Earth Spins Faster

Earth Spins Faster

This summer the Earth has spun (旋转) slightly faster on several days, for a number of reasons.

The length of a day is well known—it's 24 hours. That's the amount of time it takes the Earth to spin around completely. And 24 hours times 60 gives us 1,440minutes a day. With 60 seconds in a minute, we have 86,400 seconds in a day.

On average, that's about right. It takes the Earth about 86,400 seconds to spin around once. But that's an average. In fact, sometimes the Earth spins a little slower. And sometimes, like recently, it spins a little faster.

For most of human history, the spinning of the Earth has been used to measure time. But in the 1950s, scientists began using“atomic (原子的) clocks”to measure time. These clocks are based on the speed at which an atom vibrates. Atomic clocks are a very accurate and exact way to tell time.

Scientists had long known that the Earth didn't always turn at the same speed. But the changes are small. With the invention of atomic clocks, scientists were able to measure exactly how fast the Earth was spinning,which allows them to know when there are“fast”days or“slow”days.

▲ For example, last July 5 was the fastest day ever recorded. It took about 1.65 milliseconds less than 86,400 seconds for the Earth to make a complete turn. Several fast days are happening this summer. On July 9, the Earth turned 1.36 milliseconds faster than normal.

There are many things that affect the speed of the Earth's turning. The most important is the pull of gravity(重力) from the moon. As the moon orbits the Earth,sometimes it's closer and sometimes it's farther away.And the moon's orbit itself doesn't stay in one position—it moves in space.

Other things, such as the Earth's atmosphere and major wind systems, can also affect the Earth's spin. It may be surprising, but the Earth's core, deep inside our planet, can also affect its speed.

Despite our planet spinning slightly faster recently,in general, over millions of years, the Earth has actually been spinning more slowly. It typically loses a couple of milliseconds every hundred years or so. Scientists say that pattern is likely to continue.