Pick a time. Let's say, if I go a quarter of a circle, I expect to be moving to the left.
选一个时间,例如我走四分之一圈,我想向左走
and at that time Columbus Circle was very dirty.
那时候,哥伦布环岛这儿还很脏。
You've got to ask yourself, let the time it takes to do a full circle, that's the time period, do a full circle.
你已经问你自己了,留足够的时间让它运动一整圈,那就是运动整个圆周的时间周期
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