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I know this has been asked before but I'm going to pose the question again under slightly different circumstances and with a more specific goal. I'm trying to work out the details of how the day/night cycle would work on a planet in an S-type binary star system, AND how I can generate a calendar or method of tracking where the secondary star is at any given time. Here are the parameters:

  • A habitable planet orbits an F5.5 main sequence star, 1.2 solar masses, at a distance of about 1.5 AU.
  • The secondary star, a K-type about 0.5 solar masses, orbits the primary star at around 24 AU.

I started working this out in Universe Sandbox which was helpful up to a point but I can't really "stand on the surface" of the planet and watch the sunrise and sunset. From what I've worked out, the secondary star should have a magnitude of about -14, or a little brighter than the full moon, from the planet's surface. The secondary star orbits the primary star every 88 Earth-years and the planet orbits every 1.73 Earth-years, so people on the planet would see the secondary star moving across the sky throughout the years and decades. ("in-world" they might refer to it as the "Wandering Star" or "the star of day and night")

I think for one half of the year the secondary star would be visible and quite bright in the night sky, and then visible during the day sky for the other half of the year. But how do I work out its position when the star is also rotating around the primary star?

This calendar I found on Donjon for calculating phases of the moon and lunar calendars is quite helpful for other aspects of the world I'm building (which includes a smaller habitable moon as well as two additional small moons). Is there something similar I can set up for a binary star system? How do I figure out where the star will be and build a calendar system around that?

Is this system even plausible? I think the secondary star is small enough and far enough that it wouldn't really impact heat and climate on the planet, but might cast a sort of dark twilight during half the year.

And for those wondering, I started with a P-type system so I could have a double sunrise and sunset situation, but Universe Sandbox doesn't maintain stable planetary orbits in that kind of setup so I abandoned it and am now exploring the implications of living in an S-type system, which I find equally if not more fascinating.

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2 Answers 2

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I figure that you could have the star correspond roughly to centuries. Unfortunately, this system probably wouldn't result in a double sunset, and I would guess that the "Wandering Star" would act more as a moon than a star.

As for climate, Wikipedia says that if the planet doesn't exceed 1/5 of the B star's closest approach, the planet can be habitable.

Edit: Can't do the math, I am not an expert. I estimate that for about a month or two, the stars would set at approximately the same time, which I assume is what you mean by a "double sunset."

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A habitable planet orbiting an F5.5 main sequence star, 1.2 solar masses, at a distance of about 1.5 AU would have a day/night cycle similar to that of Earth. The secondary star, a K-type about 0.5 solar masses, orbiting the primary star at around 24 AU would be visible in the night sky but not bright enough to cast much light on the planet's surface. The secondary star orbits the primary star every 88 Earth-years and the planet orbits every 1.73 Earth-years, so people on the planet would see the secondary star moving across the sky throughout the years and decades. ("in-world" they might refer to it as the "Wandering Star" or "the star of day and night")

I think for one half of the year the secondary star would be visible and quite bright in the night sky, and then visible during the day sky for the other half of the year. But how do I work out its position when the star is also rotating around the primary star?

This calendar I found on Donjon for calculating phases of the moon and lunar calendars is quite helpful for other aspects of the world I'm building (which includes a smaller habitable moon as well as two additional small moons). Is there something similar I can set up for a binary star system? How do I figure out where the star will be and build a calendar system around that?

Is this system even plausible? I think the secondary star is small enough and far enough that it wouldn't really impact heat and climate on the planet, but might cast a sort of dark twilight during half the year.

And for those wondering, I started with a P-type system so I could have a double sunrise and sunset situation, but Universe Sandbox doesn't maintain stable planetary orbits in that kind of setup so I abandoned it and am now exploring the implications of living in an S-type system, which I find equally if not more fascinating.

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