Although the Zodiac is best known for astrology nonsense, it has a purpose in astronomy too. The constellations of the Zodiac define the plane of the ecliptic: the region where the Sun, Moon and planets appear to travel through the sky. What are the constellations of the Zodiac, and how do astronomers use them as waypoints?
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Female Speaker: This episode of Astronomy Cast is brought to you by Swinburne Astronomy Online, the world’s longest-running online astronomy degree program. Visit astronomy.swin.edu.au for more information.
Fraser Cain: Astronomy Cast Episode 319: The Zodiac
Welcome to Astronomy Cast, our weekly facts-based journey through the cosmos. We hope you understand not only what we know but how we know what we know.
My name is Fraser Cain. I’m the publisher of Universe Today and with me is Dr. Pamela Gay, a professor at Southern Illinois University, Edwardsville, and the director of CosmoQuest.
Hey, Pamela. How are you doing?
Dr. Pamela Gay: I’m doing well. How are you doing, Fraser?
Fraser Cain: Great.
How is your weather there?
Dr. Pamela Gay: It is gloriously sunny, with orange and red leaves and it’s cold!
Fraser Cain: Is it?
Dr. Pamela Gay: It is.
Fraser Cain: Yeah, it’s strangely warm here. Normally, it’s a lot colder. We’ve been getting some – we got our first frost just a couple of days ago but it’s just been, you know, so warm outside. Ahh, it’s been great.
Dr. Pamela Gay: We’ve hit the time of the year where you occasionally find critters that, like, didn’t find someplace warm enough before the sun went down just frozen in place.
Fraser Cain: Ohh.
Dr. Pamela Gay: Yeah.
Fraser Cain: But you got a frog pick, right?
Dr. Pamela Gay: I did, I did. I got a glorious – it’s a toad.
Fraser Cain: It’s a toad.
Dr. Pamela Gay: It’s an American toad that likes to scare the bejesus out of me once or twice a week.
Fraser Cain: How big are they?
Dr. Pamela Gay: This one is about the size of a fist. It’s just big enough to be a challenge to pick up and move someplace that it won’t get stepped on.
Fraser Cain: That’s a big amphibian. We don’t have anything like that here.
Dr. Pamela Gay: It’s an awesome amphibian.
Fraser Cain: I think I saw some in Hawaii. Those big Cane toads? Same kind of monster –
Dr. Pamela Gay: Oh! Those are huge. Yeah.
Fraser Cain: Yeah. It was really big but don’t lick them.
Dr. Pamela Gay: No.
Fraser Cain: Alright. Well, let’s get rollin’.
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Fraser Cain: So, although the zodiac is best known for astrological nonsense, it has a purpose in astronomy too. The constellations of the zodiac define the plane of the ecliptic: The region where the sun, the moon and the planets appear to travel through the sky. What are the constellations of the zodiac and how do astronomers use them as waypoints?
So, I get to make this joke every few episodes. So, Pamela, can you tell me my horoscope?
Dr. Pamela Gay: No.
Fraser Cain: Okay. I’m a Leo. Does that not help?
Dr. Pamela Gay: No.
Fraser Cain: No? Okay.
Dr. Pamela Gay: And are you sure you’re a Leo, actually?
Fraser Cain: Oh. Is there a bit of a controversy?
Dr. Pamela Gay: Well –
Fraser Cain: I’m August 19th. Yeah. Apparently, I’m a Leo. I don’t know.
Dr. Pamela Gay: Okay. So I’m looking this up. You’re August 19th. Yeah, you’re actually a Leo.
Fraser Cain: Okay. Why did you even ask me a question?
Dr. Pamela Gay: So, one of the things that completely baffles me about how astrologers use the zodiac is they take the year and they divide it up into completely even chunks – 12 completely even chunks. And they assign each of these chunks to a constellation that, at some point during that span of time, is probably where the sun is located.
But, the thing is, the real constellations, they aren’t all equal in size and the sun doesn’t dwell in them for equal amounts of time. So, if you look up the dates that astronomy says the sun is in the constellation Leo, it’s not that nice 30-day chunk that astrologers identify. It’s actually 37 days and it goes from August 10th to September 15th, which is not what astrologers say.
Fraser Cain: Right. So they mean it is actually located within the boundaries of the constellation of Leo, with all of its strange, jaggy lines.
Okay. So let’s go back a bit in, sort of, back to the beginning here. So, what is the zodiac?
Dr. Pamela Gay: The zodiac is a group of constellations that form a whole variety of different animals and humans that the sun passes through as it changes its alignment relative to the stars as the earth goes round and round it.
So the sun appears to move in the sky because of our motion and each day, if you look right before sunrise, you can see what constellation the sun’s going to be in when it comes up.
Fraser Cain: And this has been used for a long time, right?
Dr. Pamela Gay: Yeah.
Fraser Cain: This is the way that they’ve, kind of, measured the position, the months? I mean, do they – do you know when and, sort of, where the current concept of the zodiac came from?
Dr. Pamela Gay: Well, the current constellations – they actually go back so far that there’s people who argue over the linguistics of some of them thinking that the names actually go back prior to the separation of many of the languages in the Mediterranean.
You can take many of our constellations back to Ptolemy and Hipparchus and we’ve been using them for a long time. We’re basically using the Greek constellations. The Romans couldn’t be bothered to come up with their own, so they stole them wholeheartedly and just renamed a few of them.
But it’s a set of constellations that, in the Western world, astrology says there are 12. Astronomy recognizes 13 and they’re the ones the sun goes through. If you go to other cultures, you’ll see other constellations. Although, one of the things I love is the Aztecs. They see Scorpio as a scorpion as well. So there are some similes because when you look up, some shapes just stand out and Scorpio is one of them.
Fraser Cain: And so, what are the constellations that are in the zodiac?
Dr. Pamela Gay: I have to read a table here. I’m a – astronomer who doesn’t memorize things often.
Sagittarius, Capricorn, Aquarius, Pisces, Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpius, Ophiucus.
Fraser Cain: And so, the Ophiucus one – I think people won’t be familiar with it being in the zodiac.
Dr. Pamela Gay: It’s the serpent-bearer and so, what happens is Scorpius, which is the one that gets all the hype – if you look at how much of Scorpius actually crosses into the path of the sun, the sun only spends seven days going across poor little Scorpius’s bit that’s in the ecliptic and then, the rest of the time for 18 days, the sun is actually in the constellation, Ophiucus, the serpent-bearer.
I’m actually an Ophiucus, if you care about where the sun is on the day of your birth. I’m December 12th. And Ophiucus actually is the zodiacal sign for anyone who was born between November 30th and December 17th.
Fraser Cain: Wow. I wonder what their personalities are like.
Dr. Pamela Gay: Well, I – that’s the ludicrous thing about all of this – is, in astrology, they have charts. And I thought for the longest time that they were actually using real charts, like planispheres and stuff like that, like we use in astronomy and they were just making up stuff.
But they’re not even using charts that actually reflect the sky. Because they take the year and, as I said, they just divide it up into 12 equal-sized bits. And if you travel and see any of the amazing paintings that are in various places in the world that depict the different signs of the zodiac, they always have these equal bands that the sun appears to pass through and that’s not the case. So when they make charts, they’re using a chart that doesn’t actually depict the actual locations of the constellations.
Fraser Cain: So are they, like, on average, accurate?
Dr. Pamela Gay: Ehh. Uh-uh. Not really. I mean –
So, here’s the thing. There’s some that are worse off than others. You have constellations like Pisces, which is 38 days long; Taurus is 37; Leo is 37. And then you have, well, the little guys, Scorpius, that’s only 7 days long. And Virgo – Virgo is 45 days.
And so half the people whose actual sign, if you look to see where the sun actually was on the day of their birth, they’re thinking that they’re probably a Libra and they’re actually a Virgo.
So, I – like I said, this is just one of those things that baffles me. I understand that symmetry is good. We all want things to be equal and it’s way easier to cast charts if all of the time periods are the same because you don’t have to pay attention to precession and details like that. But, dang it! That’s not the way the universe works and, if you’re going to make stuff up, at least ground it in the reality of how big the constellations are.
Fraser Cain: You just ask for too much.
Dr. Pamela Gay: I do. I do.
Fraser Cain: So how do astronomers use the zodiac? I mean – you know what, this is kind of funny. So we – and I – with our app, you know, we do this phases of the moon app and we’ve included the zodiac sign that the moon is currently in, in the – so people can kind of know – they can plan their observing. They go, “Oh. The moon is in Virgo. I’m going to stay away from Virgo tonight and, sort of, try some other places.”
But people can be – give us a hard time. But there are legitimate reasons to want to know where objects are in the sky.
Dr. Pamela Gay: The best way to think of it is it’s like a county map of the sky. Each constellation is a region of sky that you can select for naming stuff, finding stuff, and it’s just an easy way to find your way around.
So, I know here where I live in the Midwest, when they say there’s a tornado in Madison County, I know which cities are potentially going to be impacted. If someone says there’s a full moon in Sagittarius, I know it’s not a good time to go looking at the center of the galaxy because its light is going to get blotched out by the moonlight.
So we use it to figure out where things are and we also use it for naming schemes. Very roughly, the brightest stars and constellations are a Greek letter followed by a Latin form of the name of the constellation. So, you have Alpha Orionis is the brightest star in the constellation Orion. You could also say Alpha Aquarii or – it’s – all of the constellations have an alpha, they have a beta, they have a gamma. They’re not necessarily – alpha isn’t always the very brightest. Sometimes it’s the one that’s the furthest to the left. It depends on where it was when Bayer named things.
We also use it for naming variable stars. The variable stars in a constellation start with AA, work their way through to ZZ and then start with V-4-something-something, but I don’t remember off the top of my head. But that’s how we name things – is like, V489 Pisces.
It’s a good way to find ourselves with a good observing run is you just look for things with the right types of names. It’s how we name galaxy clusters: The Virgo Cluster, the Coma Cluster.
Fraser Cain: We use them for comets, as well. Because the comets are often moving through the planet ecliptic as well. And so –
Dr. Pamela Gay: I think you mean meteor showers.
Fraser Cain: No, no – with the comets. Like, such-and-such comet is currently in Virgo –
Dr. Pamela Gay: Oh! Yeah, yes.
Fraser Cain: and it’s moving into – so, you know, look in this area. Because the comets are moving and so you need to know where to find them, right?
Dr. Pamela Gay: Right. But – but what I thought you were going for is it’s also how we name meteor showers. So if you want to look at the Geminids, which are up during my birthday, they radiate away from a constellation, in general, and that’s the point in the sky where the earth’s atmosphere is touching the debris stream that was left behind by a comet or an asteroid in the past. So, we use the name of the constellations for lots of different things.
Fraser Cain: Right. You’ve got the Perseids, the Orionids, the Geminids. And a lot of those match up with the zodiac constellations, as well.
So, now, I’m thinking about – I’m running this, sort of, model of the solar system in my brain. And I know – and, you know, the planet ecliptic is the, sort of the baseline, right?
Dr. Pamela Gay: Yeah.
Fraser Cain: But the moon can be above or below the ecliptic –
Dr. Pamela Gay: 5 degrees.
Fraser Cain: The planets can be above or below the ecliptic.
Dr. Pamela Gay: Yeah.
Fraser Cain: So, does – I mean, is the zodiac like a zone?
Dr. Pamela Gay: Well, each constellation has a physical size on the sky and, what’s kind of awesome is, they are big enough that, for the most part, all the solar system bodies that are bright enough to be seen by eye, and a few that aren’t necessarily seen by eye like most of the asteroids, for instance, are all nicely confined to within the zodiac. Some stray but not very much and so, in general, if you want to go out asteroid hunting, comet hunting or you just want a good chance of randomly stumbling across one of the known planets, looking in the zodiac is a great place to start.
Fraser Cain: So we’ve talked about how the earth’s axis is changing over time with this procession of the equinoxes. What impact is that having on the zodiac?
Dr. Pamela Gay: Well, this is where you hear things like the Age of Aquarius. So, as things slowly change over time, the way they align with dates changes with time, as well.
So, when we have the solstices is very gradually changing and this lines up with making great songs, apparently.
Fraser Cain: So, I don’t understand. So like, the Age of Aquarius – I don’t know what would be after the Age of Aquarius. So, how – what’s lining up here?
Dr. Pamela Gay: So, what ends up mattering is the position of special days of the year. In this case, it’s the Spring Equinox. So if you look at the dates we have for the Age of Aquarius – Well, it doesn’t quite line up right now but that’s because these ages are getting defined by astrology. But you look to see where is the Vernal Equinox and, right now, Aquarius is February 16th to March 11th in reality but the Vernal Equinox is in the constellation, Aquarius, if you follow astrology, which uses dates that don’t reflect the actual position of where the sun is in the sky.
Fraser Cain: Right. And, I mean, the – We’ve talked about the procession of equinoxes and – I’m trying to remember. It’s, like, 90 seconds every year, I think.
Dr. Pamela Gay: It’s –
Fraser Cain: There’s some amount.
Dr. Pamela Gay: It’s – One human lifetime it moves 1 degree, on average, and that’s kind of awesome.
Fraser Cain: Yeah, yeah. Like 80 years or so, it moves 1 degree. And so, I mean, these seasons are shifting, these – and people don’t realize that. That, you know, a few thousand years from now, the seasons will be all skewed up, they’ll all be changed. Everyone’s not permanent.
Dr. Pamela Gay: No.
Fraser Cain: We’re wobbly.
Dr. Pamela Gay: But it works out well enough, with the current calendar we have, that we don’t stress about it too much, although we do have to keep track of our positions of the stars. So, when you look up things in old books, they’ll give you 1950 coordinates. When you look them up in new books, they’ll give you 2000 coordinates. And, pretty soon, we’re probably gonna to have to have the 2050 coordinates.
Fraser Cain: The 2050 coordinates.
Dr. Pamela Gay: Just to keep things moving. I mean, we always have to calculate them in terms of where are the stars tonight relative to the coordinate system. But for books, we give set coordinates and one of the screwy things that we have to deal with is, as the stars move, what do you do when they exit the constellation they used to live in and enter a new constellation but their name is tied to the old constellation.
So, you run into this with variable stars that were on the edges of constellations and have high precession. As they march across the borders, do you rename them? And in some cases, you end up with objects with multiple identities.
Fraser Cain: Now, I’ve got some mnemonics here. So I don’t know if you – if you have an easy way to remember your zodiac but –
Dr. Pamela Gay: I write them in a table.
Fraser Cain: Do you? Okay. I’ve got a few here. So, these are from the internet.
“As the great cook likes very little salt, she compensates, adding pepper.”
So those –
Dr. Pamela Gay: As the great –
Fraser Cain: Aries, Taurus –
Dr. Pamela Gay: Aries is where you started. Okay.
Fraser Cain: Yeah. Aries, Taurus, Gemini, Cancer –
Dr. Pamela Gay: So, you’re starting with the Vernal Equinox.
Fraser Cain: Yeah.
“Really boring teachers can live very sadly, since apples give worthless feelings.”
Dr. Pamela Gay: That is insane. Okay.
Fraser Cain: “All the great constellations live very long since stars can’t alter physics.”
Dr. Pamela Gay: That’s pretty cool.
Fraser Cain: “The ramble twins crab liverish. Scaly scorpions are good water fish.”
I love mnemonics.
It was great – this, like, has nothing to do with the zodiac but, as you recall, after Pluto was un-planeted, people had to come up with a new mnemonic for the solar system, right? So the –
Dr. Pamela Gay: I like the – replacing nine pizzas with nothing! All mothers became much crueler.
Fraser Cain: Yeah. “My very excellent mother just served us noodles.” The one I always use.
Dr. Pamela Gay: It’s much more helpful.
Fraser Cain: So – I mean, we’ve really tried to, sort of, steer clear of astrology.
Dr. Pamela Gay: Yes.
Fraser Cain: And – except to just make fun of it non-stop. And we’re gonna do that right now but I want to, sort of, get a sense of, like, where does astrology – how does this – how did this happen? Why do people even think this kind of thing and, like, why the zodiac and why the sun signs? Like, debunk it.
Dr. Pamela Gay: So, if you look at it – each culture has their own way of looking at these things. Chinese culture cares about the year of your birth. Greek culture, you still look at the constellation of your sun sign. And each of these try to tie certain characteristics of personality.
And you have to imagine that people started by imagining that they noticed people born in certain months all shared similar characteristics. There’s often some sort of, at least a made up shrivel of information, at the core of a lot of superstitions.
And, one of the things that I found really interesting was, several years ago, there was a neat body of research where they looked at how a child’s personality depends on things like: What was the weather during the mother’s pregnancy, what was the mother’s emotional state during pregnancy.
And there’s some thought now that, if you have a whole bunch of moms who suffered through the exact same miserable winter and give birth in March or April, that their children are all going to have the same disposition. If a bunch of mothers go through the same time of plenty and sunshine and joy and give birth in late October, their children are going to have a similar personality.
So it’s possible that there’s this shred of truth, based on biochemistry and what you’re exposed to in the womb that is at the heart of defining each of our personalities. But, at the same time, yeah – Then they just took it and kept going. And then it became a, “Where were the planets?” “Where were the –”
And, in astrology like I said, they aren’t using the actual positions of the sun. They’re using charts that divide the year evenly. Not all astrologers understand what it means to say that Mercury is in retrograde, which doesn’t even really make sense considering it’s internal to us from the sun.
And so you have all of these strange things that have cropped up over the millennia that move it away from trying to understand a set of: Why do these groups of people have similar dispositions, which can be explained with biochemistry; to instead, trying to find a alignment in the stars that describes why some people go on to become kings.
Fraser Cain: Yeah. I guess you can imagine, if there was a particularly harsh winter and the children were born during – and the mother was suffering during that winter, you can imagine a certain disposition for the children.
Dr. Pamela Gay: Yeah.
Fraser Cain: Not just, sort of, emotionally but, you know, physically. Just they were sicklier or they were hungrier or they were stronger. Who knows, right?
Dr. Pamela Gay: Right.
Fraser Cain: And you can imagine a cohort that shares a lot of attributes and then you could say, “Well, they were all born in this time so, therefore, that could be…” and then nonsense, right?
But then, they just take it to the next – the levels of extreme. So, they’ll say, like, what planet it’s in matters, what planet was in your sky, what – you know, where was Mercury, where was Venus, where was Mars, where was the moon –
Dr. Pamela Gay: Yeah. My favorite practical joke of all time was David Lambert at McDonald Observatory, who’s now actually the director of the observatory, called up Bill Cochran, who’s one of the planetary scientists at McDonald. And Bill had just found a planet around a nearby star. It’s actually a binary system. And David Lambert was pretending, because he can do a very good Indian accent because he works there a lot, to be an Indian father trying to get the position of the extra solar planet so that he could get an accurate horoscope cast for his children’s wedding.
And so you can just imagine, as we are now upwards of thousands of suspected planets and near a thousand known planets – Yeah. At what point do you stop caring and how do you take into consideration all the asteroids that come much closer to earth than the planets and have more of a gravitational impact. What do you consider needed and not needed and – Yeah. It just starts to become ludicrous.
Fraser Cain: Yeah.
Dr. Pamela Gay: It’s a chaotic system.
Fraser Cain: Right. So now, I mean, is there any possible way that you could get any kind of influence, gravitationally, from anything? But that’s the only thing I can think of, right? Gravity.
Dr. Pamela Gay: The moon does have a biological impact on the planet because of tides. But, beyond the moon, not so much.
Fraser Cain: Right, right. So it’s just – It’s just nonsense.
Dr. Pamela Gay: Yes.
Fraser Cain: Did you have anything else you want to bring up that you prepped?
Dr. Pamela Gay: Well, it’s – So I can pass on the table of information that I’m looking at is actually now posted over on CosmoQuest’s Educator Zone. So, if you go to cosmoquest.org and click, “Educate”, there is a new post up on a science café that I got to do with some amazing people from Discover the COSMOS in Greece, of all places, this summer.
And there’s nothing quite as interesting as debunking astrology in the country that’s responsible for our current zodiac. And we were up in the village of Milies, which has a 18th century church with an amazing whole set of paintings, including one that depicts the zodiac. So we had people stand up and the first 12 of them evenly spaced themselves in a circle, locking hands around our human sun. And then we had them rearrange themselves and throw an Ophiucus in the actual spacing, just to show how very different the actual spacing is compared to the equal spacing of astrological charts.
Fraser Cain: I wonder if anyone will actually use the real astronomical spacing, as opposed to the astrological one.
Dr. Pamela Gay: I’m sure they’re probably are some people out there who do that but they’re not the ones who are writing for the newspapers or your major astrology magazines.
Fraser Cain: Right. They’re already making stuff up. They might as well make everything up.
Cool! Well, thank you very much, Pamela.
Dr. Pamela Gay: My pleasure, Fraser.
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Duration: 28 minutes