Ophiuchus

Note: I published this on my website back in 2021, removed it, and then lost the original publication date.

Every few years people are weirded out by the news that there are 13 zodiac constellations. However, with a bit of a thought experiment and applying some logic, it's easy to understand why astrology only uses 12 zodiac signs. 

Astronomers have always understood that there are 13 constellations on the ecliptic. And no, NASA didn’t suddenly discover this fact. The ancient Greeks knew about it. Astrologers were aware of this too. For example, Vettius Valens, a second century CE astrologer, specifically mentions Ophiuchus (Ophiouchus in the translation) when talking about how to locate parts of the zodiac in relation to other constellations. Here's how he describes the position of Taurus in the night sky.

The constellations that rise with it are (in the north) the rear of Auriga and (in the south) the rear of Cetus and the first section of Eridanus. Venus, the moon, Ceres, Mars, and Mercury. The constellations that set are (in the north) Bootes up to the belt and the leg of Ophiouchus up to the knees. In the south Orion rises with Taurus; he is belted around the waist, extends his sword in his right hand, and holds in his left hand the so-called caduceus. Vettius Valens, Anthologies, Book I (translation by Mark Riley).

So, we have to ask ourselves, if ancient astrologers knew about Ophiuchus then why are there only 12 zodiac signs used in astrology?

A thought experiment in regards to this topic can highlight the issues that ancient astrologers probably faced as they developed their methodology and why it actually makes logical sense to exclude Ophiuchus. They already knew a number of key astronomical facts. They were aware that the Sun followed a path across the sky from the observer's point of view on Earth. This is known as the ecliptic. The ecliptic is a circle (360 degrees). Which is why in Western astrology, your natal chart is presented as a circle. It’s 360 degrees and represents the ecliptic.

The next step for an astrologer (or an astronomer) would have been to figure out where the constellation boundaries were located on the ecliptic. In Babylonian times, and in ancient Greece and Rome, it wasn't exactly clear where the boundaries were. For example, Pisces is clearly a much bigger constellation than Aries. But where does Aries start?

Defining the boundaries of the constellations has been a head scratcher for thousands of years. If the constellations are different sizes, what exactly are the sizes? How can the zodiac constellations be divided on the ecliptic?

In the following IAU map you can see that on the ecliptic (represented by the blue line), Aries is on the very edge of the Pisces tether. (For an interesting article about the problems of defining boundaries and even the shape of a constellation you can go here.)

Looking at Pisces, how do we decide how much of it should occupy the ecliptic? What are the actual measurements? Is it only the portion of the constellation that resides directly on the ecliptic, or the entire width? This remained a question without an obvious answer until the 20th century when the IAU (International Astronomical Union) issued their first boundary definitions in 1930. I've rounded the figures, but you can see in the following table that Virgo is the largest constellation when measured in terms of how much space it occupies on the ecliptic, and that Scorpio is tiny.

If you’re an astrologer living thousands of years ago, you have some decisions to make. In his book Hellenistic Astrology, Chris Brennan highlights that early Babylonian horoscopes actually had variable constellation widths.

By the fifth century BCE, the zodiac became standardized to consist of twelve signs that were exactly thirty degrees in length each. Prior to this time, the zodiac was uneven because the zodiacal constellations that fall on the ecliptic have different sizes, with some being very large (e.g., Virgo) and others being relatively small (e.g., Cancer). (p. 4)

Babylonians astrologers were clearly aware of the differences in the zodiac constellation sizes, and were trying to account for this difference in their astrological charts. But there would always be the problem of how this information could be transmitted to other astrologers and students. And if the zodiac constellations on the ecliptic have variable widths, there needs to be an agreement on the measurements. Even if your fellow astrologers agree, they have to commit the figures to memory. Although the written word existed (for example, cuneiform), few people could read and write. Then there is the problem of Ophiuchus and Scorpio. The ecliptic passes through one of Ophiuchus’s feet and Scorpio occupies a miniscule amount of space.

In the IAU boundary map below, you can see this region of space on the ecliptic. The ecliptic passes through Sagittarius, the foot of Ophiuchus, the top end of Scorpio and the beginnings of Libra. This part of the sky is crowded, to say the least.

An astrologer/astronomer prior to the invention of the telescope observed the sky with the naked eye, and they had to do all of their calculations manually. A key part of astrology (and astronomy) is being able to predict the position of a planet without having to make an observation every single night. Ancient astrologers would have been faced with two choices when trying to define how to slice up the ecliptic in a way that allowed them to predict which zodiac sign a planet would fall into without having to make observations. Their first choice would be to define the width of the zodiac constellations on the ecliptic. The second choice would be to divide the ecliptic evenly. As mentioned in the previous paragraph, for the first choice everyone would need to agree to the measurements and commit them to memory. The simpler solution is slicing the ecliptic evenly, dispensing with the need to remember the size of each zodiac constellation.

The next problem is that thirteen constellations do not divide evenly into 360 degrees. The slices wind up measuring 27.6923076923. However, there’s an elegant solution to this tricky issue. Dividing the zodiac constellations into 30 even degree slices results in 12 slices. Ophiuchus and Scorpio merged together results in a slice of approximately 25 degrees anyway.

The formula becomes 360 degree ecliptic / 30 degrees = 12 slices. Extremely easy to remember, and easy to write down. Merging the tiny Scorpio with Ophiuchus and assigning a common meaning to both constellations takes care of the significations.

Problem solved!