What is a planetary alignment?

11 09 2008

So I got to thinking… planetary alignments seem to be misunderstood by a vast majority of the population.  It’s been a while since I wrote an actual original post about astronomy, so here’s some truth.

It is a common misconception that a planetary alignment is when three or more planets line up perfectly, either in line with the sun or not.  There are a few factors involved in calculating the extreme rarity of such an event, but the main one is the fact that planetary orbits around the sun aren’t exactly planar.  Here’s a diagram:

What you’re looking at is the view of the orbits edge-on.  You might not can tell from this image, but the only one truly edge-on is the Earth’s, because I set the view for that.  Everything else is in its proper place relative to an edge-on view of our path around the sun.  Kind of a mess, huh?  Let’s look a little closer…

Click on the image and get a better look.

Notice how the orbits of Jupiter and Venus aren’t even capable of lining up here.  You can also make out the orbits of the other visible planets and see the separations between them.  These orbits *do* line up in specific places, but those places are distinct points and it’s incredibly rare for those planets to actually line up three at a time.  In fact, right now I couldn’t tell you if that has ever happened since Man has walked the Earth.

To make this post current, here’s the way the sky looked in the west around sundown in October…

Seriously, click on the image and get a better look.

You’ll notice I had Starry Night put in some nice mountains in the foreground.  That’s to block the sun that is right behind the large peak.  :o)

Without any orbital references, these three planets look like they’re very close to converging.  They occupy a very small section of the sky and would, in fact, be considered in conjunction by the broad astronomical definition.  But if you look more closely at the paths they are following you see the truth:

This is the exact same graphic as the previous one with only the relevant stuff shown.  Notice that the orbits do cross each other and the possibility that two of them will line up from our perspective is actually just a matter of time, but all three do not line up at the same place from this vantage point.  Having alignments of more than two planets and the Earth is just not going to happen — it is statistically nil.  But there is a little more to this tale.

Scientists define ‘alignments’ and ‘conjunctions’ a little differently than everyday language.  We consider an alignment to be any time two or more planets get within 15 degrees of each other.  That’s a little more than the width of your fist at arm’s length.  This means that astronomers can talk about conjunctions all the time.  And that means they’re nothing special.  Now, if you can get 3 or more planets within that 15-degree space, it can look quite pretty.  And that’s pretty special.  Getting all five naked-eye planets within that region is really awesome.  And fairly rare.  It happened just a few years ago, but it won’t be happening again for many, many years.  (I quickly ran my planetarium software out for the next 50 years and didn’t see one happening.  I found a close one in 2060, but it looks like a real one might not be until 2080.  I’ll try to get an exact date and update this when I can.)

So what does this mean to you?  Hopefully it gives you pause before thinking there is something cosmically special about a couple of solar system bodies moving about their daily motions.  Though they are beautiful to witness, they have no influence over anything.  Your life can only be affected emotionally.  So that’s why I ridicule silly books like this.  They are feeding you lies and using fear to line their pockets.  Amazing what a little skepticism and independent thought can score for you.

To find out about the next planetary conjunction, I would recommend Astronomy magazine’s monthly feature, The Sky Show, written by my good friend, Martin Radcliffe.  You’ll have to buy the magazine or else a subscription to the website, but the information therein can’t be beat!

[Hmmmmm, maybe I need to do another astronomy post on how to identify a planet if you don’t know what you’re looking at.  Might be interesting, anyway.]



5 responses

12 09 2008
Nick Jones

But the astrologer in the news paper said that the planets lining up meant good luck for me. How is what he said any different from the astrologers looking through those fancy magnifying glasses say? Hmm?
Seriously though, this was a really neat and informative post that has made me late to class.

28 06 2010
Daniel Traum

Seriously Nick, I hope your just joking…if not, perhaps you should start getting to class on time. Generally speaking, just getting information on a factual level from the “News Paper” is against good advice. Also, as you say “Astrologer’s”…they don’t publish in “News Papers.” They write in science journals or teach at MIT. I am really responding under the assumption that you were just kidding in your comment, but if not…I hope your “Horoscope” for today brings you good fortune. Keep studying!

28 06 2010

Seriously dude? You are writing that about a post that was made almost two years ago?! It was a joke, calm down buddy…

25 07 2015

Nice write-up! I would like to reference this image in my blog entry on a related topic:

I don’t see any email link for you, so please contact me: photojhh at gmail dot com. Thanks.

25 09 2015

I am having trouble with the definition of conjunction… Does the term imply relativity in viewing from earth, or is it the proximity of the individual planets?… (I thought it was relative to earth)…

“Scientists define ‘alignments’ and ‘conjunctions’ a little differently than everyday language. We consider an alignment to be any time two or more planets get within 15 degrees of each other. ”

In relation to the star of Bethlehem, I ask because of a couple books that reference the signifigince of conjunctions and percentage of favorable visibility : If the definition of conjunction (and the possible number of 2 planet conjunctions) is always relative to earth, then the percentage of visibility would be 100% from somewhere on the planet (weather dependent of course)… When you factor in hemispherical visibility, doesn’t that come out to 50/50?

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