英语时差：让地球转起来（上）

We all know that humans have quite a knack for changing the natural environment. In spite of this, you probably think some things are simply too fundamental for human activity to affect. For instance, most people would imagine that the Earth’s twenty-four hour rotation would be immune to our tampering. As it turns out, this isn’t the case. We’ve sped our planet up. How fast anything spins is partly determined by how its mass is distributed. The closer an object’s mass is to its axis of rotation, the faster it spins. To picture this, imagine an ice skater doing a pirouette. When her arms are extended, she spins slowly and gracefully. But when she draws her arms inward, she spins much faster. It took more energy to start spinning with her arms extended, so as she draws them in, that extra energy is converted into faster spinning.