Scientific Astrology

You may have heard about a story circulating around the internet about astrology being wrong. In short, an astronomer named Parke Kunkle talked to the Minneapolis/St. Paul Star-Tribune about how astrology’s dates bear little relation to actual positions of the constellations and that there is a 13th constellation that isn't included. It doesn’t seem like he intended it to be big news, but it made quite an impact on the internet all the same; doing a Google News search for "astrology" turns up hundreds of articles on the topic.

If you are like me and don’t pay any attention to astrology, this did come as a revelation: why wouldn’t a system with the prefix “astro-“ and using the names of constellations have to do with the positions of the stars?

The Sidereal Zodiac

There are actually two basic ways to ‘do’ astrology, sidereal (based on the stars) and tropical (based on the seasons). Sidereal astrology is based on the positions of the planets, the moon, and the sun in relation to the essentially constant background of distant stars and the constellations they form. The “sun sign” of sidereal astrology would be the constellation that the sun would appear to be in at your birth. According to astrology, the constellations that the other planets and the moon find themselves in could also have meaning.

(Image by Tau’olunga from Wikimedia Commons)

It just so happens that there are 12 major constellations along the ecliptic—the path that the sun (and, because they are all in roughly the same orbital plane, the planets and the moon) follows across the sky over the course of a year (red, in the diagram above): Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpio, Sagittarius, Capricorn, Aquarius, and Pisces. When the Babylonians invented the system of astrology that we are most familiar with, they set the astrological calendar to start at the vernal equinox, when the sun was in Aries. They then divided the year into 12 equal segments named in the order of the constellations along the ecliptic so that the signs roughly corresponded to the constellation that the sun was appearing in. Although it should be noted that the constellations do not occupy equal portions along the ecliptic, meaning that each sign should theoretically last different lengths of time. The 13th constellation cited in the above controversy, Ophiuchus, also crosses the ecliptic (as defined by the internationally agreed upon constellation boundaries) but the major stars of the constellation are somewhat distant from the ecliptic (you can see this for yourself using the WorldWide Telescope), though it is possible that 12 was just a nice number that divided evenly into 360°.

The Tropical Zodiac

Originally, there was no distinction between a zodiac based on the background stars and one based on the vernal equinox. But there was a slight problem with basing it on the equinox: precession of the Earth’s axial tilt (see next section) causes the location of the sun during the vernal equinox (where the projection of the Earth’s equator crosses the ecliptic, at the arrow in the above diagram) to shift ‘backwards’ relative to the sun’s annual apparent motion. At this year’s vernal equinox, the sun will be located within the Pisces constellation, as it has been for many years, and will within the next ~100 years end up in Aquarius (and hence “Age of Aquarius”). The net result is that the entire astrological calendar is increasingly misaligned with the actual constellations. The tropical Babylonian system stuck with the vernal equinox, while other sidereal systems maintain the alignments to the constellations.

(Image by Dbachmann from Wikimedia Commons)

Precession

What’s causing this movement? The Earth of course is not spinning perpendicularly to its orbital plane; instead, it has a ~23.5° tilt, which is responsible for generating our seasons. But because the earth is not perfectly spherical (it bulges out at the equator), the gravitational interaction of the sun and moon with the Earth apply a torque (rotational force) to the earth. This force causes the axis to slowly shift around the perpendicular position, or precess (see diagram below). And because the direction of the tilt defines our seasons, this causes the stars to slowly shift along the ecliptic from year to year.

(Image by NASA)

None of this is news to astrology "experts", but it certainly came as a surprise to casual followers of astrology, including the at least 25% of Americans believe in astrology (as of 2001). As I said before: Why wouldn't astrology be based on the constellations it claims it is?

What’s wrong with this whole picture?

So astrology claims that any person’s personality and even the future can be determined, or at least strongly influenced, by the positions and movements of celestial objects. Let’s pretend for a moment that positions of the heavenly orbs do, in fact, modify some undefined mystical cosmic energy flow that affects life on Earth. Alright. Why would this ‘energy flow’ divide into 12 distinct and evenly-spaced sectors along the ecliptic? Why would the characteristics of the ‘energy’ in a given sector have any relation to an imaginary figure in the stars based on human mythology? And if they are related, why wouldn’t the sections shift with the shift of the constellations? Add to that the fact that there is no clear reason why the birth date, especially in an age of induced and suppressed labor, and C-sections should so sharply divide one type of person from another if it falls on a border between signs. Now let’s stop the silliness of thinking that there is some vast cosmic energy flow, because there is no evidence that such a thing exists.

Astrology as practiced simply has no validity. I would hope that this whole internet firestorm will make more people realize just how inane the whole idea is. But I don't hold out much hope for that.

What’s a budding young astrologist to do?

My advice to someone who truly believes that the movement of the planets, moon, and sun affects life on Earth is this:

1. Throw out everything you think you know about astrology and start from scratch. That mythological baggage is holding you back.
2. Collect copious amounts of information about the personalities and birth dates and locations of anyone and everyone—you’ll need a *lot* of data from all age groups over an extended period of time. Make sure you get a statistical cross-section of the population, preferably world-wide, though a single country would do to start.
3. Recording national and world events, crime statistics, and the like would also be useful, going back as far into history as you can.
4. Run an analysis of all of your data against the measured (or predicted, if necessary) positions and movements of all of the planets, the moon, and the sun. If you’re looking for energy-flow stuff, you’ll need to control for mundane sun and moon effects. And you’ll also want to control for people who strongly believe in current astrology, as that could bias your findings.
5. Examine any strong correlations that result. Rigorously verify that the strong correlations can’t be accounted for by other unrelated activity.
6. Take your new correlations and see if they hold up in the future.
7. Congratulations! You now have a scientifically-rigorous astrology, and probably have some hints as to what could be causing the correlations, because that would lead to new physics. So congratulations on the Nobel Prize as well!

Is there a kernel of truth?

In fact, there may well be some small truth to the tropical zodiac’s sun sign. Gestation and infancy are important formational times in any person’s development, and it would not be so surprising that these processes could be impacted by the changes in the environment caused by the seasons.

The most direct effect of time of year, and the one most constant from one year to the next at a given latitude, is the amount of light in the day. In fact, a study published in Nature Neuroscience this month by Ciarleglio et al. demonstrated that the initial photoperiod (length of daylight during a day) that newborn mice are exposed to ‘imprints’ their circadian rhythm, which is controlled by a group of neurons in the brain called the suprachiasmatic nuclei, and that this imprinting affects the later ability of the mice to adapt to changes in the length of daylight. However, as the authors note, they only examined the mice after seven weeks, and can’t make the conclusion that it necessarily lasts for the entire life of the organism. It is hard to say how altering the circadian rhythm might impact other aspects of behavior. And of course not everything found to be true in mice holds true for humans, but the circadian rhythm system is fairly well conserved (that is, it hasn’t changed all that much) over evolution, so it is fairly likely that the same thing happens for us.

However, even if a person’s personality is affected by the seasonal conditions into which they are born, people at different latitudes (and especially those on opposite sides of the equator!) would experience very different conditions from each other, and you can be assured that the effects the seasons have on personality have absolutely nothing to do with the constellations in the sky, and they certainly can’t predict the future.

Unless otherwise noted, the information for this post was obtained from Wikipedia information that was properly cited to an external source.