Venus and all that

The year 2000 was a leap year. If you ask anybody, you will most likely be told that it’s because it’s divisible by four, and years divisible by four are leap years. Case closed. But it’s wrong. Years divisible by 100 are not leap years, so the year 2000 should not have been. But years divisible by 400 are. So you can be right although you are wrong.

Similarly, when you ask somebody why Venus is hotter than the Earth, you will most likely be told that’s because it is closer to the sun. But it’s wrong (well, not completely).

Venus has a very high albedo (0.65) compared to that of the Earth (0.3), because it is covered with white clouds. A simple formula which gives the black body temperature for the distance (0.72AE) gives the temperature T = 252.6K (-20.6C). For the Earth, we get T = 254.9K (-18.3C). So, Venus should actually be slightly cooler than the Earth, even though it is closer to the sun. Of course, both values are false, because the black body temperature is only comparable to the real surface temperature for planets (or moons) without any atmosphere.

The average temperature on Venus is 461.85 °C, and ~15 °C on Earth. Both effects are explained by the greenhouse effect. Which is a good thing on Earth, not so much on Venus (if we want to move there). That doesn’t mean we should increase it on earth.

And you know what? Mercury is closer to the sun than Venus, and yet it is cooler.

So, why am I writing this? Because Tim Blair does not understand it. That’s why.


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