Tonight – December 9, 2014 – if you stay up at least until mid-evening, you can catch the dazzling planet Jupiter at a turning point in its year. On December 9, Jupiter is said to appear stationary in front of the constellation Leo the Lion. That means it begins its retrograde motion in front of the stars. And that means the best time for watching Jupiter is here! It’ll be easiest to spot in our sky over the coming months. Tonight – and for the next several nights – the moon is near Jupiter in the sky. Let the moon show you Jupiter tonight, then enjoy it for months to come.
In astronomy, the start of retrograde motion of an outer planet like Jupiter always means the planet is soon to be at its best in our sky. It happens when that planet seems to stop and then change its normal eastward direction of motion in front of the background stars.
Normally, Jupiter moves toward the east in front of the stars as seen from Earth. Tonight, the planet appears poised in front of the stars – moving neither east nor west. Astronomers call this called Jupiter’s stationary point. Afterwards, Jupiter will begin moving westward in front of the constellation Leo, as seen from our earthly vantage point.
What’s happening here? Has Jupiter really changed its direction of motion in orbit? No. What we’re seeing is an illusion, which baffled the early astronomers, but which now seems perfectly understandable.
Think of Jupiter in orbit around the sun. Jupiter may be a giant planet, but in contrast to Earth it lumbers along like an oxcart in the race around the sun. The Earth’s average speed is about 67,000 miles an hour, while Jupiter moves at less than half that speed, or about 29,000 miles an hour.
Because of its faster speed and shorter distance to go around its orbit, our Earth laps Jupiter about once every 13 months. It’s a lot like a fast race car in the inner track passing a slower car in the outer track.
The race car analogy works well to explain the phenomenon of retrograde motion. Suppose you are the driver of an inner, faster race car (Earth). As you move faster than a slower race car (Jupiter) on an outer track – say, just before passing it – you see the slower car appear to slow down even more as you watch it against the more distant landscape (perhaps a grandstand filled with onlookers). Just as you are passing the slower car, you see it appear to reverse its motion and appear to move backward against the grandstand. The slowing – and reversed motion – of the slower car is purely geometric illusion. The car doesn’t really slow down to let you pass; its driver still wants to win the race and is pushing forward as fast as it can. But you are moving faster still, and your faster motion as you pass the other car creates the illusion that it slows and even moves backward for a time. You’ve probably seen this happen on a highway many times, as you pass a slower car.
So Earth and Jupiter are planets on the great racetrack of the solar system. For the next several months Jupiter will appear to drift slowly westward from the bright Leo star Regulus. You guessed it … we’ll soon pass between Jupiter and the sun.
On February 6, 2015, Jupiter will be at opposition. At opposition, the Earth passes in between the sun and Jupiter, at which time Jupiter lies opposite the sun in Earth’s sky. If you could look down upon the solar system plane on that day, you’d see the sun, Earth and Jupiter making a straight line in space, with Earth sitting in between the sun and Jupiter. Because Jupiter is opposite the sun at opposition, Jupiter will be in the east at sunset in early February 2015, soaring to its highest point in the sky at midnight and setting in the west at sunrise.
In the months around opposition, Jupiter will be moving westward – in contrast to its normal eastward motion – in front of the stars. It ends its westward or retrograde motion on March 6, 2014. How can you notice its westward motion? You can notice it – over a period of several weeks – if you note Jupiter’s position relative to some nearby stars. Then after April 9, 2015, you can watch it shift eastward again.
Right now Jupiter is in the eastern sky by mid-evening. It’s rather high in your southern sky before dawn. On the up side, Jupiter is very easy to identify – just face east before bedtime tonight and look for the brightest object in the sky. Also, on the plus side, the King of the Planets is now passing close to the star Regulus, which ranks as a 1st-magnitude star. Watch how Jupiter moves relative to Regulus in the weeks and months ahead. Right now, Jupiter shines in front of the constellation Leo, to the west of the Leo star Regulus. Jupiter will cross over into the constellation Cancer by early February 2015, and at the end of Jupiter’s retrograde in early April 2015, you’ll see Jupiter fairly close to Cancer’s Beehive star cluster.
If you start tonight (or sometime soon), and plot Jupiter’s apparent position every week or so until early April 2015, you should notice Jupiter’s retrograde or westward motion in front of the backdrop stars.
Bottom line: The giant planet Jupiter begins retrograde, or westward, motion in front of the stars on December 9, 2014. Its stationary point, where it is poised motionless in front of the stars, happens on this date. Also, beginning around December 10, you will notice the moon near Jupiter in the sky. Let the moon show you Jupiter on this date!