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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.

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