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In the setting I'm working on a giant mechanism opens on every equinox and solstice to record the sky and use the data for calculations that are required for the story plot.

For astronomical reasons, is there any justification that make watching the sky on these occurrences (equinox/solstice) better than any other day?

The reasoning is that it would be a: let's check if all is well and keep updating the calculations for the mechanism.

Imagine that this mechanism has to locate a very specific point in the sky.

Would it be believable that it can only work on these days because it is when another celestial body out there is out of the way and so not blocking its view?

Also, planet is like earth or probably earth in a very distant future.

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  • $\begingroup$ If you draw a line from the sun through the Earth, then those stars visible along the line when drawn on an equinox or solstice would be, I presume, most visible with the least amount of solar distortion (if it matters at all, astronomers?) simply because the Earth is optimally between the measuring device and the sun. However, drawn on each solstice and equinox means you're looking at four very different parts of the night sky. But if you have but one point to look at, I don't know if there is a good reason. I'm interested in the answers you'll get. $\endgroup$ Jan 25 at 19:33
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    $\begingroup$ @JoinJBHonCodidact: The antisolar point is actually the center of the gegenschein or counter-glow, perfectly natural light pollution due to the backscatter of sunlight by interplanetary dust. I don't know what "solar distortion" could be. $\endgroup$
    – AlexP
    Jan 25 at 23:33
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    $\begingroup$ @AlexP You might have provided the best answer the OP can expect - something in space (whatever it may be) that is highlighted by the gegenschein only when the Earth is at one or more of the four points he/she is interested in. Be it something in front of the back scatter (causing a shadow) or something within the back scatter than can only be seen with the gegenschein effect. Heck, if my comment was so completely out in left field that it resulted in this - thanks! $\endgroup$ Jan 25 at 23:47
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    $\begingroup$ You have painted yourself into a corner because equinoxes and solstices have no astronomical significance at all. They are simply the four measurable, verifiable times each year that the Royal Astronomers used to update their top-secret technology called a "calendar." The King, in turn, used the updated calendar to declare planting and harvest times...and seem godlike by being correct. The planets move rapidly. The stars move over millennia, Only the sun is in the same location at a solstice or equinox...since that's the definition. So either the machine is looking at the sun, or at nothing. $\endgroup$
    – user535733
    Jan 27 at 5:29
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    $\begingroup$ @AlexP you seem to have come round to my original point. Welcome. $\endgroup$
    – user535733
    Jan 28 at 15:06

2 Answers 2

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is there any justification that make watching the sky on these occurrences (equinox/solstice) better than any other day?

Not really. The winter solstice has the advantage that it is a nice long night (assuming you're not observing from the equator, or your planet has a negligible axial tilt, etc) but that's about it.

Would it be believable that it can only work on these days because it is when another celestial body out there is out of the way and so not blocking its view?

It would be rather odd that something should move out of the way precisely on those four days, and then immediately move back. Obviously if the something is technological, that some other explanation can be made for its behavior, but it doesn't sound natural.

Imagine that this mechanism has to locate a very specific point in the sky.

One thing, visible from four widely spaced points on the planet's orbit, but not visible at any other time? I'm not really buying it, but I suspect I'm a lot more nitpicky than most, and have a harder time suspending my disbelief.


So what's to be done?

One solution that allows for nearly arbitrary amounts of contrived situations is to have something technological, that does some specific odd things at specific strange times for whatever reason. This may or may not fit your theme, and it does also shift the problem from "why observe at these times" to "why do a thing at these times", so it doesn't really fix the underlying problem.

You can handwave in some other process that occurs four times a year that just so happens to coincide with the solstices and equinoxes. The period of a conjunction with an inner planet, possibly. I don't know if having the timing be a coincidence is something you'd find particularly satisfying, though.

If you were happy to have it fire once a year, then it would be reasonable to have something with a slightly odd structure, say a deeply buries observatory of some kind, with a viewing shaft that only points at The Thing at a specific time each year, and during the rest of the year it just isn't visible. Such a system might have been built to last over a very long period of time (perhaps like the clock of the long now), hence its slightly unusual structure. It can't have been built to last for too long though, because axial precession is a thing and over the timescale of millenia the apparently fixed stars will move... either your target will not be visible through the device at all, or it will appear on a day other than the solstice, etc.

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  • $\begingroup$ Thanks, you actually hinted at something as it stands in the setting right now. The mechanism that makes the observation is buried in the ground. What I´m really looking for is a reasoning for why it only opens on a specific date and trying to align this with astronomical facts. So, a specific point in the sky in a specific day or a short of period of days is fine. But then again this window of observation would only present itself next year? $\endgroup$
    – mcbecker
    Jan 25 at 19:48
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The Sun has gone white dwarf, and you're using it for a telescope

There is a scheme afoot to send a probe out to 550 AU to use our Sun as a gravitational telescope to see a detailed view of exoplanets. However, in the "very distant future", the Sun will eventually go white dwarf, reducing to 1/100 of its size. In the higher gravity close to the dwarf, the light will bend a little more. And the Earth, to have avoided destruction at the red giant phase, has been moved out a bit, and no longer depends entirely on the Sun for heating. Summary: the solar gravitational telescope now works from Earth.

It is also readily imaginable the atmosphere may be less of a trouble to astronomers by then. Those things are hard to hold on to.

Now, either the Earth was used because its transits are in a line with the broadcasting star, or the broadcasting star was chosen to be in a line with the Earth. Either way this isn't going to work if the Sun misses your spot of interest in the sky.

But if it passes right over it, then a large ring around the Sun provides (with some image processing) an intensely magnified view of whatever is behind it. Signals can be received that would never have been detected by the equipment otherwise. Of course, you need to block out the star - easy if there is no atmosphere, but otherwise, perhaps there is a satellite in orbit that passes over the station at the designated time to act as a starshade.

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