The zodiac controversy…

Okay, so I had planned to wait until the weekend to post as usual, but this sudden zodiac outrage must be addressed.

For those four people out there who don’t know what all the commotion is about, it all started with an astronomer out in Minnesota (I know, random) who mentioned something about a commonly accepted astronomical phenomenon in an interview with the Minneapolis Star-Tribune last weekend. The now infamous Parke Kunkle, a member of  the board of directors of the Minnesota Planetarium Society and astronomy professor at Minneapolis Community and Technical College, told the Star-Tribune that the Earth’s relation to the Sun has changed since the Babylonians first created the zodiac. THIS IS NOT A NEW STORY! Astronomers and astrologers alike have known about this phenomenon since Hipparchus first realized it occured back around 130 BC. That’s right BC, that’s a millennium and a half  before most people accepted that the Earth was round or realized that the Earth was not the center of the solar system. As much as I love the internet and modern technology, I blame the recent outcry over the zodiac on Twitter (and other social media) and modern day journalism. This was one poorly written article that has snowballed to insane proportions; if you google “Parke Kunkle” one of the top five links is to a blog post titled “Parke Kunkle must die”. Seriously? It’s not like he made the Sun move. As news stories and Twitter posts pertaining to the zodiac skyrocket, I wanted to set the story straight, exactly what the zodiac is and why it’s “all of a sudden” different.

I laid out the basic history of astrology in a post back in September. The twelve commonly accepted zodiac signs were selected out of the 88 constellations because they basically form a line through the sky. If you look at this star chart below, you can see the curve* that the bold zodiac constellations make through the sky, this curve follows what astronomers know as the ecliptic plane, or the plane of our Solar System (that dashed line running through the bold constellations in the chart). The orbits of all of the planets in our solar system occupy basically the same plane in space due to the conservation of angular momentum as the solar system formed. Because of this fact, when we look out at the Sun and other planets in plane of the solar system from Earth they appear to follow a line through the sky, the ecliptic. Now Earth doesn’t sit exactly straight with respect to the ecliptic; in fact, the 23.5 degree tilt of its rotation axis with respect to the ecliptic plane is what causes the different seasons on Earth (it has nothing to do with the Earth’s orbit getting closer to the Sun) and the different length of the day throughout the year**.

We all know that the Earth rotates like a top on its axis once every 24+ hours, but think about what happens as you spin a top. First, you start with a nice tight spin, but as the spin starts to slow, the top begins to loosen the tightness of its spin and “precess” outwards before it falls; this is what Earth’s rotation is doing because of the gravitational effects of the Moon, Sun, and other planets. Because the stars in the sky are in a fixed position with respect to Earth (for the most part over extremely long time periods) we don’t see them move in relation to one another and the stars and the constellations themselves won’t change as the Earth’s orbit precesses. What will change though throughout the 26,000 year cycle of the precession of the Earth’s axis is how the stars seem to move through the sky. The north and south celestial poles are imaginary lines which extend out into space along Earth’s rotational axis; they indicate the point in space around which the sky seems to rotate every night. The north celestial pole points almost directly at Polaris, the North Star. It’s because Polaris sits so close to the north celestial pole that it is so special; it does not move in the night sky as the Earth rotates under it (and the rest of the stars in the sky appear to move around it). But the north celestial pole hasn’t always pointed towards Polaris and it won’t continue to; in other words, Polaris isn’t always the North Star. Over time, as Earth’s rotational axis precesses, instead of the stars appearing to rotate around Polaris, they’ll appear to rotate around other stars and for most of the time no particular star at all. If you look at this image, you can see the path through the sky that the north celestial pole follows throughout its 26,000 year cycle (known as the Great or Platonic Year in astrology); in roughly 13,000 years, the bright star Vega in the constellation Lyra the Harp will appear to be the North Star.

So that’s precession, but you might be saying, “well that was fun, but what about the zodiac?” Well, remember, the Earth’s tilt is fixed at 23.5 degrees, that won’t change, so as the north celestial pole moves in the sky, so will the alignment of the ecliptic plane with respect to the background stars and constellations. As that happens, the annual position of the Sun in the sky during the year (the entire basis of the astrology) will change. But this change is on a time span of thousands of years, so even though you could say two people born on the same day in 2009 and 2010 were both of the same sign, you couldn’t necessarily say it about persons born on the same day of the year in 10 AD and 2010.

Kunkle also threw an extra wrinkle in the argument when he noted that the Babylonians, who originally developed the zodiac, had initially selected thirteen signs for inclusion in the calendar, not twelve. Kunkle noted the that the Bablylonians had long-since discarded Ophiucus the Snake-bearer (seen in the chart above as “Oph” just above Scorpius and Sagittarius on the left) in favor of a twelve-sign zodiac.

For some reason now (most likely nerdiness and boredom), Kunkle and his colleagues at the Minnesota Planetarium Society decided to figure out exactly which constellation the Sun is in each day in the modern era. These new dates of course differ from the classically accepted zodiac sign dates, which were already “fudged” because the Babylonians had nixed Ophiucus.  So the “new” modern alignment of the zodiac dates is:

Capricorn: Jan. 20 – Feb. 16
Aquarius: Feb. 16 – March 11
Pisces: March 11- April 18
Aries: April 18- May 13
Taurus: May 13- June 21
Gemini: June 21- July 20
Cancer: July 20- Aug. 10
Leo: Aug. 10- Sept. 16
Virgo: Sept. 16- Oct. 30
Libra: Oct. 30- Nov. 23
Scorpio: Nov. 23- Nov. 29
Ophiuchus: Nov. 29- Dec. 17
Sagittarius: Dec. 17- Jan. 20

But remember, astrology is still trying to use the stars to predict your future, so not really a very accurate science anyway. In any case, I’ve done my part in trying to explain this phenomenon. I hope you understand now and I hope your new sign works out for you (if you believe in that sort of thing).

* The line on the chart is a curve because the map is attempting to show a three-dimensional shape, the spherical sky as we see it, in two dimensions.

** The Sun’s altitude in the sky or it’s height above the horizon actually changes throughout the year as it rises above (in the summer) and below (in the winter) the ecliptic plane. It’s this movement with respect to the ecliptic plane (as a result of Earth’s tilt) and how it affects the length of the path it travels through the sky that that determines how many hours the Sun stays up during the day.


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