Question:

“I’m interested in the stars and planets. One thing I always wondered about is if the planets in the solar system ever totally align with each other, so I mean not just a few planets but ALL of them. If this happens, how often does it occur? And is there a name for it? Thank you.”

Note:

This question comes up every once in a while. Partially it’s due to random doomsayers, and partially it’s due to Lara Croft: Tomb Raider, in which there is a planetary alignment causing widespread … actually, I don’t know what it causes, I haven’t seen the movie.

Answer to “Does this happen? How often?”:

This is a difficult question to answer. First off, as an astronomer I have to point out that the planets aren’t all in exactly the same plane. If one of these alignments occured, when you looked down on the solar system from above you would see the planets in a line, if you looked from the side at the same time, some would be a little higher, and some would be a little lower.

Everything I’m reading is pointing to the fact that even that sort-of alignment probably won’t happen. On May 5, 2000, the Earth, Sun, Mercury, Venus, Mars, Jupiter, and Saturn were close to a line, but it wasn’t perfect. Check out this simulator, you’ll have to type in 2000-05-05 in the date field.

Planetary Confluence from wonderquest.com

Phil Plait over at Bad Astronomy says this: “As it turns out, planetary alignments are fairly rare. Getting more than three lined up is difficult; getting them all lined up is rare indeed. But 5 billion years [the length of time the solar system has been around] is a long time! Alignments may be rare, but given enough time they do occur, and the Earth is still here. Even more, what most doomsayers say is an alignment is really more of a confluence, or loose gathering, of planets.”

You’d think that with the planets going around the Sun for 5-10 billion years we’d get a perfect alignment at least once, but it looks like the math just doesn’t work out.

Answer to “What is it called?”:

The article from almanac.com calls this a “great conjunction,” which is a pretty good word. Conjunction just means that from our point of view the planets end up close to each other in the sky.

Phil Plait would call these things “confluences,” since they’re not a perfect line.

The other word I can think of to describe this is “syzygy” which refers to the alignment of at least 3 celestial bodies. It doesn’t really apply though to the 5/5/2000 conjunction, because the planets aren’t in a straight line. So, save up the word and use it next time you play Scrabble.

Update 2/22/2009:

Amazingly, this is one of my most popular posts of all time. I’ve also gotten several questions recently about the “upcoming” planetary alignment. Hmm. I don’t know exactly which one y’all are interested in, but a quick web search turned up March 26th, 2009 as something the astroLOGERS seem to be interested in. Here’s what that alignment looks like according to the simulator I linked to above:

Planetary Alignment March 26th, 2009

In case you can’t tell – Earth is an icon underlying Venus in this picture. Doesn’t mean the Earth is under Venus – it means the icons in this picture are too big. Looks like the big deal is the Earth, Venus, Sun, Mercury “line.” Good luck all – you’re not going to be able to see a thing for this one – the blasted Sun is in the way! :)

Where’d I Get My Info?

Planetary Orbit Simulator
Tomb Raider’s Science is Wrong
Phil Plait’s Article
Nothing Special Will Happen on 5/5/2000
Syzygy

Update: 6/26/2009

P.S. Here is the “alignment” of the planets on 12/12/2012:

On 12/12/2012 the planets will be in this position.

Hmm.

Here’s what that looks like in Stellarium – at noon on 12/12/2012

Don’t forget – that line is the ecliptic

That looks pretty lined up – but don’t forget – the line those planets (and the Sun and Moon) are all aligned on? It’s the ecliptic. The Sun, Moon, and planets are all ALWAYS lined up along that line somewhere. Always. Even right now. Go turn on your Stellarium, turn on the ecliptic, and set it for today, right now. You’ll find every planet on that line. And the world isn’t ending. Hmm. And you won’t see it – this is during the day.

Update: 7/7/2009

Charity asked about the conjunction on 8/8/1423. Here’s what it looks like:

August 8, 1423 8pm – from Stellarium

August 8, 1423 – Planetary Orbit Simulator

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