# What would a large and habitable constant twilight zone on a rotating planet look like?

I want to build a world with inhabitants living in a constant twilight zone, although the planet itself should still be rotating and not be tidally locked with the sun. Optimally I'd want it to be a constant sunrise/sunset or civil twilight with the sun still visible.

In my original idea, the inhabitants would not be measuring time in morning, noon, etc. but in compass directions depending on where in the sky they currently see the sun.

Now thinking about how such a world could exist, my assumption would be that it would have to be an earth-like planet without axial tilt (obliquity) and the inhabitants living on either pole. But then compasses wouldn't work, right? Does anyone have other ideas on how to construct such a world?

If not, how would the inhabitants tell time then?

I would also like the story to take place in a climate warmer than earth-like poles. Let's say around 15-20 celsius / 60-70 Fahrenheit mean temperature. If other areas of the planet become uninhabitable, so be it :) What parameter would have to change to achieve that?

The scale of this twilight-zone should be around 300 miles / 500 km wide. Would the scale of the planet have to be adjusted? By how much?

What else would change on the planet? Would there still be seasons? What would the other areas of the world look like?

If we go with the no-axial-tilt way, there have already been discussions of effects on climate & technology and ecosystem - but what would change if we adjust for pole temperate and planet scale?

• For starters, lovers would have to come up with something else than a sunrise/sunset to look at... – Theik Sep 10 '15 at 9:21
• the geographical poles and the magnetic poles do not need to be in the same spot. On earth they aren't. So, your compass would still be (more or less) useable. Not very precise, but good enough to determine a general direction. – Burki Sep 10 '15 at 9:40
• with no axial tilt (the primary cause of seasons on Earth), seasons will be limited to orbital eccentricity (distance from the sun at different points in the orbit). This would be fairly negligible, but you could make the orbit more or less extreme to adjust (if significant eccentricity, the sun could get noticeably larger/smaller throughout the year). – pluckedkiwi Sep 10 '15 at 20:10
• @pluckedkiwi That is a good point. Although I could imagine that an orbital eccentricity large enough to create earth-like seasons would have other side-effects as well, right? – KonstantinK Sep 11 '15 at 10:24

On a planet with an axial tilt of near 90° tilt, the people living on the equator would see the sun forever staying in the north/south horizon at sunrise/sunset.

The people living on the pole directly facing the sun would live in a perpetual midday.

I think a 90° tilted planet can be effectively considered as a tidally locked planet.

You can also use tides to mark the time. Moon(s) of the planet would create tides in the oceans, which can be markers of time for the early civilizations.

Or you can use the concept of moon's position for marking time. If you examine the moon closely at night, you would notice it goes all the way across the night sky.

The months would definitely have to be lunar, not solar. Or you could try to get solar years by position of stars in the sky. With the planet's rotation around it's sun, the starts position in the sky (at twilight zones) would vary slightly, depending on the position of the planet in it's orbit. This concept can be used to mark years.

If you be specific about the planet type (size and water+atmosphere composition), I could provide you with a more specific answer.

• Your descriptions of how you envision the sun moving in the sky from the perspective of people on a planet are difficult to parse, and I suspect you should think these examples through more (especially the appearance to those on the equator). – pluckedkiwi Sep 10 '15 at 20:07
• You're right. I rechecked and found the equator phenomenon wrong. I have edited the answer. Thanks for pointing out. – Youstay Igo Sep 11 '15 at 2:35
• An effectively tidally locked planet, huh? I'll consider it. To specify the planet type: Let's assume the size being that of earth (12,756km diameter), because answering part of my own question I found out that earths twilight band more than fits my specified 300 mile / 500 km requirement. The Atmosphere should be roughly equal to earth, scaled up to a point where the surface temperature of the poles (twilight area) has a mean temperature of around 15-20 Celsius / 60-70 Fahrenheit. This would probably mean that the sun-facing sides become uninhabitably warm. – KonstantinK Sep 11 '15 at 11:28
• Considering xy to be the "horizontal" plane of the planet, it could be tilted on either axis in 3 dimensional space. If it is tilted on x-axis by 90° one pole would be in a perpetual mid-day condition and the other would be in a perpetual midnight. This is akin to being tidally locked. If the planet is tilted on y-axis by 90° then both poles would lay perpendicular to the star and be in a perpetual dawn/dusk while the equatorial belt would get normal days (the planet would spin at z axis). – Youstay Igo Sep 11 '15 at 13:12

The north of the earth, within the arctic circle, has long periods of twilight, with months of almost entirely lacking in contrast between day and night, as I understand it, this can be psychologically intense, and can result in Seasonal Affected Disorder or SAD.

• An interesting point, although it would probably only be relevant if the people of that world moved to the twilight from an area with more clearly defined cycles. To a native inhabitant the twilight would be the norm. – KonstantinK Sep 11 '15 at 10:29

Okay, I believe I have an answer you will find usable. Google: "Eyeball Planet". Venus could have become an eyeball planet with a little luck. Many exist, most around red dwarves. It is a planet that is tidally locked to its sun and the light side is hot (desert) and the dark is frozen (think Greenland in the dark). Between is a Twilight Zone where water flows from melt water near the twilight line and runs in rivers toward the hot side and evaporates to fall again as rain and freeze on the dark side. Good luck.

• "with a little luck"? – Hohmannfan Mar 9 '16 at 7:34
• That … is actually exactly the type of thing I was looking for! Thank you! – KonstantinK Mar 9 '16 at 10:19