Earth is zipping past the outer planets like a race car on the inside lane of the track. We raced past Jupiter last month, and we’ll take on Mars next month. And on Wednesday, we’ll pass Saturn, the faintest of the three worlds.
Saturn is low in the southeast as night falls, and looks like a bright star. Its luster is dimmed by the gibbous Moon, far to Saturn’s upper right. Even so, Saturn outshines all but a few other objects in the night sky, so it’s hard to miss.
Earth passes by these planets because it follows a smaller, faster orbit around the Sun. We’re the third planet out from the Sun, while Mars is fourth, Jupiter is fifth, and Saturn is sixth. As Earth catches up to these worlds, they line up opposite the Sun — an alignment known as opposition. The planets are closest to Earth about then, and they shine brightest for the year.
At the same time, the planets appear to move in reverse against the background of distant stars. It’s like passing a slower-moving car on the highway. When the car is way ahead of you, it appears to move forward against the background of buildings and trees. As you pass it, though, it appears to move backward against that same background for a while.
So Saturn and Jupiter are in the “retrograde” phase of their motion across the sky now — appearing to move in reverse as Earth speeds past them. And Mars will enter retrograde in a few days.
We’ll have more about Saturn’s opposition on Wednesday.