How come?

[ajtribick]

That the black-body temperature of Venus = 231K, which coincidentally happens to be the same as the black-body temperature of Mars, and is in fact lower than that of Earth?

Is that all due to albedo?

[Evil Dr Ganymede]

I make the BB temperature of Venus to to be 327K (orbital distance 0.723 AU, luminosity 1 Sol). The albedo should be 0.0 if you're calculating a blackbody temperature.

[granthutchison]

I think chaos syndrome is referring to the temperatures displayed by Celestia, which are based on blackbody emission calculations, with albedo factored in to adjust the absorbed radiation - this gives a better fit to real-world visible-surface temperatures than the simpler blackbody calculation.
Chaos syndrome, you're right. The current match between the two Celestia values is because of their differing albedos, and it's a transient coincidence because Mars is currently at its closest to the Sun. A planet's temperature varies as one over the square root of its distance from the Sun. With Mars currently at 1.382AU and Venus at 0.720AU, you'd expect Venus' blackbody temperature to be 1.385 times higher than Mars'.
But Venus absorbs only 23% of the radiation falling on it, whereas Mars absorbs 85%. The temperature varies in proportion to the fourth root of the absorbed radiation, which would make Venus 1.386 times cooler than Mars for the same irradiation levels. So the two effects are cancelling each other out, leaving the two bodies with same calculated temperature.

Grant

[Evil Dr Ganymede]

Well, yes. It's just that whenever anyone says 'blackbody temperature' I just assume that's based on the equation that only has luminosity and distance from the star as variables. But as soon as you factor in albedo and greenhouse effect then you're not looking at the 'pure' BB temperature.