Recently, I thought about using a red dwarf instead, and while this approach gives a nice large sun in the sky, there are other disadvantages. Since Mercator is the size of Venus, it is likely to be tidally locked to the sun. With the planet orbiting a red giant, its rotation can be arbitrary. Tidally locked, its rotation will be slower over the course of days.
With that in mind, I considered making it a habitable moon around a Neptune sized gas giant. This imposes some restrictions as well.
The star for this solar system is 0.153 solar masses, and 0.044 solar radii. The Neptune is at 0.037 AU, and has an period of 6.65 days. As I am using Neptune's planetary characteristics, this gas giant's Hill radius sphere of gravitational influence isn't very large compared to our Neptune due to masses of the star and planet and its small semi-major axis, so a Venus sized moon needs to orbit close, otherwise it will be pulled into its own orbit around the sun. The moon Mercator's semi-major axis is just inside the Hill Radius at about 235,150 km, giving it a period of three and a half days. Whether or not Mercator would be tidally locked to the Neptune, I'm not certain. I gave it a 3:2 orbital resonance (like Mercury), which gives it a period of about 2.4 days. The original red giant version I gave it 20 hours.
Thus I did the spreadsheet math, and made files to render in Celestia.
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| Trial system is tiny with respects to our solar system. |
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| The "Neptune", tidally locked to the star. |
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| Looking down at the Neptune's north pole. Mercator's orbit is very close. The Roche limit happens to be inside the Neptune for the density of moon, so no threat of breakup and ring formation. |
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| Cloudy Mercator. |
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| Clouds removes for clarity. Red curve is orbital path. |
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| The Neptune looms large in the sky, and will fill it, making for quite a remarkable view. |
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| The sun is also seen larger than our own, and larger than the red giant version. |
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| Mercator casts its shadow. |
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| Rising Neptune and sun. |
While orbiting around a red dwarf gives an impressive sky, it would require reworking clock and calendar time and keeping up with planet shine and eclipses. As I have all of that worked out and in my head for the original red giant orbiting, I don't want to redo all that work. Maybe this can be used for another world that I do have orbiting a red dwarf, so the research bears fruit.
