Exposing PseudoAstronomy

June 20, 2010

Astrology vs. Astronomy: What Coordinate System to Use?


Introduction

As probably anyone who reads my blog knows, I have recently done a few blog posts related to astrology, specifically criticizing the astrologer Terry Nazon, both for her bad astronomy and for her threats and harassment towards me and a fellow blogger. My compatriot and much more famous and well known anti-astrologer (among other things) Phil Plait was even kind enough to mention me on his blog, though I’ll admit that I did point out my problems with Ms. Nazon.

This post is not about Ms. Nazon. This post is about astrology in general, and specifically whether my criticisms have been fair given that astrologers – some Western astrologers, anyway – use a different coordinate system and different definitions than the world’s astronomers.

What Is Precession?

Also, those who have been reading my blog know that I’m teaching an introductory astronomy class this month for college undergraduates. The second and third day of class were spent going over the motions of the the sky — the stars, sun, planets, moon, etc., over the course of one day, several days, a year, and then thousands of years.

One might think that going to thousands of years is a bit much — shouldn’t the sky look the same year after year? Actually, no. Earth spins on its rotational axis once every 24 hours. It goes around the sun once every 365.25 days (approximately). And during that time around the sun, the rotational axis is pointed at the same spot in the sky (hence why we have seasons).

That’s the year-after-year motion. However, this rotational axis direction does change. Just like a top spins on its axis, but then that axis wobbles in a circle, so does Earth’s axis. Over the course of approximately 26,000 years, Earth’s rotational axis makes one complete “wobble,” returning to the same spot it was before in the sky. This is the mechanism behind what you may have heard about the ancient Egyptians — they had a different North Star than we do today.

You also may have heard that many thousands of years ago, Vega was our North Pole Star, and that it will be again in many more thousands of years. This is all due to precession.

On a short timescale, say, a person’s lifetime, precession will not be noticeable in any meaningful way. But, astrology was not invented one person’s lifetime ago. Western astrology was codified nearly 2000 years ago by Ptolemy, but its origin really dates back more towards ancient Babylon, around 2500 years ago. 2500 is nearly 1/10th of 26,000, and that is a significant fraction.

Practical Effect of Precession

A practical effect, besides the north pole star (and south pole star) changing, is that the sun will appear to be in a different part of the sky on the same date from one year to the next.

For example, this year on the vernal (Spring) equinox, the sun will be in the constellation Pisces. But, 2500 years ago, when Western astrology was being developed, the sun on the vernal equinox was in the constellation Aries. And in roughly 700 years, the sun on the vernal equinox will be in the constellation Aquarius.

The bottom-line: The stars have moved!

To Precess or Not to Precess?

Pretty much all the criticisms of my posts about Ms. Nazon’s astrology focused on the basic fact that many western astrologers do NOT take into account precession. Astronomers, on the other hand, do. And so do many other western astrologers. So is it fair of me to criticize when we’re not, as the saying goes, comparing apples to apples?

In different terminology, astrologers use what is called the “tropical” zodiac instead of the “sidereal” zodiac. (At least, based upon my understanding these terms are interchangeable — tropical zodiac is non-precessed, sidereal is precessed.)

Definition of a Constellation

Another criticism I received is that astrologers do not use the same constellation system that astronomers do. As far as the Zodiac is concerned, astrologers have divided it into 12 equal parts, with each constellation exactly 30° (360° in a circle / 12 = 30°). Astronomers, on the other hand, follow the definition of the International Astronomical Union, as codified in 1930. The constellations do not have fixed 30° intervals, and they of course move with precession.

Here is an area will I will definitely admit that my criticism should at least have been tempered, somewhat. After all, constellations are arbitrary boundaries around pareidolia-ed points of light in the sky as seen from an arbitrary vantage point. Who’s to say that the astrologic system of dividing the Zodiac is any better or worse than the IAU’s?

However, I will unequivocally state that my criticism was not specifically about her boundaries, but more that she was off by about 10% of the sky (precession), rather than a degree or two due to different constellation boundaries.

What Does Astrology Claim to Be?

Don’t worry, I’ll get back to the discussion of precession and the criticism I received …

Believe it or not, I had difficult finding this — what the real root belief is about astrology — so I am going off of what I have read on many astrologers’ websites and my own understanding. So, please correct me if I’m wrong here:

The basic premise of astrology is that the apparent arrangement of stars, planets, moons, the sun, and other objects in the sky, as observed from Earth, all will have an effect on a person (or nation, or company, or whatever) based upon where they were relative to each other on the minute the person/nation/company/thing was born/created, and then where they are now (“they” meaning the astronomical objects).

Okay, even with my rather loose understanding of an exact mechanism (and no two astrologers actually agree on a precise mechanism — but that’s a different post), I think most would agree that that’s the basic idea of how astrology is supposed to work.

So, you need to know, say, where the planet Venus is in the sky. And what constellation it’s in, so you know what personality traits will be taken on. Among other things. Right?

Returning: To Precess or Not to Precess?

Told’ja I’d get back to this. So we need to know where in the sky certain objects will be. From Earth. After all, most of us were probably born on Earth (that’s another blog post, too …).

So shouldn’t you take into account precession? If I look up at the sky, right now, I see Mars in the western sky deep within the constellation Leo. As in, the planet Mars is, in Earth’s sky, physically surrounded by stars in Leo.

Contrast that with a non-precessed sky. This time of year, around 1 B.C., Virgo is squarely in the patch of sky that is currently occupied by Leo. So instead of Mars being surrounded by Leo, it’s surrounded by the stars of Virgo. Now, I don’t pretend to know what that would signify, but I would think that the God of War being surrounded by the Lion is different from being surrounded by the Virgin. Wouldn’t that change your readings?

Besides just this, and I apologize if this may seem to be a silly question, but doesn’t it actually matter what you can see outside versus what you imagine based on what the sky looked like over 2000 years ago? Virgo’s not where it was 2000 years ago … doesn’t this change things?

Final Thoughts

Alright, I realize that this post may seem condescending to some, ignorant to others, or stupid to a few more. But I actually am in all honesty, legitimately curious here. Especially because I know that western astrologers themselves are divided over whether to take into account precession.

Maybe it’s just the way I was trained in science — I think that if stuff moves, you move with it. It doesn’t make sense to me to use something as it was 2500 years ago when everything else has changed. To me, it seems the same as, say, starting to write on a piece of paper 15 minutes ago, and you still writing away in the same spot even though someone has moved the piece of paper far away from you. You’re just writing in air now.

And that’s why I feel justified in criticizing Ms. Nazon, and by proxy, all other astrologers who do not account for precession. She’s writing on air or her desk, not the paper that was there 15 minutes ago. The stars haven’t been where she says they are for thousands of years. Shouldn’t that matter?

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