Why isn't the planet nearest the Sun the hottest?
离太阳最近的行星，为什么不是最热的？
如果给太阳系的行星排一个“最热排行榜”，你会把第一名留给谁？
If you made a hottest-planet ranking for the Solar System, which planet would you put first?
离太阳最近的水星，看起来是个很合理的答案。
Mercury, the planet nearest the Sun, seems like a very reasonable answer.
可是，比较行星表面的温度，第一名其实是金星。
But when we compare planetary surface temperatures, Venus actually comes first.
从太阳往外看，先是水星，再是金星。
Looking outward from the Sun, Mercury comes first, then Venus.
金星虽然离太阳更远，表面却比水星更热。
Although Venus is farther from the Sun, its surface is hotter than Mercury's.
这个答案提醒我们：距离并不是决定温度的唯一条件。
This answer reminds us that distance is not the only factor determining temperature.
还有一个重要条件，藏在行星周围的气体里。
Another important factor lies in the gases surrounding a planet.
这层气体叫“大气层”；金星有很厚的大气层。
This layer of gases is called an atmosphere; Venus has a very thick one.
其中大部分是二氧化碳，一种能帮助大气留住热量的气体。
Most of it is carbon dioxide, a gas that can help the atmosphere retain heat.
在金星上，大气留住了很多热量，让表面非常热。
On Venus, the atmosphere retains a great deal of heat, making the surface extremely hot.
这种保温作用与“温室效应”有关。
This heat-retaining effect is related to the greenhouse effect.
所以，比较行星表面的温度时，除了距离，还要了解它的大气层。
So when comparing planetary surface temperatures, we need to understand a planet's atmosphere as well as its distance from the Sun.
水星赢了“最近”，金星却赢了“表面最热”；两个问题，答案并不一样。
Mercury wins for “nearest”, but Venus wins for “hottest surface”; the two questions have different answers.
