Anonymous
Anonymous asked in Science & MathematicsPhysics · 10 years ago

# How fast does something have to go before the human eye can't see it?

I am writing a story and I need to know how fast someone or something would have to be moving before they couldn't be seen by the naked eye.

What I am really interested in is how fast something would have to be moving on the face of the planet, so that other people in the vicinity couldn't see it. I know that objects at great distance appear to be moving slower, as do objects of great size, but I'm more interested in the area that a human can see in a normal set of surroundings in the face of the planet.

I believe that I read somewhere that on the ocean that its eleven miles horizon to horizon that a person can see before the curvature of the earth makes it impossible to see something at the same relative height from the surface as them.

So somewhere in that range.

I want the details I put into my story to be as accurate as possible, because the fan base that I am going for will tear it apart if they're not.

Relevance
• 10 years ago

First off, these figures aren't set in stone, I'm just using some common sense. I've been to enough air shows to get an idea of what speed you would need.

The F-14 Tomcat you could begin to see at about 7 miles out (really good eyes could see it at 8 miles out) . The F-14 is 63 feet long. So using that as kind of a base, that you can see an object 63 feet long at 7 miles with the naked eye.

Drawing a 7 mile radius, with you being at one apex, you have a triangle with 7 mile long legs and your field of vision would be 60 degrees. Now it is a matter of determining how long it takes for your eyes to sweep across the horizon in a 60 degree pattern, 7 miles out.

I would suggest you need not turn your eyes at that distance (kind of like seeing a shooting star where you only see it in your peripheral vision ) and rely on how fast your peripheral vision is.

I would think you could detect a 1/20 th of a second fairly reliably.

So if you could detect an 63 foot long object 7 miles out going 7 miles in 0.05 seconds that is:

36,960 feet / 0.05 sec = 504,000 mph or 140 miles per second or 225,308 meters/sec

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Of course you would have a problem with atmospheric "burn" at a height of 2 -7 miles, so unless your vessel had a way of getting around air friction you would have to take the object at least 25 miles high. If you take it say 5 times higher, then you have to multiply its size by 5 to be able to see it (about 325 feet long) and then everything else would multiply by 5 too. Five times faster or about 700 miles a second.

• 10 years ago

Do not mix eggs with apples.

You are asking "how fast" and then giving some figures for "how far"

If you ask, "what is the DISTANCE the eye cannot see" then the answer is related by the curvature of the planet.

But, if you ask, "what is the SPEED the eye cannot see" then the answer is over the light speed.

You can see the fastest thing, LIGHT, can't you?

That travels by 300,000 km/second.

On the other hand, it also depends your point of observation. Does the object (1) approach you as it is going to hit you? (2) Or does it pass, say 20 m away from you? (3) Or does it go in a circular orbit around you?

In case of (1) limit of observation is light speed

For (2) and (3) cases, I cannot comment - I don't know..........

• K6 years agoReport

....you can see light?
Really?
You actually see undulating wavelengths of light?

• 4 years ago

tldr: it varies a LOT.

in tests with fighter pilots it was shown that they could perceive and sometimes identify plans that were shown to them for 1/200th of a second. thus a object would need to cross your field of vision in under 1/200th of a second.

as for the objects rate of travel it depends on distance and angle of travel. something moving laterally in front of your eyes a foot from you face has a much shorter distance to cover than something moving laterally in front of your face at a distance of 100 yards. like how trains look slow from a distance but fast up close.

additionally, if the object is moving in a non lateral fashion ie a car driving towards you, it would have to cover the entire "detection range" of the human eye which would require much higher velocities than the lateral examples.

also

something moving faster than the speed of light COULD be perceived, as counter intuitive as that seems. just as a bullet traveling much faster than a falling raindrop will still get wet if you fire it through a rainstorm, a faster than light object would still be struck by light, and that light would still reflect off of it. thus if it is approaching you it is outrunning the reflected light and you will not possibly see it. if it were moving laterally and it were far enough away it would appear to blink across your field of view as random reflections occurred. of course it would have to be VERY far out, and VERY large for you to see it flicker across the sky... so while possible, highly unlikely. and it would physically be gone by the time the light reached your eye, so you would be seeing after images....