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Conjunction Junction

Or, Why the World Doesn’t Come to an End When Planets Align

The conjunction of Venus and Jupiter, as visualized by the Stellarium home planetarium software. (Click on the image to get a much larger and clearer version. For more about Stellarium, see the note at the bottom of this post.)

For many nights now, if you look west after sundown, you may have spotted two very bright objects in the sky fairly close to each other: the planets Venus and Jupiter, the brightest objects in the night sky other than the Moon. This event is known as a conjunction: just as conjunctions in grammar bring together two phrases in a sentence, astronomical conjunctions bring together two unconnected objects in the sky.

Note my use of the word “unconnected”: while obviously Venus and Jupiter both belong to our Solar System, orbiting the Sun just as Earth does, they aren’t very close to each other. Venus’ average distance to the Sun is 0.7 AU, while Jupiter’s orbit averages 5.2 AU in size. Since one astronomical unit (1 AU) is the distance from Earth to the Sun, Jupiter is over 5 times as far from the Sun as we are. Venus is never as close to Jupiter as it is to Earth, even when Venus and Earth are on the opposite sides of the Sun. To make a long story short, conjunctions are illusory, created by our perspective.

The sky looks like a dome to us: every star, every planet, every galaxy appears somewhere on this dome, but there’s a bit of missing information. We have no immediate way to reconstruct the third dimension: how far away a planet or star is from us. How bright an object appears is a good first guess—after all, if it seems bright, it’s likely to be close to Earth—but that’s not always reliable. For example, the closest star to our Solar System is Proxima Centauri, which is too faint to be seen without a telescope. Similarly, Jupiter always appears brighter than Mars, even though Mars is often closer to Earth; this is because Jupiter is far larger and has a much higher albedo due to ice crystals in its clouds.

Thankfully, measuring distances in the Solar System is relatively straightforward compared to measuring them for stars and galaxies. Over the centuries since Copernicus and Kepler, our data about planet orbits can be considered extremely reliable: we know where they are at any moment. The image below shows the relative positions of Venus, Earth, Mars, and Jupiter as of yesterday (March 15).

"Top view" of our Solar System as of 8:30 on March 15, 2012. The planets all orbit in the counterclockwise direction from this perspective; Earth also rotates on its axis in the counterclockwise direction, so Venus and Jupiter appear to set in the West a few hours after the Sun does. Mars rises in the East a little before sunset, so it's very visible throughout the night.

For a conjunction to occur, Venus and Jupiter have to be on the same side of the Sun. For a conjunction to be visible to us on Earth, this must happen when both are on the same side of the Sun as we are, or else the conjunction will be lost in the Sun’s light. Since Venus is closer to the Sun than we are, we only see it either soon before sunrise or soon after sunset. That limits the number of conjunctions we see, even though alignments happen more frequently. The next visible conjunction of Venus and Jupiter is in 2014, for example.

How conjunctions happen: even though Venus and Jupiter are far apart, our line of sight connects them.

All of this is to say that alignments are not portentous. In the old geocentric view (which still shows up in astrology), the Earth is the center of the universe, so conjunctions were seen as particularly important, but as the figure to the right shows, they are simply a matter of perspective. Jupiter’s gravity influences Earth’s orbit slightly, and Venus tugs on us slightly less…but that’s it! No other force of nature allows either planet to play a significant role in Earthly affairs. To take this further, the idea that the world will end this year due to some mystic alignment (supposedly predicted centuries ago by Mayan astrologers) is even more absurd: conjunctions have happened quite often in the Earth’s 4.6 billion year history, and yet Earth still endures.

Conjunctions are beautiful events, and when they result in eclipses can even provide information about atmospheres and sizes of the bodies involved. As a scientist and lover of nature, that’s good enough for me.

Appendix: Stellarium

To create some of the images in this post, I used the free program Stellarium, which is available for Mac, Windows, and Linux. (I run version 0.11.2 on both my Ubuntu Linux desktop machine and my Mac laptop.) It’s very versatile: you can use it for viewing star charts, locating planets, and suchlike, but you can also create whole planetarium programs with it, as I have done in the past. For today’s post, I set Stellarium to show the sky at my location (Richmond, Virginia) at 8:30 PM on March 15 for the first image. For the second, I switched to “Solar System Observer” view, which lets you see the entire Solar System from a vantage point somewhere above the Sun’s north pole.

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