Pictured above is a series of images digitally stacked so that all of the stars images coincide. Here, Mars appears to trace out a loop in the sky.

上面图是一系列数码照片叠加而成的，让恒星重合，而火星貌似在天空中画了一个环。

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The problem arose because astronomers had noticed that some planets in the sky seemed to stop, move backward and then go forward in a loop. Others appeared brighter at times and less bright at others.

这样，问题就来了，因为天文学家以前发现过，天上有些行星停顿下来，往后移动，然后再成环状向前移动，而其他行星看上去有时亮些，有时又不怎么亮。

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The problem arose because astronomers had noticed that some planets in the sky seemed to stop, move backward and then go forward in a loop.

这个问题也就产生了，因为天文学家已经发现有些行星在天空似乎停止，向后移动，然后在一个循环。

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