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So I was thinking of having religions prophesy of a day where there'd be a massive beam of light coming down from on high at a specific holy spot. How would this be possible with lens orbiting the star of the system?

For convenience, assume the planet is Earth-like in size and location to a star that is similar to our sun (unless it would work better for it not to be). I was thinking of having multiple lenses orbit the star at different distances, almost like inner planets between this Earth-like one and the star. There would be one day that the inhabitants to calculate and have on their calendars when these lenses happen to all line up and focus enough light to start melting the ground in this one spot.

I'm also curious about what this would look like the rest of the time outside of a special convergence - like if one lens would suddenly make the day brighter as it passes between the star and the planet. Or if you would be able to see the lenses during the day or at night with the help of a telescope. I think the size would also be important, and depending if they were large enough, if you could theoretically see them with the naked eye.

And lastly, if it were possible, would the light beam only last for a split second? Or could you increase the size of the lenses in order to lengthen the time of this divine occurrence?

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    $\begingroup$ Can you edit this to ask a single question. We have a strict one question per post policy. $\endgroup$
    – sphennings
    Oct 13, 2022 at 23:46
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    $\begingroup$ The answer to this is a combination of "it would have to be magic" and "If you aren't relying on known physics, you can make any rules you want in your world." It isn't really multiple questions so much as one big one that the OP doesn't have enough understanding to ask. I filled in a few gaps. $\endgroup$ Oct 14, 2022 at 22:40
  • $\begingroup$ VTC Too Story-Based. You don't explain why the lenses are where they are. In the case of this question, it's important to know why they're there. Can one or more orbiting lenses do what you want? Sure. But how you rationalize that answer depends on your story. For example: your planet's denizens don't remember it, but they're actually the descendents of a slave class to a long lost civilization. That civilization placed the lenses, which rotate on their axis preventing plantary exposure save for the one day in a period where they charge a solar stack - that's been covered over the centuries. $\endgroup$
    – JBH
    Oct 16, 2022 at 1:53

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This couldn't happen unless the lenses were powered and controlled. Here are the obstacles you'd run into:

Tumbling

Unless the lenses were actually monolithic crystal spheres, the lenses would rotate. You can't tide-lock an object on a lesser axis, so it would tend to rotate such that an edge was pointing at the planet. You could probably compensate for this by giving them counterweights that extended forward and back, but I'll leave that for you to do the math.

Orbital variation

I'm presuming that you are thinking that the lenses would be on orbits that line up between the star and the planet on a schedule. For instance, if there were three of them, lens A and B would line up every third month, but lens C only aligned with them every ninth year or some such.

I suggest you look at the paths that the moon's shadow traverses over the Earth on a solar eclipse. As you can see, just getting ONE satellite to pass over a single point reliably would be nigh impossible.

Perturbations

Even if you manage to get everything lined up the way you want it to, natural satellites would mess things up for you by pulling the lenses out of their perfect orbits. Even without natural satellites, the other planets would mess things up for you. If there are no other planets, then the solar wind will push them around enough to make the alignment fail.

In conclusion, this would have to either be done through magic or technology advanced enough that it's basically magic. It could work if you had an active community maintaining engines that keep the lenses in the right place, but that makes them far less mysterious.

Also, if there are people on the space stations, you can accomplish the same thing with a single mirror set, or even with an orbital laser cannon.

How would it work?

Let's say you have an order of mystic astronauts that keep the lenses in the right place. At this point, you would have the "when single shines the triple sun" situation. The nearest one would be moving the fastest, so let's presume that it performs a final culmination of an otherwise wide beam. Let's also assume that all lenses are either convex transparent or concave mirrors.

For the big lens case:

The larger lenses would look like dark spots in the sky most of the time, getting brighter when they passed directly overhead. The furthest out one would have to be really huge to be worth the effort. When they were on the "full moon" side of the planet, they would either be invisible or they would have a tiny reflective spot that looked like a star moving slightly faster than itself. You would probably be able to see a ring of the outer edge most of the time.

For the mirror case:

For a pass-through reflection, you'd need a big outer ring that reflected light to a smaller inner circle. This would be a circular shadow most of the time.

Regardless of what they looked like the rest of the time, when aligned, the device would probably a line in the ground, tracking the nearest mirror's trajectory, instead of just hitting a single spot. Even if you had all of the orbits offset from each other, that would only make the line less intense and more wavy leading up to the final burn. If you absolutely NEEDED it to just hit one spot, you might consider having an opaque shutter object that kept light off of the nearest lens except for when it hit the critical point.

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Lenses... would probably be rather hard. Mirrors in satellites would be fairly easy. (I suspect easily doable with today's technology.)

The satellites could only deploy their mirrors at the appropriate time, and thus look less interesting the rest of the time. Multiple satellites would give you redundancy, so if you lose a few things still work. And mirrors could have their aim adjusted over time to give you hours of light beams.

The downside with either lenses or mirrors is that the target are would probably need to be very small. Big enough for the altar, but not for the congregation. (Actually, I think it would be about the size of the mirror/lens.)

One possible bonus: priesthood with appropriate communications technology and security codes could call for the light show whenever and wherever they want it.

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