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Just how dark is Pluto from orbit? How about Saturn?

  1. Oct 28, 2012 #1
    Quick question that maybe someone here could answer. I could probably dig up a bunch of math equations and answer it too, but I'm guessing someone here already has a good idea about the answer.

    Placing the Sun at the center of a sphere whose radius is equal to the distance of the Sun to Pluto, and then comparing it to such a sphere whose radius is the distance from the Earth to the Sun would lead me to believe that there is significantly less light hitting some place like Pluto than there is hitting the Earth.

    Now if there is something like 1600x less light hitting Pluto, would that make a very noticeable difference in perceived brightness? What about Saturn, receiving something like 100x less light? I came to those numbers using the fact that Pluto and Saturn are 40x and 10x further from the Sun, respectively.

    I feel like something receiving 100x, let alone 1600x less light than something else would be like day and night, but being that one could see Saturn with their naked eye from Earth at night, it must not be that much different. Is perceived brightness on such a scale that orders of magnitude aren't hugely different and that 100x isn't that much to the human eye?

    This question kinda stems from the observation that movies always show space ships as bright when traveling through space, when in fact space is actually extremely dark. Just made me wonder how dark our solar system is in terms of perception.

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  3. Oct 28, 2012 #2

    Simon Bridge

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    Welcome to PF;
    "Percieved brightness" of what? You spend a lot of your post talking about the photon flux at the orbit radius but you seem undecided if you want the illumination of ones surroundings or the brightness of celestial objects.

    To give you an idea:
    You'll notice that we can see Saturn from the Earth.

    Saturn is pretty big so, although the light-flux is low compared to the Earth, there is more of Saturn to reflect it, and Saturn is pretty shiny.

    So if you were on a moon of Saturn, Saturn would be a big bright object in the sky.
    You could have a go seeing how bright it would be compared with the Sun at that distance - in terms of lighting up your surroundings. I'd suspect Saturn - being close by and very big, would significantly light up the ground.

    But bear in mind that you don't see the light flux - you only see the light that hits the back of your eyes. There are lots of places that can come from.
  4. Oct 28, 2012 #3
    Thanks. I'm talking about the brightness of a celestial body, like how bright the planet would look from orbit compared to how Earth looks from orbit.

    Earth from orbit is a gradient of light to dark, or rather day to night, assuming you're looking at it from that angle. I'm curious to know how a planet as far away as Pluto would appear. It would obviously exhibit a gradient too, but how much darker I am unsure.

    Would Pluto in direct sunlight be like Earth a 4pm? Twilight? Midnight?

    I've just looked up lux and I see that the lux on Earth in direct daylight is maybe 100000, while a very dark, overcast day is 100. Assuming the photon flux at Pluto's distance was 1000x lower for easy math, would it be right to assume that Pluto in direct daylight would look like a very dark overcast day on Earth?

    Eyes adjust, graphs aren't always linear, etc. A number could be 1000x bigger, but I don't really know what a 1000x brighter/dimmer actually looks like. I'm just trying to get an idea of how much less light the planet receives and how to put those numbers in perspective.

    Last edited: Oct 28, 2012
  5. Oct 28, 2012 #4


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    Pluto direct sun illumination would look like average residential indoor lighting at night, or a typical 100 W tungsten light at 8 feet.
  6. Oct 28, 2012 #5

    Simon Bridge

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    You want the reflected light from the body - to your eyes - then, not just the amount of sunlight incident on the body. How bright something appears to the eye depends on it's angular size to the eye and it's albedo[1] as well as the amount of light shining on it (also the portion of the spectrum reflected).

    Compare Moon's brightness from the Earth vs earth's brightness from the Moon.

    BillSaltLake has the specific answer you are looking for.
    (I'd have liked to see the reference for that though.)

    [1] "albedo" - look this one up - the results will explain a lot about what makes some objects brighter than others.
  7. Oct 28, 2012 #6


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    100 W bulb puts out about 5 lightwatts; zenith sun at earth is about 300 lightwatts/m².
  8. Nov 4, 2012 #7
    Indeed, a measure of brightness as described by a human can only be a very rough approximation. Iris adjustments and neural circuitry will mess with perceived brightness levels and humans can see well in a large range. Few people realize that outdoor sunlight is enormously brighter than indoor lighting, which in turn is much brighter than full moonlight. Humans can see well enough in all three scenarios. As others have said, the surface of Pluto would be lit as well as average indoor lighting. Even with a very dark albedo Pluto should be easy to observe from orbit by human eyes.
  9. Nov 5, 2012 #8


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    I'm not as astronomer but don't we use Apparent Magnitude to describe relative brightness? Pluto is around 14 and I understand that the limit of naked eye vision is about 6 (see this wiki link). That would suggest that Pluto is far too dim to see with the naked eye.
    I have a feeling that the above calculations have not taken into account the distance back to Earth from Pluto when estimating its visibility. Standing ON PLuto, the illumination would be about 1/1600 of noon on Earth but that's only half the story.
  10. Nov 5, 2012 #9


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    No, not at all. The light must make it back to us, further reducing it's intensity. Pluto's average apparent magnitude is 15.1, which is FAR too dim for us to see it with the naked eye, even from orbit. The best eyesight for a human can barely make out something at mag 7, which is about 1500 times brighter than mag 15.
  11. Nov 5, 2012 #10


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    The OP asked about the relative surface brightness of Earth vs. Saturn vs. Pluto when viewed close. Obviously an object will reduce in brightness if viewed from far away (specifically if the angular size is < 1 minute of arc, it cannot be resolved by the eye, so the farther away it is brought, the dimmer it looks).
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