I was reading a World Anvil article which has a world that experiences world wide seasonal changes.

Specifically quoting the article,

During the month of Frimense, the Frostkin thrive, as the whole world descends into 49 days of winter. However, as the month of Igmense arrives, . . .

Is it even remotely possible for an entire planet to experience world-wide seasons on such a short and regular pattern?

  • $\begingroup$ Isn't world wide season what we have on Earth? $\endgroup$ – L.Dutch - Reinstate Monica Jun 25 '18 at 5:30
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    $\begingroup$ @L.Dutch Because our seasons are based on axial tilt, Summer in the Northern hemisphere is Winter in the Southern hemisphere. OP seems to be asking for no axial tilt, so that it is the same season in both hemispheres at the same time. $\endgroup$ – Chronocidal Jun 25 '18 at 7:48
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    $\begingroup$ @L.Dutch to add to what chronocidal said, as you get closer to the equator, the more similar all seasons are. Some people say there are no seasons at the equator, but that is technically not correct. $\endgroup$ – Aethenosity Jun 25 '18 at 14:43
  • $\begingroup$ Related (but not duplicate): worldbuilding.stackexchange.com/q/110478/21222 $\endgroup$ – The Square-Cube Law Jun 25 '18 at 15:55
  • $\begingroup$ A Deepness In The Sky (Vernor Vinge) is essentially this taken to an extreme. A planet at a variable star. $\endgroup$ – TLW Jun 26 '18 at 2:17

In theory, the answer is actually yes. However, unlike our own planet, whose seasons are created through tilt, the planet in question would have to have an elliptical orbit that brought it closer to its sun during certain parts of the year and further away during other parts of the year. The regularity of the orbit would create stable season times. This would likely have some unexpected effects on both the stability of the planet in question(Tidal effects from the sun would be weird), and the ecology of it, but it is certainly possible.

Popular Science has an article on a similar subject, here: https://www.popularmechanics.com/space/solar-system/a27956/what-are-seasons-like-on-other-planets/ I believe that your planet would have seasons akin to Pluto's, but on a shorter time scale.

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    $\begingroup$ @JaccovanDorp Not necessarily twice: If the planet's sun is at one end of the elipse (like it is for most comets) rather than at the centre then you would still only have 1 summer (at perigee) and 1 winter (at apogee) $\endgroup$ – Chronocidal Jun 25 '18 at 9:22
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    $\begingroup$ The central body is always in one of the focal points in an elliptical orbit. The sun cannot be at the center of the ellipse. $\endgroup$ – Daniel Darabos Jun 25 '18 at 13:00
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    $\begingroup$ Note that in those highly elliptic orbits the part far away from the star is much longer than the part near the star, so you will have long winters and short summers. $\endgroup$ – Paŭlo Ebermann Jun 25 '18 at 21:07
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    $\begingroup$ @wedstrom, Pluto certainly does have seasons, with periodic changes in atmospheric density and composition, and changes in ice distribution: en.wikipedia.org/wiki/Atmosphere_of_Pluto#Seasonal_changes $\endgroup$ – Mark Jun 25 '18 at 21:21
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    $\begingroup$ @wedstrom Also worth noting, while the sun appears ~1000 times dimmer from Pluto, that's still ~10 million times brighter than Sirius, the second brightest star in the sky. That's why Pluto has distinct light and dark sides in the pictures from New Horizons. I swear there was some NASA article that said the sun would look like an ordinary star from Pluto (so you're not dumb), but it's not remotely true. Some intern was exaggerating, perhaps. $\endgroup$ – kjgoebel Jun 27 '18 at 4:16

An even simpler explanation is simply for there to be no land in one hemisphere of the planet. If it’s summer in the southern hemisphere, then no one will care that it’s winter in the northern hemisphere if there is no land and therefore no people. Seasons will be stated as affecting the whole planet even if it’s not technically true. They might affect a few long-distance shipping routes, but nothing else.

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    $\begingroup$ This is not far from our current situation where Aussies still have to explain that they celebrate Christmas during summer. $\endgroup$ – pipe Jun 25 '18 at 12:51
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    $\begingroup$ @pipe what about South Africans, Chileans, etc.? Australians are just weird, climate has nothing to do with that :) $\endgroup$ – jwenting Jun 26 '18 at 4:37
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    $\begingroup$ @jwenting they presumably have the same problem as the Australians? $\endgroup$ – gmatht Jun 26 '18 at 5:19
  • $\begingroup$ @gmatht never seen them trying to explain anything :) $\endgroup$ – jwenting Jun 26 '18 at 5:50
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    $\begingroup$ @pipe more like you Northern Hemisphere people having to explain why you are backwards and have Christmas in winter. Xmas = BBQ & Beach ;) $\endgroup$ – Skuld Jun 26 '18 at 9:02

An alternate explanation would be a binary star system. While the two stars orbit eachother, they get closer and further away from the planet, producing more and less heat.

This allows for seasons of equal length, while an elliptical orbit would have a short warm season, and a longer cold season, as the orbital velocity of the planet is greatest at its perihelion and slowest at its aphelion.

Wikipedia also has some notes on the habitability of such systems. In short, they may be habitable depending on many factors.

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  • $\begingroup$ A fair few scifi stories have two or more suns. Also since each sun may not be the same size (one may be smaller/less powerful) that could also help with the seasonal variation. $\endgroup$ – Wilf Jun 25 '18 at 9:40
  • $\begingroup$ One of the stars need not be a proper star at all - it could be a compact object, like a neutron star or a black hole. This would give you the greatest seasonal variation. $\endgroup$ – Skyler Jun 25 '18 at 17:56
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    $\begingroup$ @Skyler - you could even have your world be a moon of a gas giant. This would also potentially end up with eclipses... note that there are other issues with this though. Namely atmosphere loss. The mechanics of such a world are weird... you'd end up with a nontrivial amount of tidal heating, and it'd likely be tidally locked to the gas giant. $\endgroup$ – TLW Jun 26 '18 at 2:15
  • $\begingroup$ See 3 Body Problem by Liu Cixin (English version translated by Ken Liu). $\endgroup$ – Paul Williams Jun 26 '18 at 15:01

First it needs to be noted, that for the entire planet to have same season it needs to have its rotational axis be perpendicular to its orbital plane.

As for how it can have changing seasons there're a couple ways:

  1. Previously mentioned in one of the answers elliptic orbit.

    For earth-like seasonal difference the furthest point should be maybe 1.4 times further from the sun than closest.

    Problem with it is planet moves faster the closer it is to the sun, so the "summer" would be somewhat shorter than "winter".

    One of the comments mentioned, that it's possible to have each season twice in one rotation, but orbital mechanics doesn't work like that. Sun is always in the focal point of the orbital ellipse.

    Also shorter seasons (winter is 49 days) would mean shorter year, which means faster rotation and closer orbit. For reference, Mercury has a 90 day year, so for such a planet to be habitable, it will need a different class of star.

As a perk, the color of the sun might have minor Doppler shift, becoming slightly greener in spring and redder in autumn, because planet moves towards/away from it.

  1. Variable star can be an easier explanation, albeit not as fun. They're very flexible, as their period can be anywhere from hours to years. And they're different from pulsars, which have periods in milliseconds.
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    $\begingroup$ Reading this and the other answers, I read "summer" as the nice warm growing season, and "winter" as freezing. A clear case of my New England centricism. Having a short summer would be great if summer were the dying season that life endured, with winter being the easy-living time. So, I'll stop trying to think of how to extend the summer. Let the summer be short and difficult. $\endgroup$ – cmm Jun 25 '18 at 15:11
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    $\begingroup$ Re: doppler shift. It wouldn't be detectable by the naked eye, alas. At least unless the central body is something exotic. (Escape velocity of the sun at Mercury's orbit, for instance, (~67km/s unless I did something silly) only shifts spectral lines by something like 0.2%) $\endgroup$ – TLW Jun 26 '18 at 2:13
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    $\begingroup$ Opening sentence is simply wrong. You can get uniform, planet-wide seasons regardless of axial tilt if the change in received light and heat due to variance in orbital distance (in the elliptical orbit case) or stellar intensity (in the variable star case) is larger than the change due to axial tilt. In such a case, it might be a little "more summery" or "more wintry" in one hemisphere or the other, but it would still be summer or winter in both at the same time. $\endgroup$ – Dave Sherohman Jun 27 '18 at 7:13
  • $\begingroup$ @cmm This is kind of how the climate is in Queensland, Australia (the top-right corner of the continent). In the summer, conditions become tropical and it gets extremely hot. In winter, it cools off to a balmy Mediterranean-style climate. In fact, the hotels in the coastal region have their high-season in the local winter-time. $\endgroup$ – Oscar Bravo Jun 27 '18 at 7:17

Here's a solution that doesn't require elliptical orbits, super fast planets or unusual stellar bodies for the sun. Giant dust cloud.

How giant you ask? Imagine that during the formation of the solar system something went horribly wrong and instead of Mercury and Venus forming as is, they instead tried to form one planet but ended up breaking apart in to dust cloud orbiting the Sun.

Spread out enough to not clump under gravity but dense enough to cause a noticeable drop in solar radiation when it's between the Earth and the Sun. Maybe something similar in density to Saturn's rings but spread over a wider area? Place it at a bit of an angle from the Earth's orbital plane so that the ring can be consistent density all the way round but still produce a variable masking.

Might require fudging the numbers a little to get a reasonable set of values, but it's not the most ridiculous stellar formation I've ever heard of. I suspect the main hurdle would turn out to be solar wind and flares disrupting the structure, but maybe there's a way round that with composition hacks?

Plus, how bad ass would that look :p

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I have tried to imagine a way that a planet's seasons might be self-generated and self-regulating through dynamics on the surface, either of weather or of plant life. I'm not quite there, though, but here are some ideas:

  • deciduous trees in the worldwide forest have white leaves; when the foliage peaks in summer, it increases the planet's albedo, reflecting sunlight into space, and beginning a cooling phase
  • as it gets cooler, winter begins, these trees drop their leaves, exposing the shorter evergreen trees, which are dark and absorb sunlight, beginning a warming cycle

The problem with this idea is that snow is white, so if there's a lot of snow, that would increase albedo and create a positive feedback loop where the "cold gets colder" and summer doesn't come again. Can anyone suggest a way to improve my idea?

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