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What is the most optimal earth's axial tilt in terms of variation of seasons? What would be optimal axial tilt for earth that life would exist and change of seasons would be at minimal level?

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This is really a question of biology. Of course that life could adapt to greater or smaller (or no) seasonal variations, too. It would be a different life, after billions of years. The seasonal cycles are used by many animals and especially plants in various ways - but they're largely non-essential. Even the existing species could adapt to much stronger or weaker variability but they would evolve in a certain different way if exposed to the new environment. –  Luboš Motl Apr 26 '13 at 13:11
    
Uranus has an axial tilt of 97.77 degrees. I can imagine an exoplanet being discovered with a similar tilt. It would fall to physics to discover the tilt and then try to predict what impact this would have on climate. If the planet was otherwise earth-like there would be great interest to understand the impact of axis tilt on habitability. –  Mark Rovetta Apr 26 '13 at 16:36
    
<pedantic>The word "optimal" means "most favorable"---optimality can't be relative, either something's optimal (given some criteria) or it isn't. So "most optimal" is redundant. –  Ahmed Fasih Oct 9 '13 at 19:11
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The premise of this question that "optimal" can be defined "in terms of variation of seasons", or that the strength of seasons influences the existence of life, seems unsupported. –  Floris Sep 26 at 4:30
    
@Floris: I meant that what would be the most optimal axial tilt for earth that change of seasons would be at minimal level eq. at "height" of Helsinki? Is it actually $23,5^o$ or $15^o$? –  laovultai Sep 29 at 21:17

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If Earth had 0 inclination to the ecliptic, then life could still exist and there would be no change in seasons. In fact, at latitudes greater than $-23.5^\circ$ and less than $23.5^\circ$, the seasons are already irregular compared with the rest of the planet; they have two points where the Sun shines directly on top of them, effectively making them have two summers. At the equator, there is practically no/minimal variation in seasons already.

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Small point: even when the inclination is zero, the elliptical orbit of the earth will give rise to some "seasons" - cooler at aphelion, hotter at perihelion (possibly with some lag due to heat capacity effects - just as it's hotter in July even though the solstice is in June). –  Floris Sep 26 at 4:32
    
@Floris the perihelion is closer to february. I covered the reason for it being hotter in July than in June in a different thread –  Jim Sep 26 at 13:39
    
my comment about "lag" referred to the stronger effect of inclination on seasons - I see now it could be interpreted differently... I meant to say "if earth had zero inclination it would have mild seasonal variation due to elliptical orbit but the days would not be hottest at perihelion but slightly after". –  Floris Sep 26 at 14:22
    
@Floris, the intensity of the Sun at perihelion is only about 6% greater than at aphelion. So, you'd have a slightly warmer than mild spring and a slightly cooler than mild spring in between two mild springs –  Jim Sep 26 at 14:29
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My favourite kind –  Jim Sep 26 at 14:40

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