365DaysDate: April 8, 2009


Title: Drifting Through the Zodiac

Podcaster: Christopher Crockett

Organization: Lowell Observatory

Description: Flip through your local paper and you’re bound to find a daily horoscope whose Zodiac signs share their names with twelve constellations. But have you ever wondered what’s so special about these constellations? And how did astrologers arrive at those particular dates for the signs? Today’s podcast will take us on a tour of the zodiac. My reading of the horoscope won’t promise success in your career, but it will reveal something about the motion of the Earth and uncover why you might want to start reading the horoscope that precedes yours in the paper.

Bio: Today is Christopher Crockett’s birthday. Happy birthday!!!

Christopher Crockett is a University of California, Los Angeles graduate student currently working as a predoctoral fellow at Lowell Observatory. His research involves searching for planets and brown dwarfs 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.

Today’s Sponsor: This episode of “365 Days of Astronomy” is sponsored by Astrocamp Summer Mission of Idyllwild, California. Help introduce a child to the world of Astronomy. Learn more at


Hello. This is Christopher Crockett from Lowell Observatory.

Today is my birthday. I mention this because, according to today’s horoscope, being born on April 8 makes me an Aries. And not just me, but also the hundreds of millions of other people that have been born between March 21 and April 19. This puts me in such notable company as Thomas Jefferson, Adolf Hitler, and Elton John. According to astrologers, being born an Aries makes one adventurous, enthusiastic and a courageous leader as well as headstrong and easily offended. We’re also apparently prone to headaches and sunstroke.

The idea behind western astrology is that the time of your birth determines your personality and can even predict future events in your life. The calendar is divided into twelve parts or “signs”. Which date you were born determines which of the signs you belong to. But have you ever wondered where the signs come from? Or what about the dates assigned to each sign? Why is March 21 through April 19 set aside for Aries? Or October 24 through November 22 designated as Scorpio?

The twelve signs of the zodiac take their names from twelve constellations: Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpius, Sagittarius, Capricornus, Aquarius, and Pisces. But why these twelve? If you look at a star map and find these constellations, you’ll notice that these twelve sit in a line tracing out a great circle on the celestial sphere. This line is known to astronomers as the “ecliptic” and marks the path that the Sun appears to travel over the course of a year.

Everyone is familiar with the daily east to west motion of the Sun; the annual motion of the Sun against the background stars is a little less obvious. The two motions arise for different reasons. The daily path of the Sun arises from the Earth rotating on its axis. As we are carried from west to east on the Earth, the Sun appears to move in the opposite direction in the sky.

But if you could see the stars during the day, you’d notice that over the course of a year, where the Sun appears relative to the stars changes from day to day. This apparent motion results from the Earth traveling on its orbit around the Sun. Imagine you are on a carousel. While riding, you look towards the center of the carousel. As you go around, what appears to be behind the center column changes. At first, maybe you see a food vendor. Then a minute later, you’re looking towards the restrooms. A minute later, you see a tree on the other side. And so on until you’ve come all the way around and you’re back to seeing the food vendor. From your vantage point sitting on the ride, the scenery behind the central column keeps changing as you go around and around.

The same thing happens to an observer on the Earth looking towards the Sun. As the Earth swings around on its orbit, the scenery behind the Sun changes. In this case, the scenery is the background stars. This means that, over time, the Sun appears to be in front of different constellations. One month, the Sun may appear to be “in” Gemini; the next month it may appear to be “in” Cancer, and so on.

You might be able to guess, then, where the horoscope dates originally come from. As the year progresses, the Sun will appear to be sitting in the constellation Aries for a while and then in Taurus, and Gemini, and so forth. According to astrology, the dates listed for each sign are the dates during which the Sun appears in that constellation! So March 21 through April 19 are set aside for Aries because that’s when the Sun is sitting in that constellation! Your sign is therefore defined by where the Sun was when you were born.

Unfortunately, that’s not quite true.

The modern dates of the zodiac were defined roughly 2000 years ago. And, at the time, the dates were correct. But astrology hasn’t taken into account the fact that the dates during which the Sun is in a given constellation changes!

In my January 11 podcast on the North Star, I talked about “precession”. To recap, think of the rotating Earth as a spinning top. Like a top, the Earth not only rotates, but it also wobbles! This leads to a drifting in the direction our axis is pointing. Over the course of 26,000 years, the direction in which the poles point traces out a circle in the sky. The most obvious effect of this is to change which star we call the North Star or if there even is a North Star!

Another effect is a bit more subtle and has to do with how we define our calendar. I’ll try to explain it by example. June 21 is one of the solstices: it marks the beginning of summer in the Northern Hemisphere and the start of winter in the Southern Hemisphere. On this date, the Sun appears at its most northerly location in the sky at local noon. This results from how the tilt of the Earth’s axis aligns with the Sun; on that day the North Pole is tilted fully toward the Sun.

Now consider the effects of precession. Over the course of one orbit around the Sun, the direction of the Earth’s axis drifts ever so slightly. This means that where along our orbit the solstice occurs also changes by a very slight amount. It works out to about 20 minutes earlier each orbit.

Our calendar is tied to the solstices – it’s defined by the so-called “tropical year”, the time it takes for the Sun to return to the same position in the sky as seen from Earth. This is different from what’s called the “sidereal year” which is how long it takes the Earth to complete one orbit around the Sun relative to the fixed stars. Because of precession, the tropical year is 20 minutes shorter than the sidereal year. And, because our calendar is defined by the tropical year, where the Sun sits relative to the background stars on a given day changes! On the June solstice 2000 years ago, the Sun was sitting almost halfway between Gemini and Cancer. On this year’s June solstice, the Sun will be sitting between Gemini and Taurus.

The punch line to all this is: the dates on the horoscopes are wrong!! The Sun appeared to pass in front of Aries from March 21 to April 19 two thousand years ago. This year, on my birthday, the Sun can be found smack in the middle of Pisces! The precession of the Earth’s axis causes the Sun, on any given date, to move over an entire constellation every 2167 years – which is roughly how long ago the modern western zodiac was invented.

This means that all the horoscope dates are off by one sign!

What’s great is that you don’t have to take my word on this. You can verify it for yourself. While you can’t directly see what constellation the Sun is sitting in right now, you can go out shortly after sunset and figure out what constellation is sitting just to the east of the Sun. To make it more precise, you can go out just before sunrise and determine which constellation is sitting to the west of the Sun as well. Knowing which two constellations the Sun is between, you can use a star chart (freely available on many websites) to figure out which constellation the Sun is sitting in. If you’re feeling particularly ambitious, do this once a month for a year and you’ll be able to track the Sun’s apparent motion against the stars for yourself! Sounds like a great student project!

The other important thing to remember is that the constellations themselves are completely arbitrary. The stars that make up a constellation are not, for the most part, physically related. The constellations are just patterns that our ancestors saw as they gazed skyward and tried to make sense of it all. What defines the “official” constellation boundaries wasn’t laid down until 1930 by the International Astronomical Union. Using the current constellation boundaries, one finds that there are actually thirteen constellations that lie along the ecliptic (the extra one not listed in any horoscope is Ophiucus, the serpent bearer, who sits between Sagittarius and Scorpius). And the constellations that are widely known are really only specific to ancient Greek culture; most of them were defined by the Greek astronomer Ptolemy in the 2nd century. Different cultures have seen patterns in the sky unique to their history. Some constellations are shared by many cultures (Orion is a notable example), but most are not.

So the constellations are arbitrary, human inventions that highlight the history and mythology of all the Earth’s cultures but have no cosmological significance. The zodiac constellations that are meant to predict your personality are derived from twelve of the thirteen constellations specific to Greek custom that lie along the Sun’s apparent path. And the dates listed in your horoscope are all off by a month. If you still insist on reading your daily horoscope, you might have better luck with the sign that comes before yours. But I wouldn’t bet on it.

End of podcast:

365 Days of Astronomy
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