All the planets of the solar system orbit around the Sun in the same direction. From our vantage point on the Earth, this makes them appear to march across the sky from season to season in the same direction. While you can’ t discern this movement with your eyes in a single night, you can capture it if you pay close enough attention: one night the planet will be around a certain group of stars, and many months later it will be around a new clump of stellar friends.太阳系悉数行星皆以同向绕日而行。自我们于地球上所处的有利位置定神细视，行星季复一季，在苍穹之上，朝着同一方向迈进。虽然你无法在一夜之间用肉眼去辨别这种运动，但倘若你目不转睛，便可捕捉其运行规律：某个夜晚，行星会围绕某一群恒星而行。而数月之后，该行星又会绕着一群新的恒星结伴而行。

But sometimes the planets appear to retrace their steps and move backward over the course of a few days or months, a phenomenon known as retrograde motion. If all the planets are moving in the same direction, then what gives?

但有时，这些行星仿佛会在数天或数月的时光中，收回行进的步伐，并沿着其运行轨迹后退，这种现象被称为“逆行运动”。如若所有行星都朝着同一方向运行，那缘由为何？The appearance of retrograde motion is due to our vantage point on the Earth. Just like all the other planets, we are also orbiting around the Sun at our own speed. From our point of view, sometimes we “catch up” to another planet in its own orbit, making it appear as if it’s moving backward relative to us.

逆行运动的出现与我们在地球上所处的有利观测位置息息相关。与其他行星别无二致，我们人类所在的地球也在以自身特定的速度绕日而行。自我们所在的角度观察，有时，我们能在另一颗行星的运行轨道“追上”它。所以相对于我们所在的位置，该行星好像是在向后退。Think of runners on a racetrack. From the point of view of the stands, everybody’ s running in the same direction. But from the point of view of one of the runners, their fellow racers may be moving away from them or coming closer to them, depending on their speeds and where on the track they happen to be.想想那些在跑道上赛跑的选手们。从看台上望去，所有的选手无一不是朝着同一方向而冲刺的。但从其中一名选手的视角来看，其对手与其本人之间的距离可能忽远忽近。这取决于他们的奔跑速度与各自在赛道上碰巧所处的位置。You may have recently seen some articles talking about how six planets are currently in simultaneous retrograde. What does this mean for you, personally?最近，你可能品读了一些文章，文中对六颗行星当前是如何同时处于逆行的状态而议论纷纷。但这于个人而言，又意义何在？Nothing. It means nothing at all. One planet moving in retrograde doesn’t mean anything either–it’s literally just a planet moving in its orbit, doing the same thing that it’ s been doing for the past four and a half billion years. Multiple planets moving in retrograde is just a random coincidence. If those runners on the racetrack were going at it for eons, circling around and around without end, these kinds of coincidences would be bound to happen.

这无关紧要，毫无意义。一颗正处于逆行状态的行星本就意义全无———而不过只是一颗行星在其轨道上运行罢了！日复一日，与它在45亿年来的所作所为分毫不差。因此多颗行星同时逆行也只是一个充满随机性的巧合。假如跑道上的选手没有可供跨越的终点，一圈又一圈地绕场而跑，无休无止；那么这种巧合定然在所难免。If anyone had asked an astronomer from centuries ago, they would have been able to predict with extreme precision when this simultaneous retrograde would occur. In fact, you can fire up any free astronomy software and do the same thing yourself, finding out exactly when the next one will happen, and the next, and the next, for millennia to come.

若向数百年前的天文学家发问，他们定当精确无误地预测出多颗行星同时逆行的时间。实际上，你完全可以打开任一免费的天文学软件，以自行解决该问题，并丝毫不差地查明下一次多行星逆行的确切时间，甚至是再下一次以及再下一次的下一次……在未来的几千年内，上演多行星逆行的时日，你都能不差分毫地予以预测。