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There have been several questions about the construction, operation and destruction of a Dyson sphere. I'm interested in something a bit less exciting: how far out can you still see it from?

Assume it is not habited and exists purely for collecting energy. Also disregard telescopes.

My civilization is a little... primitive. Not too advanced and not exactly aware that their ancestors traveled the stars. They're also easily excited and quick to jump to conclusions. I'd imagine a visible alien megastructure enveloping their source of life would be a topic for conversation, even if it is completely incomprehensible to them.

I've read that one can see sunspots with the naked eye. Supposedly they played a part in ancient Chinese astrology. I've also read that a Dyson cloud would alter the light emitted by the star but it would most likely be too small to be noticed on Earth.

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    $\begingroup$ How thick the Dyson ring is is key. A thicker ring would obviously be more visible. $\endgroup$ – Braydon Sep 3 '17 at 1:31
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    $\begingroup$ You write that "not exactly aware that their ancestors traveled the stars" -- does this mean that the ancestors built the Dyson ring? That would mean that it's been in the sky all the time, so the race would think that it's a normal heavenly body (if it's visible). Same point applies if it was built by aliens not-so-recently. $\endgroup$ – cutculus Sep 3 '17 at 2:48
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    $\begingroup$ @theindigamer Why would the fact that it's always been there mean they wouldn't find the Dyson ring interesting? Indeed the Stars, Sun, Moon, planets, rings of Saturn, etc, have been around since long before the dawn of humanity, and yet for all of our history we've found all those things absolutely awe-inspiring. Entire religions, beliefs, and professions have been built around them even though they are "normal". My point is, a visible dyson sphere would have a very marked effect on a civilisation, even if it had always been there. $\endgroup$ – AngelPray Sep 3 '17 at 9:43
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    $\begingroup$ @AngelPray Yes, but the issue here isn't necessarily that it would be unremarkable. The issue is their thoughts would be no different or more nuanced than they were about any other celestial body. They wouldn't draw any distinction between the man made ring around their sun and the non man made rings that can be found around planets. $\endgroup$ – Braydon Sep 3 '17 at 16:51
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As quite often, if not always, is the case, the answer is: "it depends".

First variable is the position of such a sphere with respect to planet, second is if it's complete or not, third it's albedo (there are others like planet's atmosphere, but let's assume it's like Earth's):

  • if it is between planet and sun:
    • if it's complete it probably won't allow life because it would block most, if not all, the light from the sun.
    • otherwise (incomplete sphere) it would appear as a black (assuming low transparency) body soften eclipsing the sun.
      • if albedo is low (dark body) they may spot stars sometimes being visible, sometimes not.
      • if albedo is high (luminous interior) then they would see large shapes dancing in the night sky, often eclipsed by nearer black shapes.
      • in any case how much of the sphere is visible will depend on how close it is to sun with respect to planet, going from quasi-invisible in the glare of dawn/sunset (close to sun) to filling half of the sky (close to planet).
  • if it is beyond planet orbit:
    • if complete they would have no view of Universe beyond it.
      • with low albedo they would have completely black night with possible exception of planets orbiting inside the sphere, if any.
      • high albedo would mean a gray/white sky at night, possibly drowning other planet's (if any) light.
    • incomplete sphere would give "windows" in which to see the stars with dark or light objects (depends on albedo) eclipsing them in a more-or-less regular pattern.

Interpretation "naive" dwellers can do to all this is open to speculation (after you decide what configuration you like most).

@Innovine: asked where goes the energy captured by sphere.

Dyson spheres are built for that exact purpose: harness all energy produced by a star.

This means radiant energy should be converted in some other kind of energy and shipped somewhere to be used (possibly leaking something in the form of infrared radiation).

If this is not true (i.e.: the transformation/transport system is not working anymore for any reason) then all energy will be converted to heat and the sphere will start radiating (black body radiation) till it reaches an equilibrium between received energy and radiated energy (unless heat destroys it first, of course).

At that point you would have a sphere much bigger than photosphere radiating the same amount of energy as black body (more or less, it would depend on sphere material, to a point), which means it would radiate at a much longer wavelength, most likely in the deep infrared.

Details depend on how big the sphere is (how far from star).

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    $\begingroup$ if it blocks all the light from the sun, where does that energy go to? $\endgroup$ – Innovine Sep 3 '17 at 10:57
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    $\begingroup$ @Innovine: I updated answer to explain. $\endgroup$ – ZioByte Sep 3 '17 at 11:08
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Freeman Dyson's conception of a Dyson Sphere was actually a swarm of orbiting satellites, energy collectors, habitats etc. which would gather a large portion of the star's energy output. While he never imagined a solid shell, that has somehow become the standard conception in the SF universe, but such a shell would be dynamically unstable over eons of time and eventually drift into the star.

An orbiting swarm of satellites would be visible from Earth, however. If the swarm was between Earth and the Sun, there would be a constant stream of "transits" as the satellites crossed between the Earth and the Sun. How large they would be depends on the size of the satellite and the distance from the sun, so there may not be any perceptible changes except through a powerful telescope, or there may be a constant flickering as the billions of satellites pass between the Earth and the Sun absorbing a large fraction of the Sunlight.

enter image description here

The Transit of Venus was visible with the naked eye on Earth

If the Dyson swarm is farther away from the Sun, it will be visible as light is reflected from the parts of the swarm. Now the Swarm might be treated to prevent reflections, but then you would have a constant flickering of the stars at night as they were occluded by the passing satellites. Otherwise, they would be visible as "stars", but ones in puzzling regular arrays.

enter image description here

Time lapse photo of the ISS passing overhead

A truly stunning sight might be a geometrically regular array of satellites (possibly Statites using solar pressure to remain in fixed relative positions)

enter image description here

This stage set gives you an idea of what a fully assembled geometric array of statites in fixed relative positions might look like from the ground

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