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The Question Let's try this again. Again. One concise question: "What significant and regular calendar-worthy events would either nomadic or stationary humans perceive on a distant future Earth that (long ago) stopped spinning about its axis (but is otherwise identical to our current Earth)?"

I cannot stress this enough but the following is not asking a different question, only providing examples of helpful answers: I am specifically interested in understanding how astronomy events would be changed because of Earth's lack of axial-spin (moon, stars, other planets, etc) and how significant and regular weather patterns would form on the planet (seasons, season length, temperature, precipitation, storms, etc).

Assumptions Please assume the following

  • The lack of Earth's spin eliminated our equatorial bulge, flooding the oceans to North and South, creating a large North Ocean and a large South Ocean and one planet-spanning mega-continent along the equator.
  • The lack of spin created 6 (current) months of "day" and 6 (current) months of "night". There will be two twilight periods in between: dawn and dusk. I do not know how long they will be (but would like to <-- mods, not asking a new question). This is NOT a tidally-locked scenario, so please don't direct me to those sources or talk about the planet's "day" side and "night" side.
  • Assume humans have found ways to survive the temperature, storms, radiation, and agricultural difficulties. I'm not asking how they survive but what they will experience by surviving in this situation.
  • Assume there are 3 human populations experiencing this situation: (1) those that stay at a fixed location somewhere along the equatorial mega-continent, (2) those that travel along the dawn twilight band, (3) those that travel along the dusk twilight band.

Relevance to others The guys says to add this. These answers will help anyone working in the same no-axial spin scenario I am or anyone working in a scenario that has disturbed the Earth's axial rotation speed or anyone working on a scenario that requires an understanding of the climate or astronomical effects of Earth's axial spin.

Conclusions so far Feel free to correct any of the below if you think otherwise.

  • Stars. For the fixed-location humans, they would always see the same stars. For the nomadic-humans, they would see new stars as they travelled around the Earht.
  • Sun. The sun will be above the horizon 50% of the time, but sunlight will appear more than 50% of the time (atmospheric diffraction). . Moon. With no axial spin we will see the moon less often. Rather than seeing it once every 24 hours (due to our spin) we will see it based on its orbital period of 27 days. As it will be out of view for approx half that, we would see it for 13.5 days at a time. During that period, it would go through its normal phases because our axial tilt hasn't changed. (Really unsure about the moon, but this is what I got so far).
  • Wind. There will be a very strong wind going night side to day side (at both sunrise and sunset) on the planet's surface. At some elevation (don't know where) the opposite would be true; i.e., a very strong wind going day to night side (at both sunrise and sunset).
  • Flooding. Sunrise/dawn will create significant flooding. Small creeks will thaw before big rivers/oceans, so water will flood all over.
  • Mega-Storm. At noon (and some degrees to either side), there will be a mega thunderstorm.
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    $\begingroup$ What a culture considers calendar-worthy is a matter of opinion, and subject to drift anyway. $\endgroup$
    – rek
    Sep 16 '20 at 19:24
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    $\begingroup$ @rek. ... Yes. I don't disagree. So we agree "calendar-worthy" events are relative but that does not make them random. They will be based on what those people experience. Which is why I'm asking what significant and regular climate and/or astronomical events will they experience in this scenario. $\endgroup$
    – Twelvfth
    Sep 16 '20 at 19:48
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    $\begingroup$ @Twelvfth They aren't random, but they are a matter of opinion. There's nothing inherently significant about the Solstice (frex) that every calendar would note it or be shaped by it. Ditto for all other phenomena. You're asking for a list of all things all potential cultures might find significant enough to build their calendar around to some degree, undoubtedly including contradictions and incompatabilities. Opinion. $\endgroup$
    – rek
    Sep 16 '20 at 20:50
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    $\begingroup$ @Frostfyre to be honest I never got the point of that rule. $\endgroup$
    – KeizerHarm
    Sep 16 '20 at 20:54
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    $\begingroup$ But I think that even restricting the question to events that recur once or a fixed number of times a year, thus dodging the subjective "calendar-worthy" qualifier, would be borderline over-broad because it's asking for all possible events that share one property (recurrence) in the domains of geology, astrology, and meteorology. Restrict it to recurring events in just astronomy, as an example, and maybe then the hivemind will let your question live. I can sympathise with your frustration, this site has some of the most nebulous rules of any stack exchange. $\endgroup$
    – KeizerHarm
    Sep 16 '20 at 21:00
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Concerning the lack of rotation, this obviously depends on the frame of reference. Zero rotation with respect to what? Given that the planet is not tidally locked, this would be with reference to fixed stars rather than the sun. So with reference to the sun the planet would rotate exactly once a year.

The Stars seen would depend upon the location on the globe. Roughly half of the stars would be visible from any location. At each pole a different set of stars would be visible. At the equator half of the stars seen at each pole would be visible, the exact stars depending on the location around the equator.

Anyone moving around the equator would see a different set of stars. As they progressed around the equator stars would sink below the eastern horizon and rise above the western horizon or vice versa depending on the direction of motion of the travellers. Those on the dusk terminator would see stars offset by 180 degrees to those on the dawn terminator.

There might be some annual variation in star position due to parallax; however this would be unlikely to be noticeable to the naked eye unless a nearby star was very close.

Dawn and dusk aren’t very discrete periods, however if we assume on our Earth a dawn / dusk period of approximately 1/24 of a day (each) with the sun taking 1 hour to transition from light to dark or dark to light then on your planet the same thing will occur over 180/24 = 7.5 “days”. What counts as twilight, day or night light levels is an open question.

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If the Earth has no spin, it is not, as you stated, locked. Because locked means the same side always faces the sun. In this case, the day-night cycle (because that's what it is) takes a full rotation. 6 months in the dark and 6 months' day. The transition zone is moving across the planet, but very slowly. It takes 365 days for a full rotation. The earth has a circumference of 40.075 km so the transition zone moves at 109.8km a day, which means it moves at 4.5km/h or around 1.25m/s. Again, slow.

So for your question, it could be a local holiday whenever the shadow moves across. The transition zone is big, but I could imagine it being a holiday or a free weekend or whatever.

Keep in mind, however, that the shadow isn't exactly sharp. Due to light scattering (diffraction is something different) the shadow will be quite big. I don't know exactly how big, though.

Sunrise and sunset too would be worth mentioning. This also wouldn't be too long. The size of the Sun in degrees is around 1/2, and the sky is 180 degrees large. It takes 132.5 days for the sun to travel 180 degrees across the sky, meaning it travels at 1.358 degrees/day, which is 2.716 times its size. So from the moment the lowest part of the sun touches the horizon, it takes about 8.8 hours for the sun to disappear.

This could be yet another holiday.

Other than that, there is not much going on. The night would be just like our night, except really cold. The day would be just like our day, with the minor difference that everyone would be dead because all the water would be gone.

Really, the main problem you have is that the transition zone will have storms, and that the night and day zones are not quite worth dying for.

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    $\begingroup$ "It takes 132.5 days for the sun to travel 180 degrees across the sky" Pretty sure that this is wrong, and it makes the rest of the calculations in that paragraph wrong, too. There's about 365.25 days in a year, and if you divide that by two, you'd get 182.625 days per 180 degrees. $\endgroup$
    – nick012000
    Sep 22 '20 at 1:37
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On the dayside, the time can accurately be measured by taking the height of the sun over the horizon. The people can construct whatever calendar dates they need from this.

On the night side I only see a lunar calendar or counting days since the last sun was seen.

Fixed groups could base their social activities on a lunar calendar (as the intervals when the moon is not seen are shorter) and their agricultural ones on the sun.

Mobile groups could base their entire lives on a solar calendar (always moving to keep the sun 2° above the horizon) and use artificial milestones natural landmarks to mark special recurring occasions.

That said, travelling 100 km/d sounds like something you'd rather do by ship than on land, making the whole landmark stuff difficult. And when you move on the sea, you could move further north or south to achieve an even more leisurely pace.

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