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The International Space Station is roughly the size of a football field and can be seen with the naked eye from Earth, usually as a tiny dot in the sky.

In the setting I'm working on, there is an O'Neil Cylinder style space city (well, it is really more like a stack of Stanford Torus Rings, but never mind) in a similar orbit as the ISS that is just over six kilometres long and one kilometre wide.

How would something of that size appear from the ground?

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marked as duplicate by a CVn, Vincent, James, Hohmannfan, Brythan Jun 9 '16 at 13:57

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    $\begingroup$ Why not figure out what angle that would be on the sky, and compare that with other objects/distances in your experience? $\endgroup$ – JDługosz Jun 7 '16 at 17:31
  • $\begingroup$ Putting your structure in a similar orbit as the ISS would be a bad idea. Something that size would experience significant drag from the small amount of atmosphere present at that altitude (even the ISS has to periodically perform rocket burns to maintain orbit). A higher orbit would be needed. The classical approach is to use a Lagrange point for a stable orbital position. $\endgroup$ – Josh King Jun 8 '16 at 19:11
  • $\begingroup$ Drag yes, Too much drag, No. Larger objects feel drag as a smaller proportion of their mass. The thing would still need a boost sometimes. $\endgroup$ – Donald Hobson Jun 9 '16 at 9:14
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ISS is an only just visible dot of 100 meters in size which at 400 km height covers a one arc-minute angle.

Your big space station would appear as a cylinder 60 times the length and 10 times the width of ISS, so it would be 60 by 10 arc-minutes (60'x 10'). And will be highly visible at the times it catches sunlight against a darkening sky.

To compare, the moon is about 30' across. Your construct probably rates a nick. Space cigar?

References:
https://en.wikipedia.org/wiki/Angular_diameter
https://www.quora.com/Are-the-satellites-visible-from-Earth-by-the-naked-eye
https://www.quora.com/How-large-would-a-satellite-in-Earths-orbit-need-to-be-for-the-shape-to-be-visible-and-identifiable-with-the-naked-eye-from-the-ground

In hindsight I kind of covered @JDługosz's suggestion. Hope it helps.

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This is more of a comment, but because of the length I'm putting it here.

While the size of the space station determines the angular distance that it occupies in the sky, the main reason that it would be visible at all has to do with the reflective surfaces which might be attached to it. The ISS is quite visible to ground viewers becsue the sun reflects off the vast sail like solar panels which provide power to the station.ISS STS-124

A large space colony will have a lot more reflecting surfaces either as solar panels, or as mirrors to bounce sunlight into the colony. A Stanford Torus (or a series of torii as you propose) is generally marked by a large "top hat" mirror reflecting sunlight into the central axis, where it is further reflected by mirrors into the colony itself (through a giant slot cut into the radiation shielding facing into the axis)

Stanford Torus

Since the mirror itself is the diameter of the torus (one kilometre in your case), it will certainly be highly visible, and the one thing which attracts attention long before the actual station itself. Under some circumstances, the glare from the mirror might actually obscure the colony itself from viewers on Earth, so ground observers will see an extremely bright "star" passing overhead, rather than the colony itself.

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At the distance a 6 km space station would not be much more vissible to us than the ISS, it would still just be a spec in the sky. Possibly a clike a grain of rice, but still, not very noticible. I gathered this from using google earth and looking at a distance of 6 km from the height of ISS. A larger station might also be positioned further out than the iss for various reasons. Which again would make it appear smaller.

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