If you stay up until mid-evening on November 7 – or get up before sunrise on November 8 – you can catch the dazzling planet Jupiter at a turning point in the year. Jupiter is said to appear stationary in front of the constellation Gemini the Twins, beginning on November 7, as it begins its retrograde motion in front of the stars. 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, in this case in early 2014. It happens when that planet seems to stop and then change its normal eastward direction of motion in front of the background stars.
But that’s not all you can look for tonight. Before Jupiter rises in the east on the evening of November 7, look first for the waxing crescent moon and the super brilliant planet Venus in the southwest sky at dusk and early evening. Can you believe Venus looks so bright? In fact, it’ll get brighter still before reaching its greatest brilliancy in early December.
So you’ll be looking for the moon and Venus shortly after sunset. Then, after the moon sets in the west this evening, look for Jupiter to climb over the east-northeast horizon shortly thereafter. It should be around mid-evening. By that, we mean about midway between sundown and midnight.
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 Gemini, 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 ordinary.
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 among the stars of the constellation Gemini the Twins. You guessed it … we’ll soon pass between Jupiter and the sun.
On January 5, 2014, 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 January 2014, soaring to its highest point in the sky at midnight and setting in the west at sunrise.
What’s more, the 2014 opposition of Jupiter will present Jupiter nearest to Earth until the opposition on July 14, 2020.
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 March 6, 2014, 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 through Gemini, which is a particularly prominent constellation. Watch how Jupiter moves relative the bright stars Castor and Pollux in the weeks and months ahead. Right now, Jupiter shines fairly close to the Gemini star Wasat. (Binoculars may help you to see this star by Jupiter.) At the end of Jupiter’s retrograde in early March 2014, you’ll see Jupiter close to the Gemini star Mebsuta.
If you start tonight (or sometime soon), and plot Jupiter’s apparent position every week or so until early March 2014, 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 November 7, 2013. Its stationary point, where it is poised motionless in front of the stars, happens on this date. Jupiter will move westward until March 6, 2014. Its apparent westward motion is caused by the fact that we will pass between Jupiter and the sun on January 5, 2014. At that time, Jupiter will be said to be at opposition to the sun. This will be Jupiter’s closest opposition until 2020.