Hey, Ophiuchus

It all started when a newspaper in Minnesota published an article informing the public that they had to say hello to their new personalities.

It had, for some reason, gone viral. If you’ve been a good citizen of the internet lately, news of grand zodiacal realignment may have already reached your screen. Not only is there a shifting of the dates of the astrological signs, but a dark horse sign comes along. Just what is going on here?

From Wikipedia: an illustration of axial precessionThe Earth undergoes several quasi-periodic movements, including rotation on its axis and revolution around the sun. Of the many planetary motions*, axial precession is largely to blame for the change in zodiac signs.

Axial precession is the cyclical change in the orientation of a planet’s rotational axis. The rotational axis would more or less trace a small circle at the end of a cycle. Notice that the entire rotational axis of the earth would trace out a cone. If you happen to have a top right now, go ahead and spin it. Precession is the wobbling motion that becomes very obvious once the top is about to fall.

For the earth, precession is an extremely slow process, and the time it would take for its rotational axis to completely trace a circle would be about 25,772 years. Thus, the position of the stars that we see at night relative to where we see the sun in the sky changes gradually.

When Western astrology was first introduced by the Babylonians, the signs were based on which constellation the sun crossed when it rose**. Over the course of three thousand years, axial precession has caused the background stars to shift, leaving us with shifted zodiac dates as well. Perhaps Bill Nye can explain it better:

If this is the case, why then did the astrological system last as long as it did? Did astrology suddenly decide to drop its central dogma?

Astrology, like many other belief systems, branched out into different systems over time. Although we now have a multitude of ways one can interpret one’s birthday, we can group most of these astrological systems into either tropical or sidereal.

The tropical system is the most common twelve sign system with fixed dates for the signs. This is because it is based on the location of the equinoxes and solstices in the calendar. Though precession changes the dates of the equinoxes and solstices (which are dependent on the orientation of the earth’s rotational axis) slightly, it is corrected for during leap years***, and so the dates more or less remain constant.

The sidereal system uses the original system, and determines the zodiac sign based on what constellation the sun crosses upon rising. What the astrologer was mentioning on news were the mechanics of the sidereal system, which was being practiced by some astrologers all along.

Well that explains the shift in the dates, but what of Ophiuchus?

The sun had always been crossing Ophiuchus, even during the time when astrology was first written down. The sun would intersect with Scorpio and Ophiuchus for about eight days each, before to moving on to Sagittarius. Having thirteen signs was rather lopsided and awkward, and was difficult to properly match with other archetypes such as elements. So the Babylonians decided to eschew Ophiuchus and keep Scorpio, probably because Scorpio look more “aligned” with the other signs.

Several western astrological systems do use Ophiuchus though, further splitting the Sidereal system into either one that incorporates Ophiuchus or one that ignores it.

This recent zodiac sign crisis was no doubt a result of sensationalized media and distortion of information augmented by the force of the internet. Perhaps we should learn a bit more before taking the media’s word, or before heeding random predictions about behavior based on arbitrary shapes from a changing mural of stars.

An illustration of Precession and Nutation*Other planetary motions related to precession that could influence the ecliptic** include nutation and polar wander. Nutation is a repeated “nodding” of the earth, and causes the rotational axis to trace a circle with “frills” instead of a smooth circle.

Polar wander is when the geographic north pole, and therefore the south pole, slightly changes its position. Both these motions are very small and slow, and only make noticeable changes over a long time. There are many more quasi-periodic movements that the earth undergoes, and collectively these are known as the Milankovitch cycles.

From Wikipedia: The Earth in its orbit around the Sun causes the Sun to appear on the celestial sphere moving over the ecliptic (red), which is tilted on the equator (blue).

**The apparent path the sun takes across the sky as we see it from here on earth over the course of one revolution is called the ecliptic. In other words, the constellation that the ecliptic intersects with on that date is that day’s appointed zodiac sign. The ecliptic lies in the ecliptic plane. In the image to the right, The large red circle is the ecliptic.

The ecliptic gets its name from the fact that when the moon intersects with the ecliptic, an eclipse would occur. (if the moon gets in front of the sun’s path in the sky)

***Our leap days are there to correct how long it takes for the earth to revolve one period around the sun, which is approximately 365.25 days. This is known as a sidereal year. Due to planetary motions such as axial precession and nutation, we would notice that the dates and positions of the equinoxes and solstices would change, and therefore we would slowly see the seasons come earlier and earlier in the year. This would be rather disorienting for humankind, when snow starts falling in Japan in June. The effects of precession and nutation are corrected for, however. This explains the existence of an additional rule in determining leap years, where all years that are multiples of 100 are not leap years, unless they are multiples of 400. On average, the time it takes to get from one summer solstice to the next is about 365.242 days, slightly shorter than the sidereal year. This is known as a tropical year.

12 comments

  1. "The ecliptic is not to be confused with the ecliptic plane, which is the plane where the earth’s orbit lies on. Due to the axial tilt, the constellations on the ecliptic plane are not the same ones that intersect the ecliptic."

    I didn't get this one. Why would the axial tilt have a bearing on what constellation intersects the ecliptic?

    cheers!

    • The ecliptic is dependent on how we people on the earth see the sun up above. Since the earth is rotating we see the sun move across the sky. We see the sun move when we look up in the sky, and this path is called the ecliptic. I think it is clear that the earth's axial tilt would change the way we see the sun in the sky.

      Try this: Hold up an object in front of you, maybe about four inches from your eyes. With one eye closed, look at the object and turn your head slowly left and right. You'll notice that it's like the object is moving left and right against the background. Now, tilt your head and turn your head left and right again. You'll notice that this time, the object is moving diagonally against and background.

      This is how axial tilt influences the ecliptic.

      There are additional clarifications with the image above, I hope that helps.

      • That would be true but the zodiac will not change with respect to the ecliptic plane. The sun appears to travel along the ecliptic because the ecliptic is the intersection of the ecliptic plane with the celestial sphere, with the sun and earth both lying in this plane, but the stars (and the zodiac) are "fixed". The axial tilt affects how "low" or "high" is the ecliptic vs. the horizon but the zodiac will still be along the ecliptic. We may see some "effect" of the orbit of the earth with how we see the sun against the zodiac (like in your reply above) but not the axial tilt.

        • Actually, I doubt my own sentence above about the earth orbit… Try to imagine the solar system with respect to the zodiacs. We can assume this configuration is fixed. Now put the earth and let it move around the sun. The sun will appear to travel through the zodiacs as the earth revolves around the sun. Now, tilt the earth slightly, what will change is the view from the earth how "low" or "high" the ecliptic will be vs. the horizon but the zodiacs will still appear in their "fixed" location with respect to the ecliptic plane. The zodiacs were so chosen because they lie along or close to the ecliptic.

          • but the earth's wobble goes around a conic path… so the perspective changes on all X,Y, and Z axis. wouldn't this also mean that the zodiac's perceived position relative to a point in the horizon would also float around on all three axis?

          • Yes it does affect how high/low are the zodiacs/sun vs. the horizon but the sun and zodiacs will be affected in the same way. If the wobble of the earth makes the sun just a wee bit lower in the horizon than without the wobble, so does the zodiacs in the same degree and in the same direction. The wobble of the earth has a more perceived effect on the location of the north star.

        • I'll have to correct myself in the article: the ecliptic lies in the ecliptic plane. I think that may have been confusing.

          However, the axial tilt will influence what constellations intersect with the ecliptic. The zodiac is defined as the constellations that line the ecliptic. If I tilt the Earth sideways like Uranus, we would observe a whole new set of constellations that the sun will cross.

          The ecliptic plane makes a 23.5 degree inclination with the orbital plane, and that is due to axial tilt.

          • Ah, there lies your error. Whether you tilt the earth 23 degrees or like Uranus (tilted by 98 degrees) doesn't change where the zodiacs lie wrt the ecliptic, and so the sun will still travel against this same backdrop of stars. You look up to the sun, and whether it lies low in the horizon in winter (if you are in the northern hemisphere) or high in summer, the zodiacs also lie low along with it.

            Another way to visualize this is, if the ecliptic plane is a flat circle, the ecliptic is the edge of this circle, with the sun travelling along this edge, and with the earth "fixed" at the center. You can divide the edges of this circle by 12 arcs corresponding to the zodiacs. (There goes the effect of the earth's orbit with respect to viewing the zodiac as the earth's orbit is an ellipse.)
            The tilt of the earth only affects how "low" or "high" the sun/zodiac will rise/set vs. the horizon.

          • Yes. The equatorial plane and the ecliptic plane are not the same but they align twice yearly during the equinoxes. During the equinoxes, if you are standing along the equator, say somewhere in Indonesia, the sun will be directly overhead at noon (sun's zenith). During the winter solstice, if you are again in the same spot in the equator, the sun at it's zenith will be lowest on the horizon compared to other noons of the year on the same spot and it will be a bit southerly. At this time, if you are at the same longitude in the northern hemisphere, along the Tropic of Cancer, the sun will be directly overhead. Additionally, the sun rises and sets a bit southerly. In the summer in the northern hemisphere, during the summer solstice, the sun's path (ecliptic) is most northerly. These are just some of the observational effects of the earth's tilt relative to the ecliptic. Other ovservational effects of this tilt is in when do we see Venus and Mercury – sometimes at sunrise and at other times at dusk (Mercury's and Venus' own orbits around their own orbital planes which are not the same as the ecliptic plane also contribute to how/where we see the two planets)

  2. thanks for clearing that up. The article on GMA News seems to have gotten it wrong. http://www.gmanews.tv/story/210677/think-changing
    quote: "the Earth's precession or "wobble" around its axis, caused partly by the Moon's gravitational pull, makes the years grow longer over time. "

    their article seems to make it out that our years were getting extra time so Ophiuchus was inserted to fill in the gap. so Ophiuchus was just one of the many early constellations pruned away only as a matter of convention.

    Now we open another can of worms when talking about the Milankovitch cycle. Is it having more of an effect on global climate change than man-made factors?

    • Yes, that would be a very long discussion. The Milankovitch cycles no doubt influence the climates in our planet, as well as many other natural phenomena. We are experiencing global warming, but can we can so quick to attribute it to man-made factors? The IPCC's stance, and therefore the official stance, is yes. It is based on computer simulations and observations regarding the earth's temperature in modern times. It is a fact that gas emissions from vehicles and such do in fact contribute to the greenhouse gases, which help trap heat in the planet.

      However, studies related to the causes of global warming are being conducted, and it is obvious that there is more to it than just increasing concentrations of greenhouse gases due to human actions. Whether the global panic due to global warming is justified is also one of the questions being researched. The predictions of the IPCC are definitely being questioned. The earth's ecological systems are extremely complex, with numerous factors that need to be taken into account before we make a sweeping conclusion.

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