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Planetary alignments: They’re the favorite astronomical scenario of kooks, con artists, and Hollywood producers everywhere. But has it ever happened? And what would it do to Earth if it did?
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Sources:
http://helios.gsfc.nasa.gov/qa_plan.html
http://www.universetoday.com/34076/planetary-alignment/
http://www.nasa.gov/topics/earth/features/2012.html#planets
http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/961207a.html
http://science.nasa.gov/science-news/science-at-nasa/2000/ast30mar_1m/
http://helios.gsfc.nasa.gov/qa_plan.html#flip
http://curious.astro.cornell.edu/question.php?number=203
http://www.badastronomy.com/bad/misc/planets.html
http://content.time.com/time/health/article/0,8599,2070503,00.html
http://www.fourmilab.ch/cgi-bin/Solar
It's one of the most beloved astronomical scenarios of cooks, con artists, and Hollywood producers.  A planetary alignment!  When the planets line up with the sun to form a sort of celestial Newton's Cradle and the laws of physics presumably use the solar system as a little play thing.

You've probably seen this scenario in a movie or read one of the conspiracy theories online.  The gravitation of the aligned planets would create a catastrophic force causing earthquakes and tsunamis on Earth, throwing other planets out of their orbits and probably other bad stuff!

Well, obviously this Mayan apocalypse version of events has never come to pass.  But there are conflicting opinions about whether a true planetary alignment has ever happened.  Some scientists think it just might not be possible, at least not precisely, because the planets don't travel on the same exact plane.

It's true that most of them travel at roughly the same level at they orbit the sun, an area known as the Ecliptic, but Mercury dips a little above and below that line and Pluto is not a planet, but if we were making a sci-fi adventure movie in the '80s and we wanted to cast Pluto, its orbit is so rogue and weird it'd never fall in line.

Other astronomers though argue that in the 4.5 billion years that the planets have been around, they've probably fallen into alignment at least a couple of times.  The thing is, even though we have a pretty good handle on the planets' orbits, there doesn't seem to be a single agreed upon calculation about how often all eight of them will line up closely in three dimensions.

To get a sense of how hard it is to get these eight little ducks to fall in a row, it's kind of a big deal when the planets are simply all on the same side of the sun.  That happened in March of 1982, for example, and a few hucksters managed to sell copies of their book by claiming a so-called "Jupiter effect" caused all sorts of catastrophe on Earth, especially to poor California.

Now I wasn't born yet but Hank was and he's fine, so, no Jupiter effect in 1982.

In May of 2000, meanwhile, Mercury, Venus, Jupiter, Mars, and Saturn all happened to fall into the same place of the solar system about 25 degrees across.  So we couldn't see it because they were all on the other side of the sun.

Even if all the planets were to line up with the sun in true Hollywood fashion, nothing would happen to Earth.  Space is big and the planets are small and really far apart.  The vast majority of the gravitational effects felt by Earth is from the moon and the sun, everything is negligible.

We can look at it this way.  The sun exerts less than half of the tidal force on Earth that the moon does, and the next biggest object in the solar system, Jupiter, exerts only 0.001% of the force on us that the sun does.

In 2000 NASA actually did the math and calculated that even if all the planets and moons in the solar system were lined up precisely with Earth, it would exert enough extra gravitation to raise the ocean tides by 1/25 of 1mm.  Which I think could be an interesting movie but definitely more a Terrence Malick than a Michael Bay.

But you know what would be an amazing movie with at least some appeal?!  Syzygy, when three or more celestial bodies align.  This is a lot more common because it involves fewer moving parts.  So it's easier for us to predict and understand.

Every time we can see a solar or lunar eclipse from Earth, that's a type of syzygy involving the sun, the moon, and our home planet.  Which is swell, but there are practical applications too!

In 1966 engineers of the jet propulsion laboratory discovered that Jupiter, Saturn, Uranus, and Neptune were aligned in syzygy 13 years later in 1979.

So NASA used that math to plot the course the Voyager I into spacecraft, allowing them to study all four planets as they passed by, giving us our best look yet at the outer solar system.

Not only did a disaster not happen, but something really, really awesome happened.  You guys would watch that movie, right?

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