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In this universe, an advanced race of aliens have constructed the largest object in the universe other than the universe itself, which is about 40,000,000 lightyears tall (don't worry about the massive amount of gravity this would have, it's reduced and managed through technological and magical means.). It is the shape of two 4-sided dice stuck to one another, with one of them being elongated by 2.5 times. It would look like a kite shape in a 2 dimensional perspective.

The surface has naturally developed landmasses, and the surface alone is enough to sustain most life forever, but that would require a daylight cycle, which is the problem. Due to the structure's non-spherical shape, you cannot make a ring of interconnected suns rotate around it. The light and heat energy must also come from the sky, not from heat lamps on the ground.

I have considered a sort of net of powerful heat lamps that encloses the structure, but that wouldn't work because it wouldn't allow ships to land and the maintenance would be a problem. I've also considered the aliens hiring others to operate massive "Space Trucks" that manually haul suns over areas at certain intervals, but that wouldn't work either because then it would only be a matter of time (no matter how many background checks you preform) until some lunatic steals a sun or crashes it into something important.

A crude illustration of the shape of the Ultra-structure

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    $\begingroup$ What more explanation could a magical solution need, or ever even have? It's magic. $\endgroup$ Apr 13 at 16:53
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    $\begingroup$ Note that if you're allowing magical solutions, you need to (A) include the magic tag, and (B) define exactly how your magic system works. Otherwise, anything is fair game and there is no objectively "best" answer, something the StackExchange model for Q&A requires. Questions without this tend to be put on hold for being primarily opinion-based. $\endgroup$
    – Frostfyre
    Apr 13 at 18:10
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    $\begingroup$ @Frostfyre Oh I see. Just this hexahedron is a "triangular bipyramid" and not a cube. $\endgroup$
    – Alexander
    Apr 13 at 18:48
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    $\begingroup$ Out of curiosity I calc'd mass. Given a regular tetrahedron of side length of 20Kly, composed of aerogel(1.5Kg/m^3) mass ~= 1e60Kg. Mass of observable universe 1e53 Quite the feat, be it magic or engineering. $\endgroup$ Apr 13 at 19:48
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    $\begingroup$ @Alexander oops, so mass of the aerogel tetrahedron would be another e9 more massive,closer to 1e69Kg. Might as well just pull the suns with chariots. Chariots pulling suns is well storied. $\endgroup$ Apr 13 at 21:51
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You control the gravity. That means you get to say what an orbit looks like. Sometimes stars are attracted, and sometimes they're repelled. Double and septuple stars fly across the landscape at sedate-to-relativistic speeds, chasing after one another like Keystone Kops. If there is a mishap in the choreography, fine ... reverse gravity to disassemble the star, put it back together again with a dispersed core so it lives longer, 3D print a few galaxies' worth of toasted inhabitants, and try again.

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That's no star, that's an illusion

The plants get their energy magically at day-like intervals, which keeps them alive. The landscape and air is heated in day like intervals. However, the sun is an illusion only visible to people who have that illusion cast on them. It doesn't matter where you are on the thing. The sun appears to orbit like the whole thing is rotating. The thing looks like it orbits the star at the normal solar distance, but that distance is so great people don't realize that the sun appears to crash into the thing every day. The fake sun appears to be at the same angle relative to the thing, but applied to all people. This means that if two people on opposite corners of the same face of the structure look at the sun they will see it is the same angle in relation to the face of the thing they are standing on.

Now you effectively have a sun for the day night cycle but don't have to deal with anything complicated.

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Don't move the light sources, move the blinds

Let's work through a few of the basics of this situation:

  1. A single point source (or even ring of point sources around the "equator") won't work - it/they would totally fry the centre of the structure while failing to illuminate the ends. (Assuming a light source around the magnitude of our sun, very sophisticated instruments would be required to even detect the light at the ends of the structure when it arrived 20 million years later.)
  2. Therefore, a lot of point sources are required. In order to avoid frying some points while avoiding unacceptably cold dark spots in between, probably need sun-sized sources of illumination in a grid around the entire structure spaced around 15 light minutes apart and about 15 light minutes above the structure (wild guess, happy for someone to do more exact calculations - remember that points on the surface are receiving radiation from not just the closest "sun" but all the surrounding ones as well). There is no benefit to moving these point sources around, as this will only vary the level of light, not give a day/night cycle. Without knowing the length of the short sides of the hexahedron I can't calculate how many of these point sources there will be for the entire structure, but it will be over one billion point sources per square light year of surface area.
  3. Yes, we now have a net of "heat lamps" around the structure that the OP did not want. They may require maintenance (probably not, they just stay "on" all the time), but they do not prevent landing on the structure because of their spacing - passing them requires coming within about 7-8 light minutes of a sun-analog heat source, but that is roughly equivalent to travelling along or just inside Earth's orbit. Anyone who can build a giga-structure like this can easily manage the navigational problem and at even our current tech level we (obviously) build ships that can survive that environment. Yes, this does restrict moving payloads more than a few light minutes in width, but given that the point of the exercise is to allow life to exist, why move cargos that are so massive that they will destroy that life with tidal effects?
  4. Now move huge opaque blinds a few hundred kilometres above the bits of the surface that you wish to experience day/night cycles. The blinds can be of whatever length and width required but only need to be few microns thick and should be designed to disintegrate if they even hit atmosphere (note that maintaining an atmosphere is outside the scope of the question) so it is nowhere near as dangerous as dragging suns around for either the people living on the surface or incompetent spaceship pilots. Have spare blinds on hand for when they get damaged or you get tired of the colour. The blinds can either keep moving all the way around the structure or get rolled up once they are over uninhabited areas and taken back to the start. The latter idea allows different day/night cycles and even seasonal variations to be implemented in different areas, though the borders between different blind zones may be confusing. To use the idea of @Gault Drakkor, tow the blinds with chariots for traditional reasons. (Shamelessly stealing the idea from Ringworld where sunshade squares created the day/night cycle on that structure.)
  5. Austerity measures. I have to admit I was somewhat baffled by the combination of this being a created structure and it possessing "naturally developed landmasses". Isn't all of it a deliberately constructed mega-giga-massive landmass? However, if this means that only certain areas are life-bearing then skip the artificial suns and blinds where they are not needed - which is probably the vast majority of the structure.
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  • $\begingroup$ For some reason, the idea of simply a "sun net" with larger holes never occured to me. Also, to answer your question about the naturally formed landmasses; the structure would be so large durring it's construction (before the instalation of gravity management stations) that it attracted enough planets and their mass to create it's own layer of sediments. Albeit, the landscapes are mostly open plains and rolling hills. $\endgroup$ Apr 14 at 1:30

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