Tonight – March 9, 2018 – Jupiter is poised, or stationary, in front of the constellation Libra the Scales. That’s a signal that the king planet is beginning its retrograde motion in front of the stars. And that means the best time for watching Jupiter is approaching. Normally, the planets move toward the east in front of the stars. But, at times during Earth’s year, we’ll see the planets change direction and begin moving toward the west!
You can see Jupiter easily now, if you’re up late at night, or if you’re an early riser. As seen from around the world, Jupiter rises in the east late and can be found high in the sky at dawn. It’s bright – the second-brightest planet – and this month, when it’s up, it’s brightest starlike object in the sky (since Venus sets soon after sunset). The bright star right next to it is Spica in the constellation Virgo the Maiden. Click here for recommended almanacs; they can give you Jupiter’s rising time in your location.
Jupiter (and the constellation Libra) will be rising earlier and earlier by the day, and will be well-placed for evening viewing in another month or so. In two more months – on May 9, 2018 – Jupiter will be at opposition, and out all night long, from dusk until dawn.
What is retrograde motion? 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 (108,000 km) an hour, while Jupiter moves at less than half that speed, or about 29,000 miles (47,000 km) an hour.
Because of its faster speed and smaller orbit, our Earth laps Jupiter about once every 13 months. In this sense, Earth is 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). The apparent 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.
Earth and Jupiter are planets on the great racetrack of the solar system. Over the next several months, from our earthly vantage point, Jupiter will appear to drift slowly westward in front of the stars.
Why? You guessed it … because we’ll soon pass between Jupiter and the sun.
In 2019, Jupiter’s opposition comes on May 9. 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 May 2018, soaring to its highest point in the sky at midnight and setting in the west at sunrise.
Bottom line: The best time to look for Jupiter is just ahead. Its retrograde motion begins March 9, 2018.