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I am revising my first draft and trying to add more evocative details.

Here, I asked whether stars in a binary system are likely to fall on the ecliptic.

As my hero hikes through the day and suns set approaches, would the suns appear to spread apart from one another from atmospheric lensing?

Possibly the lower sun becomes larger and sets, and then the upper sun becomes larger and sets, and there is no apparent change in distance for the planet-bound hero. But I'm not certain. Maybe lensing makes the space between the two stars expand, too.

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  • $\begingroup$ This depends on atmosphere density / height. For example on Earth usually celestial bodies appear higher, but sometimes lower than they are. And your atmosphere doesn't even have to be particularly earth like. $\endgroup$ – Mołot Oct 8 '17 at 21:28
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Atmospheric lensing usually makes the appearance of celestial bodies appear higher than they should. However this effect is dependent on atmospheric conditions and under some conditions the effect can even be reversed making the object appear lower. The main problem caused is inaccuracy in sighting the exact position of celestial bodies near the horizon, but the effect is usualy relatively small.

To specifically answer your question, yes the stars might appear further apart at sun set but the effect would neither be large or reliable as it depends on atmospheric conditions such as air temperature.

The size of the effect

atmospheric lensing or refraction

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I've never seen an explanation of atmospheric lensing that didn't sound like it was more than half guesswork but given the known effect, increasing the apparent size of bright objects, like the moon, that are close to the horizon, I would expect that the opposite would in fact occur. The apparent size of both stars would increase as they approached the horizon and the apparent distance between them would shrink.

As a completely separate phenomenon you might get a halo effect from the stars' gravity lensing the light from each other that would only be apparent after the star doing the lensing had dropped below the horizon. So you'd get first sunset, second sunset and then the "blazing arch" lensed above the horizon would linger for a time.

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