Date: February 21, 2012
Title: Astronomy Word of the Week: Constellation
Podcaster: Dr. Christopher Crockett
Organization: United States Naval Observatory
Links: http://www.usno.navy.mil/USNO
http://astrowow.wordpress.com/
Description: The astronomy word of the week for Februrary 21, 2012 is: Constellation. In this podcast, will learn about the history of the constellations, what they are, and how the night sky has been viewed through the lenses of different cultures throughout history.
Bio: Dr. Christopher Crockett is an astronomer at the United States Naval Observatory in Flagstaff, Arizona. His research involves searching for planets around very young stars (“only” a few million years old). It is hoped that the results from this research will help constrain models of planet formation and lead to a better understanding of where, when, and how often planets form. Chris is also passionate about astronomy outreach and education and will talk for hours about the Universe if you let him.
Sponsor: This episode of 365 Days of Astronomy is sponsored by someone who really likes saying ‘supernovae’.
Transcript:
Several years ago I attended a party and during introductions it was revealed that I’m an astronomer. The conversation went something like this:
“So is that, like, a night job?”
“Not typically. There are some times when I’ll spend a night or several at a telescope but usually it’s just a regular day job.”
“Oh. So, what is it you do then? Do you look at Orion and decide to draw a hat on him?”
“Errrr…oh look, is that shrimp cocktail?”
Without a doubt, when most people think of astronomy, their minds leap to the constellations. Orion, Hercules, Andromeda, Cassiopeia, Triangulum. Well, maybe not that last one as much (it’s a triangle). But these are names and stories with which many of us are familiar, even if we don’t know exactly why. When ever I host a star party, I’m almost always asked to point out a favorite constellation. “Where’s the Big Dipper?” “Which one is Gemini?” “Can you see Leo right now?”
A constellation is simply a pattern of stars in the sky. Humans have an amazing ability to see patterns in things that aren’t necessarily there. We see animals in clouds and occasionally the Virgin Mary in a tortilla. There’s even a word for this phenomenon: pareidolia. Our ancestors thought the sky was home to their gods and the various heros and villains with whom they deemed worthy enough to share the celestial sphere. And so out of the myriad twinkling dots that traversed the sky night after night, they came to see shapes that represented their culture and mythology. Those dots over there look something like a scorpion while those seven stars over there vaguely resemble a man. The night sky became a patchwork of stories following the thread of their history and their beliefs. In truth, the constellations may be some of the oldest relics of our humanity; their history can be traced right down to the roots of our civilization.
The constellations that we know and love – Orion, Taurus, Ursa Major, Sagittarius – are tied to a very specific culture, that of the Ancient Greeks. Specifically, the modern constellations can be traced to a compilation recorded by the astronomer Ptolemy in the 2nd century called the Almagest. There he listed 48 constellations that were known at the time, most of which have survived unscathed to today. But Ptolemy’s constellations can be traced back even further to the Babylonians circa the 7th century B.C. Remarkably, many of the Babylonian constellations are borrowed from their ancestors, the Sumerians – all the way back to 2000 B.C. It seems the constellations – like the stars which make them – were built to endure. Many of these nightly wanderers call out to us from the dawn of human history and have stood unperturbed for nearly 4000 years.
Ptolemy’s 48 remained mostly unchanged until the 16th century when astronomers began to tinker with the established canon and tried to outdo one another by “identifying” ever fainter patterns of stars. Explorers who sailed to the Southern Hemisphere made detailed maps of the southern sky to aid their navigation and, in the process, invented a plethora of new constellations.
As one might imagine, this led to much confusion since there was no official list of constellations. By 1900, depending on whose star atlas one referenced, there was anywhere between 80 and 90 constellations and no real agreement on where one constellation ended and another began. In 1922, the International Astronomical Union met for the first time and made one of their first orders of business the adoption of a definitive list of constellations and their boundaries on the sky. From their discussions, a list of 88 constellations were deemed “official”. Their boundaries were drawn along lines of constant right ascension and declination (the equivalent of longitude and latitude on the sky). The constellations were thus transformed from pictures to regions on the sky. To the modern astronomer, they are useful as a guide (“that galaxy is in Virgo”) but not much more than that.
That the constellations which make up the official 88 come to us from Greece is an accident of history. As they are products of human imagination, different cultures saw different constellations and they are as varied as the peoples who invented them. Not every culture decided to group the stars the same way and some didn’t even limit themselves to stars. The Incans, for example, identified several “dark constellations” – regions in the Milky Way where no stars appeared. To them they were animals which brought seasonal rains; modern investigations reveal that these are actually dense clouds of interstellar gas and dust which block the light from more distant stars.
Some star patterns appear in many places and times in history. The seven bright stars which make up Orion, for example, have been shared by many cultures around the globe – which isn’t very surprising given their brilliance and the distinctive pattern they make on the sky. The Greeks saw a great hunter; to the Egyptians he was Osiris, God of the Underworld and the nearby star Sirius was the home of his wife, Isis; the Babylonians called him Anu, herald of the Gods; Australian Aboriginals saw a canoe called Julpan which held two brothers who caught and ate a forbidden fish – they were punished by the Sun who sent a waterspout which shot them into the sky.
Our ancestors had no knowledge of the true nature of the stars – that they are cosmic furnaces blasting at millions of degrees in which the very stuff of which we are made is forged. We now know that the stars in a given constellation, with some exceptions, are not physically connected in any way. The stars in Orion span thousands of light years in every direction. The pattern we see is unique to our location in the Galaxy. Were we to head out on a spaceship and swing around the “side” of Orion, the familiar hunter would disappear – though entirely new patterns would emerge.
One can only wonder, if there are other civilizations out there, what stories they see in their skies – and if our Sun is a part of them!
End of podcast:
365 Days of Astronomy
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