Can distance in space be measured by human systems?

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Summary:

I am puzzled by great distances in space and how we perceive it
I'm past middle age, and it seems I should have fewer questions about life and the universe than ever. I have more now. For some reason this past year or so I've been absolutely consumed with trying to grasp the immensity of the universe and distances. For whatever reason, I'm having more difficulty with this than I did in high school physics. I'm not a kook nor by any means a flat-earther. I do believe it's more of a certainty now more than ever that life exists in many places in the universe. But mainly it's those distances that are nagging me. I find it harder than ever to imagine that we can actually see even a tiny dot of light that represents Jupiter or Saturn. 562 million miles away is a staggering, unimaginable distance. Yet every night I can look up and I'm told a particular point of light is, intact, that far away and I can clearly see it.

But the fact that I cannot see the beam of a flashlight from one side of my town to the other, even if there were no atmosphere because of the very small size of the light itself, makes it harder to believe I can see Jupiter or Saturn. In fact, I doubt my flashlight's light could be seen from one moon horizon to the other for the same reason and there's virtually no atmosphere there to distort or block it. So, how is it that we can see REFLECTED light from Jupiter from millions of miles away. It doesn't generate it's own light like a star. It seems that just the distance alone, atmosphere or no, would render it so very tiny, relatively, that even when its light eventually does reach the earth it would be so small and so tiny we wouldn't be able to even see it with telescopes. I can see the rings of Saturn with just a cheap department store telescope! How is it possible that such a small magnification would allow me to even see that planet over a billion miles away??!! It makes absolutely no sense to me.

I'm not senile! ;). But why is my mind turning on me like this? We shouldn't be able to see any trace of any object that far away, telescopes or not, in my mind. Of course this is a crazy assertion. Centuries of science and the most brilliant minds have told us we can see them and how far away they are. But I sat bolt upright in bed the other night wondering if somehow our science has fooled us into thinking our systems of measurement apply to space when they really don't and our calculations have misled our conception of those kinds of distances, that things we see in the night sky are actually much, much closer than the miles and lightyears away they are. Of course we've proven those measurements are accurate, we've sent rockets to explore and it was our calculations that got our instruments there because the science we've developed are proof that we have calculated this accurately.

So, why is this persistent notion that we're somehow wrong about all this somehow plaguing me? My dad was an aircraft carrier jet pilot and then engineer for Boeing and Lockheed until he retired. He had certain top secret clearance was all he shared when asked certain questions, especially about UFOs. He and I used to spend lots of time in summer looking at the night sky, him pointing out the fight patterns of satellites, talking about why life elsewhere was fairly certain. He even told me that there were things that people reported as UFOs might or might not be. That some could be made by us and some things that weren't made by humans were indeed flying around. This was before stealth bombers were unveiled as part of our military arsenal. And when I was a kid, from the first US rocket sent men into space he let us stay home any time there was a daytime launch from NASA after those were routine enough that they no longer let us watch them during class at school. So, I've always been fascinated with what's going on up there. ;)

So, anyone feel like trying to explain why this old lady is bothered that she can see Saturn with the naked eye, from one billion miles away? Anyone? Just for fun? It might help me sleep a little better.
Cheers, Space Fans!
Denise
 

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  • #2
phinds
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Have you ever looked up in the night sky and seen stars with your naked eye? The distance to most of those stars makes the distance to Saturn be not even a rounding error it is so small in comparison.
 
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  • #3
russ_watters
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This is bothering you precisely because the distances involved are crazy. But you can verify the reality of these things with simple math. The size of Jupiter as seen in a telescope is just found by solving a triangle. Draw it and solve for the angle. The amount of light received is an inverse square function of distance and reflectivity (albedo). Note that Saturn is noticeably dimmer than Jupiter.

By the way; do you own a cheap department store telescope? If not, maybe you should buy or borrow one....it's a little easier to believe when you see it with your eyes.
 
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  • #4
phyzguy
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I think you are asking why you can see astronomical objects like Jupiter or Saturn at great distances, while you can't see a flashlight that is only a few miles away. The simple answer is that Jupiter and Saturn are hugely brighter than a flashlight. And as @phinds pointed out, you can see stars that are much, much further away than Jupiter or Saturn. The reason is that these stars are hugely brighter than Jupiter or Saturn. It really is as simple as that.
 
  • #5
Ibix
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The simple answer is that Jupiter and Saturn are hugely brighter than a flashlight.
I think it's worth pointing out that any single square centimeter of Jupiter's surface is much less bright than a square centimeter of a good flashlight (I assume - I think reasonably, but I haven't actually run the numbers). But there are a lot more square centimeters of Jupiter's surface than your flashlight. So the total brightness is a lot higher, which is why you can see it.
 
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  • #7
Have you ever looked up in the night sky and seen stars with your naked eye? The distance to most of those stars makes the distance to Saturn be not even a rounding error it is so small in comparison.
Yeah, I understand all that. But stars are Huuuuge suns that generate an enormous amounts of light! Saturn simply reflects light from a sun billions of miles from it! Look we can’t even see satellites in near-earth orbits very well. Because they’re so far away, not because they reflect less light. Even for its size, Jupiter is not hundreds of thousandS, not hundreds of millions but BILLIONs of miles away.

We shouldn’t be able to see anything at all from that far away that doesn’t produce it’s own intense light! And someone compared a flashlight size with Jupiter’s size, suggesting that Jupiter has more “sq centimeters“ more of light reflected. That‘s not how that works, either. The surface area of Jupiter doesn’t magnify the intensity of the reflected light, it just means it has a much larger surface area, it’s just bigger, is all. The light reflected from a small planet is the same brightness as a large one the same distance away. But it’s harder to see because it’s smaller.
 
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  • #8
hutchphd
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Many people's minds seem to turn inward with age. I am fighting that with every ounce of strength.
If I can be so presumptuous I ask you to celebrate your lack of complacency. I am approaching 70 and figured by now I might have more answers than questions. Thankfully the opposite is true.
 
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  • #9
russ_watters
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Look we can’t even see satellites in near-earth orbits very well. Because they’re so far away, not because they reflect less light.
Well of course we can. Some are even brighter than Jupiter. But again, the math will tell the story. How big, exactly, is a satellite in angular diameter? How big is Jupiter? How much light does each reflect?
The surface area of Jupiter doesn’t magnify the intensity of the reflected light, it just means it has a much larger surface area, it’s just bigger, is all. The light reflected from a small planet is the same brightness as a large one the same distance away. But it’s harder to see because it’s smaller.
You're contradicting yourself there. Yes, the surface brightness is a function of distance. Yes, bigger means more total light. These two effects have to be combined to get the resulting total light. And remember: it also matters if your eye can see it as more than a point of light.

Again, again, again; if you want to really know what the answers to these questions are, you need to do the math rather than just reacting to a gut feeling of incredulity.
 
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  • #10
hmmm27
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Jupiter is - on a good day - 600million km away from Earth ; and it's about 140,000km wide. Working that out and your "flashlight" is more than a foot wide for each mile of distance. City's 10mi across ? That's about an 11-12 foot diameter flashlight.
 
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  • #11
sophiecentaur
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Yeah, I understand all that. But . . . . . . .
I have a feeling that you are letting your heart rule your head if, indeed you do 'understand it all'. All of modern Science works reliably (always with a certain level of uncertainty, of course). So, if you accept that the machine you are using right now, to read this, works because that same Science actually works and you can safely accept that the numbers involved in basic astronomy can be relied on.

Comparing what you see of Jupiter with what you see of the brighter stars, it's due to the light power coming off them and their respective distances. The sums works out right, as far as the amount of light getting into your eye. They also apply to your small torch at the end of the road.

It's only in the last few years that we have managed to see Jupiter sized planets in orbit around distant stars. The (the very same) sums tell us that we need very good telescopes and some very clever computing to do what our eyes cannot.

Sums work with seeing planets, money in your bank account and miles per gallon calculations, although I would say that we can be a bit more dispassionate about the astronomy stuff.
 
  • #12
PeroK
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Yeah, I understand all that. But stars are Huuuuge suns that generate an enormous amounts of light! Saturn simply reflects light from a sun billions of miles from it! Look we can’t even see satellites in near-earth orbits very well. Because they’re so far away, not because they reflect less light. Even for its size, Jupiter is not hundreds of thousandS, not hundreds of millions but BILLIONs of miles away. We shouldn’t be able to see anything at all from that far away that doesn’t produce it’s own intense light! And someone compared a flashlight size with Jupiter’s size, suggesting that Jupiter has more “sq centimeters“ more of light reflected. That‘s not how that works, either. The surface area of Jupiter doesn’t magnify the intensity of the reflected light, it just means it has a much larger surface area, it’s just bigger, is all. The light reflected from a small planet is the same brightness as a large one the same distance away. But it’s harder to see because it’s smaller.
The human eye is very sensitive:

https://math.ucr.edu/home/baez/physics/Quantum/see_a_photon.html
 
  • #13
PeroK
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But the fact that I cannot see the beam of a flashlight from one side of my town to the other,
What sort of experiment have you done? When I am in the Alps, where you often climb at night, you can see the headlamps of climbers on neighbouring mountains many miles away.
 
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  • #14
Klystron
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When I read the title of this thread, I expected concerns about the limits of human-based measurement units applied to stellar and galactic distances.

The human body works reasonably well for measuring mundane distances on the surface of the earth. A foot is slightly larger than the foot of the largest person in the village. A yard is close to the length an adult can fold cloth; close to three feet or one adult step. A mile was approximately 5,280 feet; and so on. While useful around the village and farm, these familiar units were less useful for seafaring which developed different systems of measurement, and even less adequate for measuring stellar distances.

Two suggestions:

Consider adopting astronomical units to measure and compare planetary distances. Then, instead of thousands of millions units, Jupiter resides ~5.4 AU distance; practically next-door neighbors.* Understanding AU leads to understanding stellar parallax measurements of nearby stars, and so on.

Adopt scientific notation, particularly for very large and very small numbers. A number such as 106 is easier to handle than a million. No reason to count strings of zeroes; just look at the exponent. On an international forum, terms such as billion signify different amounts in different countries and cultures.

*From memory. Please correct any mistakes.
 
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  • #16
sophiecentaur
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When I read the title of this thread, I expected concerns about the limits of human-based measurement units applied to stellar and galactic distances.
I think the OP is unused to hopping between various distance units but she would be familiar with the fact that we measure the thicknesses of wire or needles in thousandth of an inch (or perhaps microns) and the distance to Antarctica in miles or km. We use the appropriate units. Likewise Astronomical Units are great for comparative distances in the solar system and light years or parsecs for inter stellar distances. That way we can compare like with like and avoid massive numbers of zeros before or after a decimal point.

It's a big ask to assume that, without some special knowledge (numerical values) , it would be possible to know just how visible a given object may be to an observer on Earth. After getting used to a subject, many experts can make better guesses about the magnitudes involved but, as with votes in elections, it's no good using the term 'a lot' in any sensible discussion about anything. Thereafter, the numbers can nearly be relied on.
 
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  • #17
sophiecentaur
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Look we can’t even see satellites in near-earth orbits very well. Because they’re so far away, not because they reflect less light.
It's for both reasons. They are more than 100km away and are only the size of a bus (it's mainly the size that accounts for how much energy is reflected from the Sun (and they are only visible when they are illuminated by the Sun from the side). Most of the time they are either behind the Earth in shadow or swamped by the brightness of the sky in the day. The amount of light energy that reaches our eyes is about the same as from incredibly distant stars with incredibly high light output. The numbers count in these things and you cannot afford to rely on 'intuition'.

Also you can see the ISS very clearly in the evening and morning (when it happens to be in the right place) because it is sooooo big and its massive solar panels keep pointing at the Sun. It's visible for just a few minutes at a time so you need to look up where and when to look. (Very satisfying experience and "you can't miss it" when it flies over.)
 
  • #18
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One thing you could do is check the soundness of individual steps of the distance ladder.
Jupiter is about 900 million km away near conjunction.
Moon is about 384 000 km away - it varies. So if the distance is 375 000 km (within range of variability) then Jupiter is 2400 times more distant than Moon.
Now look at crescent Moon - new or old. Does it look bright against the darkness of night sky? Do the horns stand out, even at the tips that get too narrow to resolve their width?
It is crescent Moon which is near the Jupiter near conjunction.
Moon has about 3500 km diametre. Jupiter has 140 000 km. Jupiter is 40 times bigger than Moon.

Both Moon and Jupiter are merely reflecting light of Sun. Does it look plausible that Jupiter is visible but only as a dot because it is 1/60 the angular size of Moon? If so, and Jupiter really is 40 times bigger than Moon, it follows it really is 2400 times more distant.
 
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