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PostPosted: Sat Jun 15, 2013 8:20 pm 
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Would an elliptical trajectory (0.20 eccentricity?) be able to recreate an uniform season like effect on a planet with 0 axial tilt? I really don't want to keep track of seasons depending on the player's position on the planet.

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PostPosted: Sat Jun 15, 2013 9:01 pm 
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Endorya wrote:
Would an elliptical trajectory be able to recreate an uniform season like effect on a planet with 0 axial tilt?

Yes. The aphelion would be the equivalent of a global winter,
the perihelion would be the equivalent of a global summer.


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PostPosted: Sat Jun 15, 2013 9:03 pm 
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Awesome! Thanks rust!

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PostPosted: Sun Jun 16, 2013 1:48 pm 
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Endorya wrote:
Would an elliptical trajectory (0.20 eccentricity?) be able to recreate an uniform season like effect on a planet with 0 axial tilt?
Seasons, yes, but not uniform: in accordance with Kepler's Second Law, the planet will spend much less time passing through periastron and much more through apastron. So summer will be much shorter than winter. At an eccentricity of 0.20, the difference will be fairly extreme: if I'm remembering the maths correctly, a 37-63 split. Since incoming stellar radiation varies as the square of distance, the energy received will vary considerably, as well: 1.56 vs. 0.69 at periastron and apastron, respectively, or about the seasonal variation at 30 degrees latitude on Earth. This will make your summers short but intense; your winters long and dismal.

Also note that the seasons will apply to the entire globe at once, rather than alternating hemispheres. This will affect things like weather patterns and migration of avian species.


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PostPosted: Sun Jun 16, 2013 2:29 pm 
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thrash wrote:
Endorya wrote:
Would an elliptical trajectory (0.20 eccentricity?) be able to recreate an uniform season like effect on a planet with 0 axial tilt?
Seasons, yes, but not uniform: in accordance with Kepler's Second Law, the planet will spend much less time passing through periastron and much more through apastron. So summer will be much shorter than winter. At an eccentricity of 0.20, the difference will be fairly extreme: if I'm remembering the maths correctly, a 37-63 split. Since incoming stellar radiation varies as the square of distance, the energy received will vary considerably, as well: 1.56 vs. 0.69 at periastron and apastron, respectively, or about the seasonal variation at 30 degrees latitude on Earth. This will make your summers short but intense; your winters long and dismal.

Also note that the seasons will apply to the entire globe at once, rather than alternating hemispheres. This will affect things like weather patterns and migration of avian species.


Ahhh.. You are absolutely right! Though the 0.20 eccentricity was just an example, I clearly see what you mean.
So I my question now is: Would it be possible having each season lasting about the same period of time in a planet with no axial tilt?

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PostPosted: Sun Jun 16, 2013 5:35 pm 
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As thrash says, the problem is that the planet moves faster at perihelion, so the summer will always be shorter than the winter. If you want the 'seasons' to be more similar in length, you'd need to have a smaller eccentricity, but that also makes the difference between the 'seasons' less extreme.

To illustrate:
Earth's blackbody temperature (not including atmosphere and albedo) without any eccentricity at all is a flat 278K (at 1 AU).
The variation caused by Earth's current eccentricity of 0.0167 is +2.36/-2.30K (orbital distance varies between 0.983 and 1.017 AU)
With an eccentricity of 0.05 that would be +7.24/-6.72K (orbital distance varies between 0.95 and 1.05 AU)
With an eccentricity of 0.10 that would be +15.07/-12.97K (orbital distance varies between 0.90 and 1.10 AU)
With an eccentricity of 0.20 that would be +32.89/-24.28K (orbital distance varies between 0.8 and 1.20 AU)
With an eccentricity of 0.30 that would be +54.40/-34.26K (orbital distance varies between 0.7 and 1.30 AU)

Keep in mind that the summer and winter variations we see are caused mostly by our tilt though - in northern hemisphere summer we're actually furthest from the sun, and in northern hemisphere winter we're actually closest. (see http://spaceweather.com/glossary/aphelion.html ). But if we had our tilt and even an eccentricity of 0.05 we would REALLY notice the extremes!

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PostPosted: Sun Jun 16, 2013 8:28 pm 
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EDG wrote:
As thrash says, the problem is that the planet moves faster at perihelion, so the summer will always be shorter than the winter. If you want the 'seasons' to be more similar in length, you'd need to have a smaller eccentricity, but that also makes the difference between the 'seasons' less extreme.

To illustrate:
Earth's blackbody temperature (not including atmosphere and albedo) without any eccentricity at all is a flat 278K (at 1 AU).
The variation caused by Earth's current eccentricity of 0.0167 is +2.36/-2.30K (orbital distance varies between 0.983 and 1.017 AU)
With an eccentricity of 0.05 that would be +7.24/-6.72K (orbital distance varies between 0.95 and 1.05 AU)
With an eccentricity of 0.10 that would be +15.07/-12.97K (orbital distance varies between 0.90 and 1.10 AU)
With an eccentricity of 0.20 that would be +32.89/-24.28K (orbital distance varies between 0.8 and 1.20 AU)
With an eccentricity of 0.30 that would be +54.40/-34.26K (orbital distance varies between 0.7 and 1.30 AU)

Keep in mind that the summer and winter variations we see are caused mostly by our tilt though - in northern hemisphere summer we're actually furthest from the sun, and in northern hemisphere winter we're actually closest. (see http://spaceweather.com/glossary/aphelion.html ). But if we had our tilt and even an eccentricity of 0.05 we would REALLY notice the extremes!


That means that, all else being equal, northern hemisphere seasonal temperature changes are less extreme than those in the southern hemisphere by 4.66K, right?

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PostPosted: Sun Jun 16, 2013 8:31 pm 
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I see what you mean. Thanks EDG!

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PostPosted: Sun Jun 16, 2013 9:23 pm 
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Sir Chaos wrote:
That means that, all else being equal, northern hemisphere seasonal temperature changes are less extreme than those in the southern hemisphere by 4.66K, right?


No - the number before the slash is the global (blackbody) temperature gain at perihelion (Earth's closest approach to the sun), the number after the slash is the global temperature loss at aphelion (earth's furthest distance from the sun) - the temperature at 1 AU is still 278K though. In terms of latitude, equatorial regions would still be hotter than polar regions, but that's not shown here.

So at 0.2 eccentricity, the average blackbody temperature would be about 311K (38°C) at perhelion, and 254K (-19°C). Turns out that once you include Albedo (0.3) and greenhouse effect factor (1.10), the final temperature isn't actually that different to the blackbody (but only in Earth's case) - the actual average global temperature (including those) would range between 313K at perihelion and 256K at aphelion.

Another complication is that water and land retain heat differently. It may be that with such great temperature extremes over the year the water may not 'react' quickly enough to the changing tempartures to be significantly affected. I think though that because more of the year would be spent further from the sun, it's more likely that the water temperatures could drop enough to make some pretty big icecaps - the 'hot season' might be too short to significantly affect the ice so there could be big icecaps all year around.

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