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What might the sky and star would look like to a person standing on the planet surface in the termination zone of my imaginary habitable, tidally-locked planet orbiting an M class star.

If I've done the math correctly, these are my planet and star details:

Planet

  • Radius circa .92 Earth radii
  • Mass circa .75 Earth
  • Orbit circa 0.1 AU
  • Atmosphere near Earth like mix of N2-O2 at circa same pressure

Star

  • Class M4V Red Dwarf
  • Mass range 0.08 - 0.50 solar masses
  • Luminosity .1% to 8% of our sun

1: What color would the sky overhead be versus at the horizon in the direction of the star?

Due to the star always at the horizon (much like our sun at equinox if viewed from the north or south pole), plus the red temperature of the star, I believe the sky overhead (away from the horizon) would be a deep purple. At the horizon near the star, I believe one would see reds, oranges, yellows. Am I correct?

2: How much refracted (over the horizon) light and twilight would one observe?

Given the much smaller size of the star but the much closer proximity -- compared to our sun at horizon. Would it be approximately similar to earth/sun, or close enough, for me to not worry about it for story accuracy? (Or perhaps is there a relatively compact equation I can use to calculate some numbers, such as length of twilight, from my planet and star characteristic numbers?)

If anyone has seen a simulated image created by some astronomer or astronomical agency, and can point me at it, that would be great.

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    $\begingroup$ For sky color, you should visit this post which is included on our List of Worldbuilding Resources. Please note for future reference you are allowed to ask one and only one question per post. $\endgroup$ Commented Sep 5, 2025 at 0:21
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    $\begingroup$ Thank you. The sky color post is absolutely brilliant. I'm sorry my multiple searches did not turn it up. $\endgroup$ Commented Sep 5, 2025 at 3:10
  • $\begingroup$ The sky would most likely be within the red spectrum, and "auroras" would often appear as some of the atmosphere burns at a time, probably creating flares and danger zones on the planet. I'll probably come back to this with more info. $\endgroup$ Commented Nov 20, 2025 at 22:23
  • $\begingroup$ If it's a habitable planet, that means a breathable atmosphere. Given that we're talking about a tidally locked planet that hasn't suffered from atmospheric collapse, that means you'll not be able to see the sky at all. At ground level, there will be constant hurricane force winds from the cold-side to the hot side probably carrying debris and ice crystals. At higher altitudes a hurricane force wind will be travelling in the opposite direction (wind-shear is going to be a nightmare) carrying dust and water vapour back to the cold side. $\endgroup$ Commented Nov 26, 2025 at 12:22

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