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Earthlings,

I live on a world where most places on my planet has four seasons, just like yours does. We also have one sun and one moon, presumably. It's been not even a year since I came of age (20), and we've gone through the four seasons about a dozen times. I repeat, it has not yet been a year.

The people on my planet are very similar to humans on your world, and a side-by-side comparison would show this. Our animals are much like yours also, with minor exceptions that aren't really relevant. The only difference between us and your people is that we're much more resilient to the elements, blunt force trauma, and so forth. Several times each, I've survived electrocution, being blasted in the face by fire, and falling off a tall building where I fell at least 5 stories, probably more (usually being caught by something before hitting the ground, but not in a way that would make the impact much safer) and suffered minor injuries at most from each of these. So if the reason means the conditions of our world are somewhat less hospitable compared to yours, that should still be fine, as long as our world isn't a hellscape. (Our world is actually quite pretty with rainforests, cities, crystal clear oceans, fields, and so forth.)

That said, we have holidays like you do. We have Christmas every winter (not once a year, but every winter,) and Halloween every autumn. Our birthdays, however, are only once a year, just as yours are.

I've always found this strange, though. With one sun and moon, it makes little sense that we'd have the four seasons multiple times. From my understanding, the four seasons should still only happen once... right? Is there an explanation for how this could occur naturally? If not, what about unnaturally? Could this be caused by a specific tilt to the axis?

EDIT: Some supplementary information. We are a globe planet, technically an oblate spheroid, like Earth. Not a flat world like Discworld, nor are we other abnormal shapes. Also, I'd like the answer to make sense through science, without the need of "magic" as an answer.

Additionally, we define our seasons astronomically, I presume, but I'm no scientist. I just know how the patterns change and how we celebrate holidays. We don't consider just any moment we get snow to be winter, but there are periods of the year where snow is much more likely which we refer to as "winter", but that isn't to say snow in the other seasons is impossible, just less likely.

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  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$ – L.Dutch - Reinstate Monica Oct 28 '18 at 10:36
  • $\begingroup$ "Four Seasons Occur Multiple Times In One Year?" - easy. Call your planet "Melbourne, Australia" and you will have four seasons in one day. $\endgroup$ – Gimelist Nov 10 '18 at 6:21
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The reason we have four seasons is directly related to our planet's axial tilt... the rotational axis of the Earth is currently [1] inclined at 23.5° from the "vertical" relative to our orbital plane: Summer is the season in which a given hemisphere is tilted more towards the sun, Winter is the season in which a given hemisphere is tilted away, and Spring and Autumn are the transitional periods between those states.

This is why solstices and equinoxes are so valuable in marking seasonal time: they represent the maxima, minima and midpoints of the values of day/night length for the given hemisphere.

If a planet precessed (its axial tilt relative the plane of its orbit shifting through the course of time - visualise here a spinning top tipped at an angle and that angle rotating round as the top spins) at exactly the correct fractional rate relative to its orbital period (year), it would be expected to then have two or three or four sets of repeated seasons through the course of a single year.

Bear in mind that we have observed planets with differing levels of precession of their axial tilt, and we have some good theories about the long-term stability of such situations, but we don't have long term observations with enough detail to point at concrete examples - that said, precession-based multi-seasonality would most likely be far more stable than the earlier-mentioned false seasons via cyclic weather pattern given a similar atmospheric density and composition to our own.

Given that our planet precesses (look up the precession of the Equinoxes for details) in a cycle of about 25,000 years, you could readily posit a planet whose precession speed was faster than its orbital period - giving your planet multiple seasons per year. If you are comfortable delving into this in more depth, you could posit your planet being in a period of some orbital instability around its primary [2], in which case both the orbital period and the precession periods would be in flux, and the precession might not only fluctuate in period but in vector (visualise your spinning top slowing down and the chunk of time in which it appears to almost flip around to and fro as the precession becomes unstable) and you'd have a supremely unstable set of seasons - almost aperiodic - and given we're positing this based on main orbital instability, you might have solar flares, changes in solar radiation received as distance to primary fluctuates, gravitational and magnetic storms and fluctutations - you could go for some pretty severe, apocalyptic level climactic chaos here.

I will leave it to some of the physics minded folks to give you the orbital mechanics maths details, as I know my physics studies and practical usage are now far behind me; statics I still use, but physics per se - juuuust the core concepts these days!

Hope this helps.

[1] This value changes slightly over time in a range between ~ 22.1° and 24.5°
[2] Orbits can collapse due to other orbital bodies being disrupted (asteroid impact) or coming apart (our solar system's asteroid belt was most likely once a planet) or due to direct interaction with other bodies - say a large passing comet moves past your planet with a very close approach, or due to changes in the primary itself (new mass added due to another satellite's orbital decay being severe enough to allow impact & merger) or even just tidal gravity effects robbing your planet of orbital velocity.

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    $\begingroup$ This would also explain how we still have multiples of the holidays at consistent intervals. Thank you! :) $\endgroup$ – Sora Tamashii Oct 25 '18 at 17:09
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    $\begingroup$ I noticed the addition. I'm holding off on selecting an answer until tomorrow to encourage more people to try and answer. $\endgroup$ – Sora Tamashii Oct 25 '18 at 20:34
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    $\begingroup$ That's fine. We have other historic means of navigation without relying on a single individual star. $\endgroup$ – Sora Tamashii Oct 26 '18 at 3:37
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    $\begingroup$ If it were the case that a planet had multiple seasons in a year, then how would the locals define a year? Would they define it as the amount of time it takes for the planet to go around the sun or would they define it as a cycle of seasons? On Earth the concept of a year came about because of the seasonal cycle and not knowledge of astronomy. The concept of a year long predated the model of a heliocentric solar system. $\endgroup$ – Kevin Peter Oct 26 '18 at 19:51
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    $\begingroup$ I have to take issue with your assertion that the asteroid belt in our solar system was likely once a planet. I saw that notion in science fiction a few times in my youth so I looked it up in Wikipedia a while back. Apparently astronomers are satisfied that there is nowhere near enough mass in the asteroid belt for the asteroids to have ever comprised a planet. $\endgroup$ – Henry Oct 26 '18 at 22:03
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The planet has no axial tilt.

But it anyway has seasons, because:

The star it orbits arount is an intrinsic variable star, it changes its size and brightness periodically, causing seasons that may repeat multiple times per year, since the star may change its brightness multiple times per year.

Variable star at different times:

Variable star at different times:

therefore, differing from earth, on this planet, seasons are the same on both hemispheres.

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    $\begingroup$ Regardless of tilt, you'd still have a periodic seasonal variation with an intrinsic (specifically: pulsating) variable star. Having axial tilt will just cause hot summers and mild winters in a particular hemisphere if it occurs at the same time as the expansion phase, and mild summers and harsh winters if it occurs during the contraction phase. $\endgroup$ – Stephan Oct 25 '18 at 20:28
  • $\begingroup$ @Fattie They do. en.wikipedia.org/wiki/Cepheid_variable $\endgroup$ – Geoffrey Brent Oct 26 '18 at 8:11
  • $\begingroup$ @GeoffreyBrent - you're indeed correct. typical periods might be "one or two (Earth!) months" ... so in fact that's perfect for the question under discussion. $\endgroup$ – Fattie Oct 26 '18 at 8:27
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Your seasons aren’t caused by axial tilt. They’re more accurately known as ‘weather’.

There is a system on Earth that leads to warm, humid seasonality and raised sea levels on top of the usual four seasons caused by tilt relative to the sun. It is called El Niño/ La Niña, and (to simplify greatly) it's a giant wave that bounces up and down the planet’s oceans at a pace dictated by the shape and size of said oceans.

Now, if your planet has a series of such weather systems (for example a wave that heads up and down the planet, a storm that cycles around the planet, or an ocean current that periodically switches direction) then you can experience multiple ‘seasons’ as many times per year as your weather systems will allow. They’re not driven by axial tilt, rather the resonant properties of your planet.

It is worth noting however that such seasonalities are unlikely to be stable over geological timescales, so the seasons of a thousand years hence will be different to the seasons of today. You may even have different sets of seasons in different geographical locales, which would be fun and interesting!!

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  • $\begingroup$ Ooh! Of course it's good practice hear to wait 24 hours before accepting an answer, but I like this! Not only could this explain it within reason, it'd also explain inconsistency with season lengths as well. Thanks for that! $\endgroup$ – Sora Tamashii Oct 25 '18 at 16:49
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    $\begingroup$ Edited to change to El Niño/ La Niña, instead of El Niño/ El Nina. In Spanish, El/la changes with the gender of the noun. Niño means boy and is masculine, while Niña means girl and is feminine. You also forgot the tilde on the n in niña. $\endgroup$ – John Locke Oct 25 '18 at 18:23
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    $\begingroup$ Cheers @JohnLocke. Linguistics is really not my strong suit. $\endgroup$ – Joe Bloggs Oct 26 '18 at 6:16
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You orbit a double star, one of which is much dimmer than the other.

This causes the "brighter" star to come closer/further away in a period faster than your "year".

The dimmer double star is dim enough that you cannot practically see it as a star; it is washed out. At best, it might appear as a faint ghost at dawn/dusk. Also, the dim star would rarely eclipse the brighter one.

The orbital period goes up with the 3/2 power of the orbital radius. So if the dual star is 16 times closer to each other than the planet is, they will orbit each other ~64 times faster than the planet.

A big risk here is that a dim compact dense partner (like a white dwarf star) can accumulate hydrogen from the solar wind (or worse if the primary is too close), and go nova. I'm honestly uncertain at what radius the nova risk is low enough to plausibly evolve life.

Very interesting effects could happen if the primary/secondary star axis of rotation does not line up with the planet's axis of rotation. But the more interesting it gets the less stable the planet's orbit is likely to be.

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  • $\begingroup$ That would only explain up to 2 sets of seasons in a year. $\endgroup$ – Sora Tamashii Oct 25 '18 at 20:23
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    $\begingroup$ @SoraTamashii Why so? The ratio between the frequency of the star rotation and the planet rotation can be nearly arbitrary. If the binary system is 100 times closer than the planet is, then the orbital period will be 100^(3/2) = 1000 times faster. $\endgroup$ – Yakk Oct 25 '18 at 20:28
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    $\begingroup$ Oh, I was misunderstanding! I see what you're getting at now! I'll admit I still don't like this answer due to the need for two suns, which I feel would be apparent, BUT I like that you have an out for that. I'll make that a +1 for you. Okay, nice job. I see what you're going for and that's a more clever solution than I originally was giving you credit for! $\endgroup$ – Sora Tamashii Oct 26 '18 at 3:11
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    $\begingroup$ @SoraTamashii See en.wikipedia.org/wiki/Black_dwarf -- note that these are pretty exotic, as they should require far longer than the universe has been around to form. $\endgroup$ – Yakk Oct 26 '18 at 3:26
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    $\begingroup$ As a quick and dirty answer though, it is a valid one, especially when the best answers were already made. This was a great way of making an interesting answer that still stands out. It'd be up to me to figure out how to make it work if I went with it. I like the effort put in, nonetheless! $\endgroup$ – Sora Tamashii Oct 26 '18 at 19:22
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Does your moon happen to be large and highly reflective shiny?

It sounds like where you're from, it takes more than 20 earth years to go around your star (sun? No, that one's ours!). But how fast does your moon go around your planet?

Here on earth, this happens roughly 13 times per earth year. And while our moon is the largest in our system, and appears to be quite bright, it could be a lot brighter still! If if were, then every full moon, we'd have a nice little bit of summer going on due to the extra light reaching our planet. On a new moon, we wouldn't get as much light, and temperatures around the world would drop.

On our earth, these "lunar seasons" aren't really significant at all, because the solar seasons are much stronger. But if your planet's solar seasons are quite weak (because of a small axial tilt) then the effect might be a lot more noticeable!

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There are two ways this could happen.

  • A star system composed of infinite cylinders will do the trick in some arrangements. This is discussed at length in this question:

Seasons on Infinite Cylinder Planet

Since the disc's tiny orbiting sunlet maintains a fixed orbit while the majestic disc turns slowly beneath it, it will be readily deduced that a disc year consists of not four but eight seasons. The summers are those times when the sun rises or sets at the nearest point on the Rim, the winters those occasions when it rises or sets at a point around ninety degrees along the circumference.

(...)

Since the Hub is never closely warmed by the weak sun the lands there are locked in permafrost. The Rim, on the other hand, is a region of sunny islands and balmy days.

Precisely why all the above should be so is not clear, but goes some way to explain why, on the disc, the Gods are not so much worshipped as blamed.

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    $\begingroup$ I should edit in my post that we are a globe, but I do appreciate this input. I hadn't considered that at the time. Also I'm not sure how I feel about an ICP planet. A few ICP fans at a given time are enough. :P (ICP being a band. I'm making a joke. Don't overanalyze. lol) $\endgroup$ – Sora Tamashii Oct 25 '18 at 17:03
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    $\begingroup$ @SoraTamashii I know ICP and now I am imagining a world made of magnets and magic. $\endgroup$ – Renan Oct 25 '18 at 18:27

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