Maybe you saw the moon, Saturn and Jupiter splayed across the southern sky last night. Tonight they’ll make an even tidier package. Watch for the three to gather into a compact triangle that will surely catch your eye. The waxing gibbous moon passes just 3° from Jupiter and 6° from Saturn. You might be able to squeeze the scene into the field of view of wide-field binoculars, but they’re best with no equipment at all.
Jupiter and Saturn circle the sun at their own individual pace, slowly moving eastward across the sky from one zodiac constellation to the next. Closer-in planets like Mars move more quickly compared to the distant planets. If you’ve paid close attention to Jupiter and Saturn — noting their movement relative to the background stars — you may have noticed that since late May they’ve been traveling west instead of east.
What’s going on here? Did the two giants suddenly stop and shift into reverse? I hope not. That would be a disaster. Instead, the faster-orbiting Earth recently lapped both planets. As we pass Jupiter and Saturn, the outer planets only appear to fall behind or move backwards for a time. Think of passing a car on the freeway. As you signal into the left lane and accelerate, the slower car appears to be moving backwards as you zoom by. Astronomers call this backwards travel retrograde motion. Every time Earth laps an outer planet that planet appears to slow down, move west and then loop back to the east as shown in the animated diagram.
Because Jupiter is closer to us than Saturn it moves faster and farther across the sky during both normal times and in retrograde. That’s why the separation between the two planets has been increasing in the past few weeks — Jupiter is outpacing Saturn and pulling ahead. The apparent distance between them will continue to increase until about August 29 when it reaches a maximum of 8.3° or nearly a fist. Later in September the two will slowly start to close again as they return to their normal eastward motion and faster Jupiter outpaces Saturn. That’s when the fun begins anew.
During the fall it will be exciting to watch the two planets draw closer and closer together until their Dec. 21 conjunction. On that date the two gas giants will be just 0.1° apart, so close they’ll almost merge into a single object as seen with the naked eye. Keep watch and you’ll see all this play out just outside your door.