How can we see the ISS at 200 miles away with the naked eye?

[TruthSeeker777]

TL;DR Summary

how can we see the ISS at 200 miles away?

NASA says the ISS is 200 miles away.
Planes fly at around 10 miles from the surface and we can't see them due limits to the angular resolution of the human eye.

Given the inverse square law of light and the ISS being 20 times the distance to the plane, how is it possible to see the ISS?

 

[malawi_glenn]

Not during daylight.

ISS reflects light from the sun, like the moon.

 

[russ_watters]

Not during daylight.

ISS reflects light from the sun, like the moon.

...and our eyes are shockingly sensitive. It's tough to tell because they adjust automatically, they have a very wide range of brightness we can see.

Consider how the moon during the day (the illuminated part) looks dim, but at night appears almost blindingly bright even though it hasn't actually changed.

 

[DaveC426913]

Planes fly at around 10 miles from the surface and we can't see them due limits to the angular resolution of the human eye.

Do you mean commercial airliners? What makes you think they can't be seen due to resolution limits?

And if they can't be seen, it is generally a contrast issue against the sky, or a resolution issue.

 

[TruthSeeker777]

Do you mean commercial airliners? What makes you think they can't be seen due to resolution limits?

And if they can't be seen, it is generally a contrast issue against the sky, or a resolution issue.

The angular resolution of the eye which is 0.02° and corresponds to 0.3 m at a 1 km distance. If the plane has a wingspan of 15 m then the average person should just be able to see it at 45 km. You will just about see it as dot rather than anything that looks like a plane. This is assuming very clear sky conditions.

I can't see how the eye can see the ISS which is at 20 times distance at all.

 

[TruthSeeker777]

...and our eyes are shockingly sensitive. It's tough to tell because they adjust automatically, they have a very wide range of brightness we can see.

Consider how the moon during the day (the illuminated part) looks dim, but at night appears almost blindingly bright even though it hasn't actually changed.

Your eye cannot see anything that is beyond the limits of its angular resolution. Either the object must be bigger or closer.

 

[FactChecker]

Your eye cannot see anything that is beyond the limits of its angular resolution. Either the object must be bigger or closer.

The angular resolution of any star (other than the sun) is much smaller than the resolution limit of the eye.

 

[anorlunda]

Here's an example even more dramatic than the ISS. A hand held signal mirror. The video shows it seen from 0.7, 11.1 and 43 miles away.

 

[DaveC426913]

If the plane has a wingspan of 15 m

For a typical commercial airliner, it's closer to 35m, and up to as much as 80m.

You will just about see it as dot rather than anything that looks like a plane.

Right. So you can see it. Nobody said anything about making it out to be more than a dot.
The ISS is naked eye visible only as a dot.Now that the premise of your confusion (the comparison to aircraft) has been dealt with, let's move on to the meat of it.

The ISS is seen because of its extremely high contrast, not because of its angular resolution. It is seen by direct unfiltered starlight reflecting off its very flat solar panels, so it's very bright. Far brighter than any aircraft.

And as FactChecker has pointed out, distant stars are easily visible despite being no more than points in our vision.

The crux of the issue is brightness of source against background.

 

[DaveC426913]

Thanks, that is only because its a convex mirror.

Check your facts. It looks pretty flat to me.

 

[FactChecker]

I really do not know how much the visibility of bright pin-point of light depends on some diffusing in the atmosphere to make a larger. Are the stars visible to astronauts in space?

UPDATE: Apparantly, stars are visible to astronauts in space. https://phys.org/news/2017-08-astronauts-stars-space-station.html

 

[berkeman]

Thanks, that is only because its a convex mirror. Light spreads out when its reflects off a convext mirror. If the mirror was flat you wouldn't be able to see it beyond the angular resoolution limits? Pictures of the ISS don't seem to indicates concave surfaces from what I can see?

All my signal mirrors are flat. Any curve at all to them would defeat their operation. What is the basis for you claim that the video shows a convex signal mirror? Please cite your source on that.

Also, so far in this thread, you do not seem true to your username...

 

[anorlunda]

Thanks, that is only because its a convex mirror.

It is not convex. It is flat.

 

[FactChecker]

Thanks, that is only because its a convex mirror. Light spreads out when its reflects off a convext mirror. If the mirror was flat you wouldn't be able to see it beyond the angular resoolution limits? Pictures of the ISS don't seem to indicates concave surfaces from what I can see?

A convex mirror would not appear as a single point of light over the large range of distances from 0.7 miles to 43 miles.

 

[anorlunda]

Another example. Iridium flares. They are reflections from the solar panels of Iridium satellites. I've seen them many times.
https://en.wikipedia.org/wiki/Iridium_satellite_constellation

https://www.heavens-above.com/
That website will show you where and when to look to see similar reflections from other satellites.

 

[TruthSeeker777]

It is not convex. It is flat.

ah yes, you're right its not convex. Thanks.

 

[hutchphd]

But the lack of convexity is one of the hallmarks of much of stealth design (look at the SR71!) . A flat surface will reflect a point source only over a very small range of angles. It makes it impossible to track. This is also why you must actively wobble the signal mirror.
The other suppositions about visibility from @TruthSeeker777 are incorrect as noted.

 

[TruthSeeker777]

Thanks, that is only because its a convex mirror. Light spreads out when its reflects off a convext mirror. If the mirror was flat you wouldn't be able to see it beyond the angular resoolution limits? Pictures of the ISS don't seem to indicates concave surfaces from what I can see?

CORRECTION - its not a convex mirror.

Here's an example even more dramatic than the ISS. A hand held signal mirror. The video shows it seen from 0.7, 11.1 and 43 miles away.
View attachment 304876

This is a good point. A mirror certainly reflects the vast majority of light from it. Is the suggestion that solar panels from the ISS are reflecting the vast majority of light that hits it from the sun? The solar panels I've seen do not seem to be very reflective and are grey in colour.

 

[DaveC426913]

This is a good point. A mirror certainly reflects the vast majority of light from it. Is the suggestion that solar panels from the ISS are reflecting the vast majority of light that hits it from the sun?

According to Google (which is cheating here):

"Those solar panels convert 30% of the sunlight into electricity, and another part into heat. The amount of light reflected is only about 20% (less than the reflection coefficient of e.g. grass)"

The solar panels I've seen do not seem to be very reflective and are grey in colour.

The reflectivity of the Moon is 0.12 - about that of blacktop asphalt.

The point being that, in contrast to the blackness of space - especially when your eyes are attuned to it - sunlit blacktop asphalt is very bright.

 

[TruthSeeker777]

I really do not know how much the visibility of bright pin-point of light depends on some diffusing in the atmosphere to make a larger. Are the stars visible to astronauts in space?

UPDATE: Apparantly, stars are visible to astronauts in space. https://phys.org/news/2017-08-astronauts-stars-space-station.html

This is a very good point. I have certainly seen pictures of astranauts & ISS where the sky behind them is pitch black and no stars are visible.

 

[hutchphd]

Yes the stars are visible to astronauts in space. This was one of the loony items brought up by the Apollo deniers. Why would they not be??

 

[Baluncore]

But the lack of convexity is one of the hallmarks of much of stealth design (look at the SR71!) . A flat surface will reflect a point source only over a very small range of angles. It makes it impossible to track. This is also why you must actively wobble the signal mirror.

The mirror can be flat because the Sun is over half a degree in diameter, which is greater than the angular resolution of your eye. With RADAR, the illumination angle is very much less.

 

[TruthSeeker777]

According to Google (which is cheating here):

"Those solar panels convert 30% of the sunlight into electricity, and another part into heat. The amount of light reflected is only about 20% (less than the reflection coefficient of e.g. grass)"The reflectivity of the Moon is 0.12 - about that of blacktop asphalt.

The point being that, in contrast to the blackness of space - especially when your eyes are attuned to it - sunlit blacktop asphalt is very bright.

Certainly but the moon is far far bigger than the ISS. If I use a flashlight on an object that is smaller than the angular reolution would allow for in the dark should I be able to see it?

There are two factors at play here:

1. Inverse square law - i.e an object's brightness (measured in Lumens) dereases by 1/4th every time you double the distance to it
2. Angular resolution of the human eye

 

[hutchphd]

Yes of course. But you still have to wobble your rescue mirror!

 

[Baluncore]

Yes of course. But you still have to wobble your rescue mirror!

Why? Scratch a hole in the mirror, so you can look through at your outstretched arm thumb. Point your thumb at the target, then reflect the Sun onto your thumb. Problem solved.

 

[hutchphd]

So when the rescue plane flies over Is your thumb sight good to half a degree? Also you want the Flashy Wobble for contrast. I'll use the wobbly eyeball approach, thanks!. Actually I would do both...

 

[Baluncore]

Is your thumb sight good to half a degree?

It works well. Measure the size of your thumbnail against the Moon or the Sun. A steady, bright light is easier to see from a moving aircraft than momentary flashes. I try not to need it.

 

[russ_watters]

This is a very good point. I have certainly seen pictures of astranauts & ISS where the sky behind them is pitch black and no stars are visible.

Note again that the dynamic range of our eyes is very wide. A camera taking a photo of a brightly lit astronaut won't capture the dim stars.

 

[DaveC426913]

Certainly but the moon is far far bigger than the ISS.

The point is simply that this:

The solar panels I've seen do not seem to be very reflective and are grey in colour.

is a poor yardstick.

 

[DaveC426913]

Certainly but the moon is far far bigger than the ISS. If I use a flashlight on an object that is smaller than the angular reolution would allow for in the dark should I be able to see it?

I think we've established that the visibility of stars pretty much resolve any doubt.

 

[DaveC426913]

Why? Scratch a hole in the mirror, so you can look through at your outstretched arm thumb. Point your thumb at the target, then reflect the Sun onto your thumb. Problem solved.

Anorlunda's rescue mirror is solid metal - for durability.

Otherwise though, it's a clever idea for sighting.

However, it does not obviate the need to wobble the mirror.

 

[Baluncore]

However, it does not obviate the need to wobble the mirror.

If you want to send morse CW you will need more reliable dots and dashes than wobble. I think you are assuming high contrast green grass background, not snow, or the glint of wind on small lakes.

The lights on Boeing aircraft flash once, Airbus flash twice, which better confirms their position in your vision.

If there is no hole in the mirror, use the edge of your thumb and the edge of the mirror. Practice aiming at a nearby object before you need it.

 

[DaveC426913]

If you want to send morse CW you will need more reliable dots and dashes than wobble. I think you are assuming high contrast green grass background, not snow, or the glint of wind on small lakes.

I'm not sure where morse code or background contrast factors in. I am simply assuming virtually any situation where the intended target doesn't know ahead of time exactly where to look - and all we're trying to do is get their attention, not send a (more complex) message. For example: the provided video.

A flashing point of light will be orders of magnitude more catching to the eye than a steady dot, which could easily be overlooked in almost any real world scenario (plausibly including the provided video).

 

[Baluncore]

A flashing point of light will be orders of magnitude more catching to the eye than a steady dot, which could easily be overlooked in almost any real world scenario (plausibly including the provided video).

I believe the point of light should turn on and off in a steady and controlled manner, not randomly flash or flicker.

 

[DaveC426913]

I believe the point of light should turn on and off in a steady and controlled manner, not randomly flash or flicker.

I am more confused now. This is the arc of the pertinent conversation, as I see it:

Post 24:

Yes of course. But you still have to wobble your rescue mirror!

Post 25:

Why? Scratch a hole in the mirror, so you can look through at your outstretched arm thumb. Point your thumb at the target, then reflect the Sun onto your thumb. Problem solved.

Anorlunda's rescue mirror is solid metal - for durability. Otherwise though, it's a clever idea for sighting.

However, it does not obviate the need to wobble the mirror.

Oh I think I see. Are we talking past each other?

Are you associating "wobbling the mirror" with the mirror-holder and his attempt to place the reflection on the target. (which is why your solution concentrated on accurately aiming the mirror)?My understanding is that the mirror-wobbling is to make the target see the flashing - which is far more noticeable than a steady light. For that, the holder needs to manually flash the target on and off.So, now:

I believe the point of light should turn on and off in a steady and controlled manner, not randomly flash or flicker.

Other than wobbling it back and forth - how might one do that with an inert hand-held mirror?