You Know The Big Dipper Isn't Actually A Constellation...Right?

The Stars book works hard to earn its’ subtitle “The Definitive Visual Guide to the Cosmos.” You’ll of course learn about the birth and death of stars, the stellar main sequence and what’s outside of it (Red giants! White dwarves!), but also about cosmology, astronomy, and our local solar system. But, what we’re going to talk about today is the subject that takes up almost two-thirds of the book: constellations.

If you ask most people to name a constellation, after their astrological sign, they’d probably say the Big Dipper.  But if you pull out your new, exhaustively complete Stars book to find out more about the dipper… it’s not there. Why? Because the Big Dipper is really an asterism–a star pattern that isn’t a constellation, but a fragment of a larger constellation, in this case Ursa Major or the big bear.  Another well-known asterism is the Northern Cross, which forms the backbone of the constellation known as Cygnus the Swan.

So wait, who gets to tell us that the Big Dipper isn’t a constellation?  Is it the same people who told us Pluto wasn’t going to be considered a planet anymore? (actually yes: more on that in a moment)

 But first, here’s one of the odd things about Cygnus, and almost all of the constellations you find in this book: you’ll find that there are lot of different ways of drawing them! Many include different stars or shapes for the exact same constellation. For Cygnus, the illustration in the book (page 124) shows a very common configuration; but if you look at the official drawing of the constellation from the International Astronomical Union, you’ll see the wings aren’t just a single line, but include a trailing edge, making them fully formed. Or take a look at Pisces, an astrological sign and thus one of the best-known constellations. If you check out Pisces in the book (page 154-55), you’ll see that the body of the lower fish has seven stars in it.  Now take a look online.  Is it seven stars?  Six?  Five?  (the IAU version has five). So what gives?  

It turns out the reason it can be hard to find a consensus on certain constellations is that for literally centuries, there wasn’t one. Basically, the ancient astronomer Ptolemy described 48 constellations in the year 150 CE, and other than agreeing on those originals, for roughly the next 1700 years, people pretty much had free reign to make up new ones, call them by whatever names they chose, and generally run amuck with their constellation shenanigans. And for a very, very long time, it didn’t matter! 

You see, constellations in general don’t have a specific scientific utility; so for centuries, it didn’t matter too much if there was a scientific agreement as to what they were, which stars formed them, and what names they went by.  But in the late 19th and early 20th centuries, research in certain fields of astronomy made it important that everybody agree on what the constellations were.  One example is the study of variable stars: their designated name includes the constellation in which the star is found. That’s a real problem if one astronomer thinks the star is in Orion and another in Virgo, or if they don’t agree on the name of the constellation.

So in 1922, the International Astronomical Union, a professional organization for astronomers, met for their first general assembly to adopt an official list of scientifically recognized constellations, set the names, and specify the included stars and phenomena. These “modern 88” are the constellations you’ll find in your book. Now, even though this happened almost 100 years ago, the old disagreements about the borders, names, and other specifics are still out there. One great example is the constellation known as the Scorpion.  In 1922 the officially named “Scorpious” won out over the commonly used alternative “Scorpio”–but astrologers still prefer the latter name, so Scorpio is what you see in every astrology column out there.

There are a lot other things we love about this book: one is that it shows how much our view of each constellation is a matter of perspective: that is, we’re so used to seeing the constellations reprinted in two dimensions that we often forget they really exist in a three-dimensional space.  On each of the constellation pages, there’s a fascinating illustration of how far away from each other each star really is in three dimensions.  Another is how many of the constellations are named after interesting objects – sure, you’ve got your microscope, telescope, and chisel (that’s right, there’s a constellation named Caelum the Chisel,  so maybe it could also be a wrestler) but more esoteric devices have also been honored. There are official IAU constellations named Antlia the Air Pump, Pictor the Painters Easel, and Norma the Set Square, which sort of sounds like an animated character in a Disney movie about architectural tools. 

What we’re saying is that there’s so much here–literally an entire cosmos’ worth–that we hope it feeds your curiosity for years to come!