eye and brain. This is how it happens:
- Light hits an object. Some of the light is absorbed, and some reflects off the object.
- The reflected light enters your eye through the transparent outer layer (cornea).
- The cornea bends the light rays toward the pupil (the dark center of the eye).
- Muscles in the iris make the pupil larger or smaller. This controls how much light hits the lens.
- The lens focuses light on the retina, the layer of tissue at the back of your eye. The retina contains two types of light-sensitive photoreceptors, called rods and cones.
- Rods and cones are tiny nerve cells that change light rays into electrical impulses. These travel through the optic nerve into the brain’s visual cortex.
- The brain processes the impulses to create an image.
What We Can’t See
Many internet statements claim the human eye can detect the faint glimmer of a candle flame at distances of 10 miles or more. Some claims extend up to 30 miles. However, recent research suggests you can’t.5
While testing these claims, Texas A&M scientists calculated that the naked eye can see a candle flame at a distance of 1.6 miles.5
These candle flame claims are likely based on the understanding that an object of any size should be visible at any distance as long as enough photons reach the eye.1-4
However, a single candle flame is unlikely to compete with all the other light our eyes pick up. You probably need a pair of binoculars and good night vision to see a candle flame flickering 10 miles away.
Summary
There’s no defined maximum distance to human vision. Vision scientists conclude that how far you can see depends on how many particles of light (photons) a distant object emits.
Various factors affect how far you can see, including your eyesight and the health of your eyes.
Larger and brighter objects emit more photons, which makes them easier to see at a distance.
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