Our spherical planet has two hemispheres divided at the equator. Those living in the north hemisphere experience the opposite season as those living in the south hemisphere at any given time of year. For example: New Zealand summer/Britain winter, USA spring/Australia autumn, etc. Is it possible for a planet to have the same seasons in opposite hemispheres? If so, how?
$\begingroup$ are you looking for the same seasons at the same relative latitudes (ie 30 deg N is the same as 30 deg S) or are you looking for seasons to be the same everywhere (ie 20 deg N is the same as 80 deg S)? $\endgroup$– NosajimikiMay 21, 2020 at 14:50
2$\begingroup$ I’ve created a world where this is exactly the case. There is no axial tilt and the planet has a more elliptical orbit. During my research, I’ve found that the hotter part of the year (when the planet is closest to the sun) will be notably shorter in length because of the change in velocity as it gets closer to the star. $\endgroup$– RauriMay 22, 2020 at 0:43
Yes it is possible, if the planet has little axial tilt but an eccentric orbit. Then summer will be when it is closest to its sun, and winter will be when it is furthest away, which will be the same all over the planet — at the equator as well as in higher northern and southern latitudes.
5$\begingroup$ oh, right. Ninja me by 13 seconds :-) $\endgroup$ May 21, 2020 at 14:21
3$\begingroup$ You are missing a very crucial side effect of seasons caused by orbital eccentricity. Summers will be much shorter than winters. $\endgroup$ May 21, 2020 at 16:38
6$\begingroup$ @NateWhite Shorter, yes, but probably not much shorter. It doesn’t take a massive amount of eccentricity to produce a significant difference in insolation between aphelion and perihelion, though I don’t have figures to hand. $\endgroup$ May 21, 2020 at 17:52
$\begingroup$ You'd need to be 1.8 times closer to give the same difference in solar radiation flux at noon as the difference between summer and winter that 50 degrees latitude sees for Earth. (sqrt(cos ((50-23.5)*degrees) / cos ((50+23.5)*degrees))) ; gives eccentricity of 0.28, more if total insolation per day were considered. Pluto is 0.25 so not infeasible. $\endgroup$ May 21, 2020 at 23:58
3$\begingroup$ @Pete Kirkham: Not as extreme as your calculation. Heat is transferred poleward on earth, which moderates the seasons. $\endgroup$ May 22, 2020 at 3:48
Of course. Just make sure the axial tilt is zero. Basically, 99% of our weather changes are due to the angle of solar incidence, and only a small amount is due to the distance Earth is from the Sun (elliptical orbit). Earth's axial tilt is roughly 23.44 degrees. For a given latitude, a little trigonometry will show how the angle of solar incidence (draw a line from the sun and see what angle it makes with the ground) changes as that axial tilt faces towards or away from the sun -- and note that "towards" in the Northern Hemisphere means "away" in the Southern Hemisphere.
Thanks to Matthew for reminding me that there's a significant change in the number of hours of daylight as the orientation of the axial tilt changes. That's at least as important as the angle of incidence of the sun's rays.
2$\begingroup$ In addition, the tilt has a non-trivial effect on the relative lengths of day/night. I'm not sure this is exactly what you (Carl) meant by "angle of solar incidence" (does the angle on its own have an effect? It might...), but obviously longer nights ⇒ less time when the sun is shining ⇒ less cumulative solar energy. $\endgroup$– MatthewMay 21, 2020 at 15:24
$\begingroup$ @Matthew you are correct that the length of daylight is important as well. I will edit. $\endgroup$ May 21, 2020 at 15:25
3$\begingroup$ @Matthew The angle of incidence on its own does in fact have a very significant effect. $\endgroup$– HearthMay 21, 2020 at 23:41
2$\begingroup$ @Hearth, I suspected so, but wasn't sure. Come to think of it, though, if this wasn't the case, you'd expect the poles (which alternate between months of continuous sunlight to months of continuous night) to get really hot in summer, which they don't. $\endgroup$– MatthewMay 22, 2020 at 17:09
Expanding on the other answers here, let's start with an overview of why there are seasons. I really like this description:
We have seasons because the earth is tilted (wonky) as it makes its yearly journey around the sun. The Earth's axis is tilted at an angle of 23.5 degrees. This means that the Earth is always "pointing" to one side as it goes around the Sun. So, sometimes the Sun is in the direction that the Earth is pointing, but not at other times. The varying amounts of sunlight around the Earth during the year, creates the seasons.
Here's a visualization of the orbital tilt and seasons from Wikipedia.
The Earth stays roughly the same distance from the Sun because it has a (roughly) circular orbit. But there are a lot of different paths that a planet can take to orbit a star. When a planet moves farther from the star, the planet gets colder (winter). When the planet gets closer to the star, the planet gets warmer (summer). The relevant Wikipedia article is complicated but this GIF shows several different orbits.
Imagine you're on the planet below. If the planet isn't tilted then the two hemispheres would experience the seasons at the same time. Check out this question for more detail.
9$\begingroup$ Your animation illustrates another important point: Such a world will have short summers and long winters. This is a consequence of Kepler's 2nd Law. $\endgroup$ May 22, 2020 at 4:26
Other answers are right at pointing at eccentricity as the most likely cause for global seasons. However, there is another possible cause: multiple stars.
If the planet were orbiting a double star, the distance to each star might change as the stars orbit each other. If they have different mass and bright, the total amount of heat the planet gets may change producing seasons.
A planet orbiting around one of the stars of the binary system could also get a changing amount of heat from the other star.
Interestingly, binary stars could lead to a wide range of changes in radiation and even color of light that could make interesting plot devices.
$\begingroup$ It is worth nothing that this will result in very short seasons, since the orbital period of the binary stars will be much shorter than that of the planet. I'll also note that in this configuration you can get changing seasons due to one star eclipsing the other. $\endgroup$– BBeastMay 23, 2020 at 6:34
$\begingroup$ You might want to check out Jack Vance's "Marune: Alastor 993" which is set on a world with 4 suns. The climate's not such an issue, but the colour of the ambient light at whichever of the 16 possible solar phases is a critical plot device. Vance even provides a convenient reference chart for what each phase of the day is called, and what it is customary to do in those phases. $\endgroup$ May 23, 2020 at 10:59
$\begingroup$ @BBeast - Depending on orbital parameters you tune seasons length quite freely. In an S-type orbit (planet orbiting around one star), the orbit of the other star may be very long. Even for a circumbinary stars, if distances are large enough, 1 (Earthly) year seasons cycle is possible. $\endgroup$– PereMay 23, 2020 at 13:17
1$\begingroup$ @PrimeMover - And Asimov's "Nightfall", where climate is not the plot device but continuous sunlight from sis different stars. $\endgroup$– PereMay 23, 2020 at 13:19
$\begingroup$ @Pere This is true. Although, the intersection of 'stable orbital configuration', 'habitable zone' and 'makes the seasons you want' is probably highly non-trivial. The three body problem is notorious for having many unstable configurations (there are some stable configurations, often to do with resonance). $\endgroup$– BBeastMay 24, 2020 at 4:58
Zero tilt, elliptical orbit are the requirements but there something else that is important...winds.
Winds blow due to pressure differentials due to mainly two reasons on earth, thermal insolation differences that drive sea-land vice verse winds and coriollis forces that drive planetery winds.
In the above situation even with uneven land-mass distribution you may find most winds blowing towards the poles...causing thinning of atmosphere near equater...people can correct me in the comments on this prediction I’ll be happy to edit.
3$\begingroup$ Atmosphere thinning isn't going to be much - going 100 meters up or down will expose you to higher pressure variation than moving from a high-pressure area to a low-pressure one, unless you have extreme situations (you have very low pressure in the eye of a hurricane, you'll have to move up 500 meters to get the same difference). $\endgroup$ May 22, 2020 at 8:36
1$\begingroup$ 'Is it possible for a planet in a circular orbit with no axial tilt to still have seasonality?' - localized phenomenon, coincidentally yearly, so as to fit the definition of season, +1 $\endgroup$– MazuraMay 22, 2020 at 23:02
There are five ways that this could happen:
- a very eccentric orbit with little/no axial tilt
- a pulsating star with little/no eccentricity or tilt
- the majority of the planet's energy comes from a star orbiting a black hole sharing a similar orbit with the same semi-major axis but has different eccentricity to the planet
- the planet is in a system with binary stars and little tilt/eccentricity
- the "planet" is a moon of a gas giant that gets heat from tidal locking
The planet could be a rogue planet not orbiting any star. It exists in permanent winter darkness, the same in both hemispheres. This almost certainly isn’t what the question is looking for, but I figured I would include the degenerate case for completeness.
As an example, Venus with its extremely thick atmosphere over a rocky world yields extremely similar temperatures and seasons over the whole planet.
Multiple circulation cells, very high density atmosphere, and thick clouds/dust to absorb insolation all conspire to make the surface conditions relentlessly the same.
Venus also has very little axial tilt, so it wouldn't have much for seasons even if it had only 1 standard atmospheric pressure. But you might adapt your story with Venus in mind.