The Great Twilight Planet Show rolls into town

Venus (lower planet) and Jupiter vie for attention in the western sky yesterday during twilight. Photo: Bob King

Have you been watching the light show in the western sky these past few weeks? Jupiter and Venus has been ever-so-slowly drawing closer together during evening twilight. Like two lovers in a lengthy courtship, these two planets were clearly meant for each other. They’ll prove it come March 12-14 when they’re closest together for the year at just three degrees apart. The time of closest approach occurs on the night of the 13th. Watch the show unfold from your front yard or from downtown Chicago – these two luminaries are bright enough for anyone to see.

Watch Venus and Jupiter move together and then glide past each other in the western sky at dusk over the next several weeks. Mercury joins the scene later this month. Created with Stellarium

There are two reasons the planets are approaching one another. Earth’s revolution around the sun causes the stars to drift westward with time. For example, we first see Orion in the morning sky in the east in late summer. By mid-winter, it’s straight up south during the early evening, and when April rolls around the Hunter soon disappears below the western horizon. The same westward slide applies to all the outer planets from Mars to Neptune. That’s why Jupiter has been inexorably moving westward in Venus’ direction. It was only a matter of time when they’d meet.

Venus will move farther to the left of the sun all the way through March 27, the date of its greatest elongation east as seen by an observer on Earth. After that date, it will appear to change direction and head back to the west (right) toward the sun. At the same time Venus' phase changes because the viewing geometry changes. Illustration: Bob King

The inner planets Venus and Mercury are different because they orbit between the sun and Earth. Take a look at our illustration above and you’ll see that Venus moves to the left or east of the sun when it’s in the evening sky, defying the western drift. For observers on Earth, the planet appears to move up and away from the sun until reaching its greatest separation on March 27. With Jupiter headed west and Venus east, the two planets are on a collision course. OK, not really. It only looks that way. Jupiter is several hundred million miles in the background “behind” Venus.

After greatest elongation, Venus will appear to move closer to the sun, reversing its direction of travel from our perspective, and dropping down to the western horizon late this coming spring. Notice that the planet’s phase changes throughout. Today it’s a “gibbous moon”, in late March a half moon and by May a crescent. The phases are caused by the ever-changing viewing geometry between Earth and Venus as each orbits the sun.

One great thing about the planets is how easy they are to see. Just a casual look now and again will keep you in touch with who’s doing what in the evening sky.

7 thoughts on “The Great Twilight Planet Show rolls into town

  1. I saw the huge lights in the sky and had to double check that they weren’t the lights of airplanes. Wow. I am really impressed. First time I’ve ever seen this, I think. I Googled it and found you. Thanks for sharing.

  2. I have been watching, but wasn’t sure why they were getting closer. Thanks for the post!

    On a semi-related topic, while out last night, I noticed Mars starting to come up in the eastern sky. Any chance you know of a site that shows the orientation of Mars (in terms of the tilt?). I’m trying to help figure out what I am seeing.

    Thanks.

  3. Bob,
    It looks like Venus is on it’s way “up” into the sky, while Jupiter is on it’s way “down” to the horizon. After Venus reaches it’s maximum height, and starts falling, will it pass Jupiter again? Or will the two of them fall at the same rate?

    • Great question RC. No, Jupiter will be get to the sun sooner and they won’t meet again in the evening this year. It reaches conjunction on May 13 when Venus will still be shining in the west.

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