Earth with no axial tilt question

[Guardsman Bass]

I've been thinking about what Earth would be like with no axial tilt. The obvious stuff is that the day lengths and sun inclination in the sky would never change - they'd be perpetually at what they are at the Equinoxes. The equator would always have 12 hour days with the sun directly over head, a place at 45 degrees latitude north would have the Sun only reaching 45 degrees in the sky every day, and the poles would never have direct sunlight (just perpetual twilight during the day).

But how would ocean and atmospheric currents matter in such a world? It seems like the constant, intense sunlight in the equatorial regions would drive strong currents of air and oceanic water northward and southward with heat, raising the temperature in more northerly and southerly areas above what they would otherwise be based off their sunlight. But the variation between latitudes would individually be quite small - areas 5 degrees north or south of the Equator would be slightly cooler than the Equator, and so on and so forth. I would think that would dampen heat transfer on the planet, although I'm not certain on that.

Does anyone else have ideas on whether that would work? Because otherwise, you end up with heavily banded life habitats, and eventually get to areas far enough northward and southward that the daily temperatures are constantly below freezing, making land plant life impossible aside from maybe lichen.

[B5B7]

The Sun is not a point source of light and heat - it's a huge ball - think about it.
Not that even that would make any significant difference.
With no tilt, everywhere on Earth surface receives same amount of light and heat - however the poles would still be cold due to the effect of heat hitting a more sloped surface - as is current situation. The only other difference due to day and night (and possible differences due to oceans, and mountains, and of course altitude). Also no seasons. This last is the only major difference to current status.

[Purple]

Why no seasons? I thought those were due to the earth getting closer to and farther from the sun on its elliptical path. Would the lack of axial tilt make that path circular instead?

[Terralthra]

Why no seasons? I thought those were due to the earth getting closer to and farther from the sun on its elliptical path. Would the lack of axial tilt make that path circular instead?

No. The seasons are due to axial tilt. If it were the slight elliptical path of the orbit, the southern and northern hemispheres would have the same seasons, not opposed.

[Raw Shark]

Fun Fact: We're actually the farthest from the sun that we ever get right about now, in the middle of July.

[Guardsman Bass]

I've been thinking more about this, and it gets rather weird. On the one hand, the poles basically never get direct sunlight - just twilight/dawn/dusk conditions from refraction. That would tend to make them cold, but at the same time they'd never have weeks and months of night time. They'd also probably be a lot drier, since most precipitation in the Arctic falls in the summer IIRC. Cold air doesn't hold moisture well.

It would basically come down to how warm the planet is overall from greenhouse gases. If they're strong, then you could get warmer conditions all over the globe, and the main limitations would be distance from bodies of water and available light for photosynthesis. But if the planet overall is colder, then you could have ice sheets forming in areas where the daily temperatures never get above freezing, reflecting sunlight and cooling the planet further until you get stuck in an ice age that can only be resolved with more greenhouse gases or increasing solar luminosity.

There might be a lot more deserts, as well. Little seasonal variations means you're not going to get seasonal changes in rainfall patterns - the colder or more in-land you are from large bodies of water, the drier you'd be. The interiors of the big continents might be all deserts, even drier than they are now.

[Terralthra]

There are other factors causing long-term climactic shifts. Are you cutting out variances in Terra's orbital eccentricity and precession as well as axial tilt? See here for more information.

[Guardsman Bass]

I'm only eliminating axial tilt and axial precession, so the other forms would be still in effect. Seasons wouldn't disappear completely, but they'd be much weaker.

[Purple]

What if having no tilt the tilt was variable? Like say we have the angle we are in now, this time in ten years there is no tilt and ten years from that we are back to the tilt we have now?

[Broomstick]

That would be worse - the effects on agriculture alone would be deviating. Except, of course, that if the Earth had always been like that life presumably would have come up with some sort of adaption for wildly fluctuating types of seasons.

[Lord Revan]

the funny thing is that the axial tilt is variable, it's just that rate of change is really slow.

[SpottedKitty]

the funny thing is that the axial tilt is variable, it's just that rate of change is really slow.

Something like a 26,000 year period, isn't it? And there's more than one pattern involved, I remember reading about two or three curves superimposing to give us the slowly varying tilt angle we have today.

[Lord Revan]

the funny thing is that the axial tilt is variable, it's just that rate of change is really slow.

Something like a 26,000 year period, isn't it? And there's more than one pattern involved, I remember reading about two or three curves superimposing to give us the slowly varying tilt angle we have today.

I don't have the numbers at hand atm but something like that, certainly alot longer then any person would live.

[SpottedKitty]

<nod> It isn't really noticeable until you look closely at the astronomical data of the Egyptians and Babylonians — the North Pole has shifted noticeably in the meantime, and the precessing equinoxes mean the zodiac is actually one constellation out of step. I think the medieval astrologers must have basically said "fugettaboutit"...

[Simon_Jester]

The precession of the equinoxes changes the direction the Earth's axis of rotation points (relative to the axis of the Earth's orbit around the Sun).

It does not change the angle of the axis.

However, the Earth also experiences a bit of nutation, which DOES change the angle of the axial tilt.

As Broomstick says, significant changes in the Earth's axial tilt would have very dramatic effects on climate. Seasons would become much more intense when the axial tilt was high, and much less intense when it was low. Patterns of rainfall, wind, and temperature would become almost entirely impossible to predict. Terrain would be soaked with water one year (and probably erode massively), and dried out a few years later.

The only animal species that could survive would be those which are adapted to a wide range of temperatures at all phases of their life cycle- it'd be impossible to be confident of having particularly wet or dry or hot or cold conditions during a spawning season or anything. Animals that cope with unfavorable climate conditions by hibernating would have to get used to varying cycles- at some times, winters are mild and hibernating for three months puts you at a disadvantage, while at other times such a hibernation behavior isn't enough and you wake up three months into a bitterly cold five-month winter.

Plants would have it even worse.

[SpottedKitty]

Ah, thanks, I always get those two effects mixed up.

[Guardsman Bass]

I've heard that proposed as one of the potential filters for complex life - lack of a relatively limited range of axial precession. Even just having a planet at 90 degrees tilt would be incredibly rough on life unless the atmosphere was so thick that it much more heavily redistributed the heat, or most of the planet is oceanic in surface coverage.

I'm most curious as to how the "temperate zones" would fare on such a world. The equator would have two winters and two summers IIRC, and the poles would get the full "six months of equatorial heat in summer followed by six months of brutal arctic cold", but what about the regions in between? It seems like you'd get a range, with all places getting at least one day of direct-overhead 24 hour sunlight and one day of no-sunrise 24 hours of night. Or would they get the same light issues as the poles?