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In the real world, each season (Spring, Summer, Autumn and Winter) lasts three months.

In my fictional world, I would want to make sure that each of these seasons will last six months.

What I want to do is to create the years of the calendar and divide these years into years A and years B. In years A, I would include Winter, then Spring, and in years B, I would include Summer, then Autumn. I will also plan to create fictional months and fictional holidays in my fictional world.

Do you know how can I do to provide a scientific explanation for four seasons that will last two years instead of one year?

Thanks you for your answers, and have a good day!

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    $\begingroup$ Let's make it clear - your want "even" and "odd" years to consist of different seasons? $\endgroup$ – Alexander Oct 29 '19 at 19:14
  • $\begingroup$ is there anything wrong with having a planet that takes twice as long to orbit its star? E.g. star's gravity is weaker than our Sub, or planet orbits further away than our Earth? $\endgroup$ – Bald Bear Oct 29 '19 at 19:16
  • $\begingroup$ And what will prevent your people from defining the year as a full cycle of seasons, just as we did on our Earth? $\endgroup$ – Bald Bear Oct 29 '19 at 19:17
  • $\begingroup$ Why not reduce each month to 15 days? $\endgroup$ – nzaman Oct 29 '19 at 19:50
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    $\begingroup$ I think you have a strange definition of "year". A "year" is generally defined as the orbital period of the planet around its sun, which, combined with axial tilt, is why you get seasons in the first place. If you want to somehow have each orbit have a different set of seasons... I think that's going to be really hard. OTOH, if you want your planet's year to equal two Earth years... just do that 😉. Sphinx, from David Weber's Honorverse, has a year that is about 5 Earth-years long. $\endgroup$ – Matthew Oct 29 '19 at 21:21
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Easiest way is probably to double your world's orbital length around its sun.

Our seasons happen because our planet is tilted in respect to its orbital plane (the imaginary plane we would get if we made a flat plane that included all points of the Earth's orbit). The planet's orbit causes that tilt to be in a different direction in respect to the sun as it moves around the sun. The tilt's direction isn't actually changing, but this orbital motion and the tilt mean that at different parts of the year, a different line around the planet is what will be closest to the Sun.

So, double the planet's orbital period. You'll still have the same seasons, but lasting twice as long. Of course, this now means that the orbital period is two earth-years longs, but that should be fine. Just have your civilizations count their years starting on each solstice, irrespective of whether it's the summer or the winter solstice.

The major issue with this is that your sun will have to be hotter, since the increased orbital period will also mean that the planet is farther away, but that shouldn't be a problem as long as you aren't diving into the science behind this planet's orbit. Even some light handwaving should give the desired effect of having the same climate, and the most attention you'd ever need to bring to it is that the two "years" don't have the same stars in the sky.

If you're going for a realistic star for this setting, you could ask the question of what kind of star would have a goldilocks zone that's twice the orbital period of our own.

For the distance, you will want to take the Earth's distance from its orbital point and multiply that distance by the square root of two (very roughly rounds to 1.4). That should give you the radius you need to double the orbit's circumference (and thus its orbital period). This will give you a very rough approximation because I'm ignoring the possibility of an elliptical orbit and the gravitations effects of other bodies, but it should get you in the right neighborhood at least.

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    $\begingroup$ There are some knock-on effects from having, say, summer last six months. Places with dry summers will get REALLY dry; places with cold winters might get extremely cold. Not a critique of the answer,just a suggestion of other things that might follow. $\endgroup$ – Zeiss Ikon Oct 29 '19 at 19:24
  • $\begingroup$ @MrSpudtastic : If I understand your answer, I should make the sun hotter while putting my planet nearly two times farther from the Sun? $\endgroup$ – user69450 Oct 29 '19 at 19:33
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    $\begingroup$ Not quite that far away. Double the distance will more than double the orbit, but otherwise, that's pretty much it. $\endgroup$ – MrSpudtastic Oct 29 '19 at 19:36
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A year is simply the time it takes for a planet to orbit its host star once.

Since seasons are intrinsically tied to the relationship between a planet's axial tilt and its year, you will have one full set of seasons per "year".

However, there is no reason why a planet could not have a year length that takes twice as long as a year does on Earth. Consider the length of the Martian year, which is 686.98 Earth solar days, or 668.5991 sols (Martian days).

In regards to having 6 months per season, you could easily have a moon that divides this double-length year into ~24 orbits rather than ~12 as on Earth.

Keep in mind that in order to take twice as long to orbit its star, a planet would have to be further away. If you intend for your planet to have an Earth-like climate, your star will need to be correspondingly brighter to account for the extra distance.

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