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Jupiter ends retrograde on July 10-11

Tonight – June 10-11, 2018 – the planet Jupiter pauses before resuming its usual eastward course in front of the background stars of the zodiac. In other words, Jupiter is stationary on July 11, at 4 UTC. At United States’ time zones, Jupiter reaches its stationary point at 12 midnight EDT, but on July 10 at 11 p.m. CDT, 10 p.m. MDT, 9 p.m. PDT, 8 p.m. Alaskan Time and 6 p.m. Hawaiian time.

The retrograde (westward) motion ends in front of the constellation Libra the Scales. This retrograde, or westward, motion for Jupiter also began in front of the constellation Libra on March 9, 2018. Jupiter was to the east of Libra’s alpha star, Zubenelgenubi, as the retrograde started, and now lies to the west of Zubenelgenubi as the retrograde comes to an end.

As dusk gives way to darkness, you may see the star Zubenelgenubi near Jupiter. Binoculars reveal this star to be a double star – two stars in one.

What does it mean? Only that Earth passed between Jupiter and the sun earlier this year, on May 9. That event – called an opposition of Jupiter by astronomers – marked the middle of the best time of year to see Jupiter, since the planet was closest to us and brightest in our sky around the time of its May 9 opposition.

The end of retrograde motion means the best months for seeing Jupiter are over.

And yet you might not believe it if you gaze at Jupiter tonight. Jupiter is still incredibly bright, brighter than any star in the evening sky.

Are you in the Northern Hemisphere? Then look for this brilliant beauty of a planet high in south to southwest sky as soon as darkness falls. As seen from mid-latitudes in the Southern Hemisphere, Jupiter appears high overhead or high in the northern sky at early evening.

Jupiter and its moons as seen through a telescope on August 15, 2009. Click here for the present position of Jupiter’s four major moons.

Far brighter than any star, this blazing world is even visible from a light-polluted city. You might even see the modesty bright star Zubenelgenubi close to Jupiter.

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Illustration showing why a superior planet moves with a retrograde direction – apparently backwards from its usual motion, that is, westward instead of eastward – for a portion of each year. As seen from the north side of the solar system, all the planets circle the sun in a counterclockwise direction. Image via Wikimedia Commons

Jupiter is the fifth planet outward from the sun, while Earth is the third planet outward. In the language of astronomers, Jupiter is a superior planet. In their outward order from the sun, the superior (exterior) planets are Mars, Jupiter, Saturn, Uranus and Neptune.

Like all the planets, Jupiter always goes eastward in its orbit, or counterclockwise as viewed from north of the solar system.

However, as seen from Earth, all superior planets spend a portion of each year moving westward in front of backdrop stars. The illustration above explains why.

When the Earth in its smaller and swifter orbit swings by any superior planet, that planet appears to be going backward in its orbit (relative to the backdrop stars of the zodiac) for several months. It’s comparable to passing a car on the highway, with that car appearing to be going backward relative to distant background. Of course, you know that car isn’t really going in reverse. Neither is Jupiter, during its time of retrograde motion.

Starting on July 11, Jupiter will be moving eastward along the ecliptic again – going toward Zubenelgenubi, the alpha star in the constellation Libra. You probably won’t discern much – if any – movement of Jupiter in front of the stellar background for the next week or two. But keep watching as Jupiter moves toward Zubenelgenubi, to pass just north of this star mid-August 2018.

Bottom line: Jupiter ends retrograde motion on July 11, 2018. This means the best time in 2018 to see Jupiter is now past … but you won’t believe it if you gaze at the planet tonight!

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Bruce McClure