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And if I wanted them to appear about 10 times the size of the sun and moon, would this be possible? Should they lie inside LEO or beyond it? I would like their orbit to be considerably closer than the moon's orbit. How big would they have to be, and how far away? Would we be able to feel their gravitational pull?

Thank you, new StackExchange friends.

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  • $\begingroup$ Welcome to Worldbuilding! You have indeed asked a lot of questions - we like to have posts like this split up into multiple posts so that you only have one question per post (although feel free to add links to the other posts) as this helps provide better answers to each individual question, so if you edit your post so that it only asks one question, you'll be improving the answer quality that you get $\endgroup$ – Mithrandir24601 Nov 30 '17 at 0:02
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    $\begingroup$ First off, it depends. What kind of satellite? a planet type one, or just a skeleton frame space station? $\endgroup$ – WorldCraftTrainee Nov 30 '17 at 0:12
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    $\begingroup$ Hi, MChing, I have edited your tags by deleting hard-science & replacing it with science-based. Because you are new here you may be unaware of the precise tag classification. This should provide better answers. By satellites, do you mean natural or artificial satellites? It is unlikely we could build artificial satellites big enough in the near-future to be ten times the size of the Sun or Moon. Have fun here! $\endgroup$ – a4android Nov 30 '17 at 1:05
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    $\begingroup$ The hard-science tag demands and requires mathematical and authoritative proof for an answer. Neither of the two answers I see meet the requirement. Further, you have not provided enough information to properly scope a question that expects hard-science. I am therefore voting to close your question as too broad. $\endgroup$ – JBH Nov 30 '17 at 1:52
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First: how big to appear as big as the moon if in a typical satellite orbit? Here is a fine image of the ISS passing in front of the moon and we can use it to gauge their relative apparent sizes.

from https://www.space.com/6870-spot-satellites.html the ISS and the moon

When I blew them up, I measured the diameter of the moon at 567 pixels and the ISS at 14. 567/14 = 40 so a satellite 40 times the size of the ISS would be the size of the moon. If the ISS is 100 meters this moon-size-looking satellite will be 4000 meters.

I found this excellent (but perplexing - what is this guy up to?) proposed solar shade which happens to be exactly 4 square km!

from http://www.interstatetraveler.us/Forward.Thinking/Space.Systems/Space.Shield/solar%20sheild_8x11_300%20dpi.jpg 4 square km solar shade

Space sunshades for power generation or earth cooling have been mooted since the 1980s. Yes, I meant to write "mooted", and mooted in all seriousness by serious people. The space sunshades were.

Here is another proposal. From http://environmentalresearchweb.org/cws/article/news/27020

A 1992 NAS report suggested a slightly less extreme project, which would involve lofting 55 000 "solar sails" into orbit around the Earth, each with an area of 100 sq. km. These would block out about 1% of sunlight, but avoiding collisions would be a space-traffic controller's nightmare, warns Michael MacCracken of the Climate Institute in Washington, DC.

space sunshade http://images.iop.org/objects/phw/world/22/9/33/eng2.jpg

10 times the size of the moon? Bah. 25 times the size of the moon! 100 square km makes that 4 square km scheme look pathetic! And you will note: 55000 of them are proposed in this slightly less extreme project. If these are hooked up as a unit that is 5500000 square km or ONE MILLION 375 THOUSAND TIMES THE SIZE OF THE MOON. Muhahahaha!

hmm. that seems big. What is the surface area of the Earth? 510,000,000 square km. OK, we're fine.

So yes: superlarge satellites are a plausible, fine and desirable thing. Cool the earth, generate power, and show movies.

For completeness sake: 1: How big would they have to be - we covered that.
2: How far away - we used the ISS as a reference point and it is 408 km up.
3: Would we be able to feel their gravitational pull - no, because these are wafer-thin satellites.

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    $\begingroup$ he wants to assemble that huge SHEILD the same way he fixed his typo, I assume... $\endgroup$ – L.Dutch Nov 30 '17 at 6:47
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    $\begingroup$ @L.Dutch - I felt bad about that. The guy clearly put a lot of effort into that image, but I would be slow to hire someone who can't be bothered to spellcheck. $\endgroup$ – Willk Nov 30 '17 at 12:31
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    $\begingroup$ . . . that's no moon . . . $\endgroup$ – Binary Worrier Nov 30 '17 at 13:19
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Firstly, the size

The moon orbits at ~400,000 km satellites in lower Earth orbit ~ 400 km +.

So for a satellite to appear Moon-sized, it will need to be ~ 1 / 1,000 of the Moon's diameter, or around 4 km. That's pretty big, but not impossible. To be 10 times as large, that's now 40 km which is really very large indeed (although not quite Death Star-sized, according to Google)

Anything that orbits in something other than a geo-stationary orbit should rise and set, albeit at a different rate to the Moon and Sun. So if by "like the Moon" you mean at the same rate then no, as that would require a vastly larger orbit (and thus a massive satellite); whilst LEO will give you a total orbit time in the order of an hour or two.

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  • $\begingroup$ Based on this answer from not too long ago the only way to have a rise-and-set rate equal to the moon would be for the satellite to occupy the same orbit as the moon (regardless of its size). $\endgroup$ – Draco18s Nov 30 '17 at 2:39
  • $\begingroup$ The Moon's rising and setting is more to do with the rotation of the Earth than the Moon's orbit. $\endgroup$ – colmde Nov 30 '17 at 13:05
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At 20,000 km a satellite would make two full orbits a day. If it is orbiting in the same direction as earths spin, it would appear to an observer on earth to orbit once a day, rising in the west and setting in the east. The Earth - moon Roche limit is about 10,000km so you could just move the moon closer without worrying about tidal forces tearing it apart. From 400,000 km to 20,000 km is 20 times closer so it will appear 20 times larger. The effects on the tides will be noticeable and I would expect other noticeable side effects, but the actual pull of the moons gravity when overhead will still only be one tenth of a percent of a g, so we probably wouldn't be able to notice the change.

A satellite in this orbit would need to be 23000 km^2 to match the moon in size. Borrowing from Wills answer, this size of solar panel / sun shade seems possible. This does not seem like the most practical orbit for this sort of project though.

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Is it possible for satellites to rise and set like the sun and moon? Yes it is if they are in the right orbit.

Is it possible for a satellite to be ten times the size of the sun or the moon? No. Such large objects would destabilise the whole solar system. Is it possible for a satellite to appear to be ten times the size of the Sun or the Moon? Yes but it’s not realistic.

The real problem is that to appear to be 10x the size of the Sun or Moon in a distant orbit they would have to be astronomically large, but to be small enough to be a manageable size they would have to be in far too low and fast an orbit, so they would move much faster than the Sun or the Moon. Worse still the orbit would be impossibly low and they would burn up in the atmosphere.

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Frame challenge: you do not need satellites, use balloons instead.

Current technology is fine. Costs will be steep, but not as steep as the space based solution.

Have huge balloons flying around. The closer to the ground, the smaller they can be, but higher ones will present a better illusion and be less vulnerable to being seen from the side by a plane or a tall tower.

It's not feasible in the near future for something so big and in the atmosphere to go around the world in a day. But consider the following scenarios:

  • A single balloon going around the world, rising and setting in a given city in its path one day per several months.

  • A single balloon "serving" a single city, and going back around it at night/day to rise again the next day/night.

  • A line of such balloons going around the Equator, such that each static ground observer will see a different but apparently identical balloon rise and fall each day.

  • Several such lines of balloons going around several parallels of latitude, spaced enough that no two are visible from the ground at the same time

  • Balloons in complex trajectories criss-crossing from and to population centers, such that every population center will see a different but apparently identical balloon rise and fall each day, possibly at the same time.

Yes, it's a hack, but note that the Sun and Moon rise and set by different mechanisms from one another, so I think the appearance might be the most important thing.

If this suits your needs, please ask another question for the size and speed estimates and for the detailed ways that the illusion can subtly fail.

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