I have a world with 9 year days. That is, the day-night cycle here lasts 9 years, where ours lasts a mere 24 hours.

Very similar to a tidally locked planet, except the habitable belt moves. The species that have come to live on this planet have evolved to instinctively migrate west to avoid the night. The terrain consists of very few mountains and oceans, with lakes and vast expanses of plains being more common. Towards the poles, the night side temperatures become so cold that massive glaciers carve the land and provide water to fuel non-mountainous rivers. While the day side gets less water, the land itself is more hospitable.

The astronomical features of this world include a small ring system and multiple smaller moons.

As I was designing the cultures living in this world, a question began to arise. If these people are circumnavigating the globe since before they were even self aware, would the idea of a flat earth even start in the first place? If this world's inhabitants did once believe in a flat Earth, would their migratory nature allow them to discard this idea sooner?

  • $\begingroup$ it they traveled along a line of latitude, would the eventually come across their own leavings? $\endgroup$
    – Marky
    Jul 8 '17 at 5:56
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    $\begingroup$ @Marky maybe? Assuming they never stray off course for more than a couple of miles (they have to travel ~6 miles a day) and even then the night side is like Australia and Antarctica had a weather baby, 4.5 years in that would put a lot of strain on a rushed building. (Assuming the speed ahead to get a couple months of stationary life) Finally and most importantly, there are violent storms at the dusk and twilight zones that very well could shatter any constructs. so that's a hard maybe. $\endgroup$
    – TrEs-2b
    Jul 8 '17 at 6:50
  • $\begingroup$ On @Marky's comment, I might have misunderstood, but if a day is 9 years, and they are always avoiding the night, then wouldn't they be circumnavigating the globe once every 9 years? It seems likely they would have something of a migratory path etched into the surface of the planet, very clear indications that someone had been there before. Either they would know it's themselves (planet is Round), or they would have to formulate another explanation regarding who they are following. $\endgroup$
    – Memetican
    Jul 8 '17 at 10:24
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    $\begingroup$ I misread the title and pictured a migratory herd of spherical creatures, rolling along, ignorant of their own shape. $\endgroup$ Jul 8 '17 at 17:44
  • $\begingroup$ What does 9 years mean? 9 Earth years, or 9 of that planet's years? Note that a planet's year is defined by its orbit around its sun, while a planet's day is defined by how long it takes to spin one entire time. It is possible (and truly existent) for a planet to have a day that takes longer than one of the planet's years. So, to comprehend this question requires understanding what you mean by a year. Also, how long does the species live (9 years to travel planet and 8 years per specimen = never re-covering ground), & does this species tell tales about history (like only humans do here)? $\endgroup$
    – TOOGAM
    Jul 8 '17 at 19:06

If species evolved on this planet they would have also developed built-in biological mechanisms to enable them to navigate across their world. There is quite a repertoire of such mechanisms. Observing the polarization of the sky, sensing magnetic fields, and celestial navigation turn up in a wide variety of organisms.

It is possible creatures on this planet will invariably be equipped with one or more biological navigation mechanisms. This will be essential for their survival.

One consequence may be for a sapient species on this planet that they may aware instinctively their world is round, but may lack a conscious cognitive awareness of the fact. They will understand, without having to explain it, that their lives are spent moving in great spatial circles. Each one of which takes nine years to complete. In the same way, we know each season follows, one after another, until the annual cycle of the seasons is complete and continues ever onwards.

This would make it virtually impossible for them to conceive that their world was flat. To them this might be an absolute fantasy. However, it could be an interesting moment in their intellectual history when they go from a purely instinctive understanding of being on a round planet and knowing this to be a fact.

  • $\begingroup$ I like this, the idea that they have a sort of inner voice telling them this is a fact, could even spark some interesting mythology... (+1 of course) $\endgroup$
    – TrEs-2b
    Jul 8 '17 at 6:54

If we put aside the "mythology" on the flat earth believer of ancient times, we can see that the awareness of the planet being round was well spread even before circumnavigation was possible: Eratosthenes calculated the globe's diameter when just crossing the Mediterranean was a challenge, and did this based on purely astronomical evidence (the shadow in the well). Other evidence also leads a scientific mind to conclude the planet is round and not flat, like the different duration of day and night when traveling along the meridians.

All of these phenomena would hardly be noticeable in the world you describe, as there would be no stationary populations. Indeed, the fact that after nine years a migratory population comes back to the same location is more evidence they cannot deny.

Assuming that they have the possibility to develop some sort of scientific mind, on the level of ancient Greece, I see the same chances as in our history.

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    $\begingroup$ Glad to see someone debunking the flat earth myth. in fact, the idea the ancients believed the earth was flat is recent historical invention. Devised to prove we are smarter than our forebears. The truth they were as smart as us. No contest. Plus one. $\endgroup$
    – a4android
    Jul 8 '17 at 6:40
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    $\begingroup$ Coming back to the same location is only evidence of having traveled in a circle. Could happen on both a flat Earth and a round Earth. $\endgroup$ Jul 8 '17 at 11:48
  • $\begingroup$ @CandiedOrange, not if you keep travelling east on a flat earth... $\endgroup$
    – L.Dutch
    Jul 8 '17 at 14:57
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    $\begingroup$ @L.Dutch, how do you correctly measure you're travelling “east” in a flat Earth? $\endgroup$ Jul 9 '17 at 2:20
  • $\begingroup$ @L.Dutch unless you believe the earth to be endlessly long (similar to how most people currently view the universe) $\endgroup$
    – Deruijter
    Jul 9 '17 at 6:37

It's obvious that for people that return to the same point every 9 years just by sailing westward, realising that the world is round will be a lot easier than it were for ancient humans here in Earth. If we take in account that ancient Greeks knew that the Earth was round and they even managed to measure it, it's clear that early scientists in the OP's world will soon be rather sure that their world is round.

However, is it still possible than common people or pre-scientific (pre-geometric) cultures believe that they live in a flat world in spite of circling it every 9 years?

It is harder than in Earth but it is possible.

  • Thermal an ecological conditions are like to vary a lot in the sunlit face. Some species and cultures may be restricted to just certain latitudes and/or small patches near the trailing or leading terminator. The path followed by people living higher latitudes is not very different from a circle in a flat world, so they might fail to notice that they are circling a sphere near its pole.
  • Although at most latitudes circling the Earth in 9 years would mean moving tens of kilometres westward every month, depending on how big is the planet, what latitude the people live and how fast they travel, they could manage to do so without navigating. If you are fast enough, you could just wander randomly moving to the sun when it's too cold and away from the sun when it gets too hot. This way you could circumnavigate the globe without doing any navigation nor realizing it's spherical.
  • To realize that you return to the same spot every 9 years you need to recognize some features. The world can just be featureless, like an ocean world of insondable depth, or a flat world (no plate tectonics, no vulcanism, no mountains, no seas...) where any new feature is wiped away by 4 years of scorching sun followed by 4 years of glacial age.
  • If the orbit of the planet is somehow eccentric, climate distribution can be different from one "day" to next one. Therefore migrating paths can me more complicated than just returning to the same spot every 9 years. If their path is complicated enough, most creatures won't return to the same place in their whole lifetimes and therefore they won't notice that they follow a closed path (although I must admit that they need to cross their own path at some point, but this factor could be combined with any other above to keep this crossing unnoticed or explained by a reason not involving a round world).

In summary, roundness of world it easier to notice than Earth's one, but it still doesn't need to be self-evident.


They would very quickly discover that the world is round, they would have discovered this before they even had a written language. Off topic here, but what are the conditions on the night side for 4.5 years? Because a civilisation might just build a settlement every 20 lines of longitude and remember where it is so they can use it again, unless the conditions on the night side ruin every settlement.

  • $\begingroup$ Imagine a much harsher Antarctica, the effect water has on buildings in our winters would be longer and harsher. Early man-made building have little chance of survival $\endgroup$
    – TrEs-2b
    Jul 8 '17 at 6:55
  • $\begingroup$ They'd have to build awfully fast. You don't have nine years to build, you have the length of time one particular place is habitable. And if your materials are in the uninhabitable zone? $\endgroup$
    – WGroleau
    Jul 8 '17 at 20:29
  • $\begingroup$ @WGroleau you can build basic structures in a few days. The Romans were quite famous for that. $\endgroup$ Jul 9 '17 at 2:10
  • $\begingroup$ @TrEs-2b than how would this species ever advance? Advancement usually requires standing structures and agriculture. If there stuff is destroyed all the time, they will never advance beyond the primitive age. $\endgroup$ Jul 9 '17 at 2:11
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    $\begingroup$ @VenusUberAlles simple, they either A) build underground, or B) take what they build with them $\endgroup$
    – TrEs-2b
    Jul 9 '17 at 2:18

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