Sphere's surface grows as radius squared, but volume grows as radius cubed. Hence a small terrarium will quickly freeze, and a huge terrarium will eventually fry. There is an optimal size for a terrarium, given its orbit, that keeps its internal temperature within the habitable range.

Also it would need many more plants than animals. I would rather go with an aquarium.

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jjk166 6 hours ago | parent | next [-]

That's not how space or terrariums work. A terrarium does not spontaneously produce energy out of nothing, it gets energy from the sun. Heat input from the sun is proportional to cross sectional area, while heat loss to space is proportional to surface area, which scale the same for a sphere. A larger object will have more thermal mass which would make it take longer to change its temperature, but it will still have the same thermal equilibrium. Terrariums do not need to be spheres, so the volume does not necessarily scale as the radius cubed.

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hparadiz 6 hours ago | parent | prev | next [-]

I imagine one like that in my kitchen which is currently moss, a succulent, and some weed that happened to germenate. All three are alive after two years so far. The bottom is rocks and soil. There's a clear water cycle too as water evaporates and collects on the surface of the glass and then drips down.

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tbrownaw 8 hours ago | parent | prev | next [-]

What does volume have to do with energy balance?

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thfuran 7 hours ago | parent [-]

Heat is transferred through the surface area and produced by the volume (assuming there's something going on in the system that's exothermic).

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jjk166 6 hours ago | parent [-]

Heat isn't produced by the volume. Heat may be produced by something within the volume, but it's not the volume's existence that causes heat to be produced. There is no fundamental reason a bigger terrarium should produce more heat, nonetheless that heat production should be directly proportional to volume.

	▲	thfuran 5 hours ago \| parent [-]
		Yes, obviously it'd be the stuff in the terrarium rather than the space it occupies that produces heat, but the amount of stuff you can fit in it is determined by the occupiable space. And if that stuff is producing heat, such as by decay, there's going to be more heat with more stuff. Though even if it cooks itself for a while, it should eventually settle on a temperature determined mostly by orbital parameters and material properties rather than size, since the stuff can't be net exothermic forever. But greater atmospheric depth probably still increases equilibrium temperature by reducing heat transfer through that side of the terrarium.

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askvictor 7 hours ago | parent | prev [-]

Is there a 'just right' size that neither freezes nor fries?

	▲	thfuran 7 hours ago \| parent [-]
		About Earth sized, I think. A bit bigger if the soil is low on hot isotopes.