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Want to learn more about how to do astrology? You've come to the right place!

Ophiuchus Is Not A Sign: Here's The Difference Between Sidereal and Tropical Astrology (And What the Skeptics Never Tell You)

Ashley Thiessen

Time to learn an important fact about astrology: there is more than one kind!

You may or may not remember back in 2011 when news started circulating about a new 13th zodiac sign, Ophiuchus, along with a list of new zodiac signs dates that more accurately reflected how much time the sun spent in front of each constellation on the ecliptic. This information was not new at the time - Snopes even has a page which mentions they've had people asking about it since 2002 - and it isn't new now, with the story going around again just last year. 

The crutch of this argument is based on the precession of the equinoxes and it goes something like this: as the Earth spins on an axis, it "wobbles" like a top, then a bunch of science talk, and the eventual result is that the where the sun rises on the equinoxes separates from the fixed stars in the ecliptic. So, for example, 2000 years ago on the spring equinox the sun was rising in front of the constellation Aries, but today it rises in front of Pisces. Western astrologers, however, will tell you that you are an Aries even though the sun isn't actually rising in front of that constellation on that day. Also, there are 13 (or even 14, depending on who you ask) constellations that the sun travels through. This new supposed Sign, Ophiuchus, isn't used by astrologers even though it obviously should be. What a bunch of crazies, right?

Popular skeptics have brought up the 13th zodiac Sign and the precession of the equinoxes for a long time. Among today's most popular voices are Bill NyeNeil deGrasse Tyson, Richard Dawkins, and even DNews. These are very high-profile names with a lot of weight behind them (well, at least some of them are) who are, unfortunately, educating people on astrology without knowing anything about it themselves. Which is particularly sad given that these are people who claim that their beliefs are based on reason and sound scientific research. Nobody did the minimal research required to learn the difference between a Sidereal and a Tropical calendar, or to learn that these are two systems for doing astrology with vastly different cultural and historical roots. So here I will tell you what the skeptics never do: Why Ophiuchus is not actually a Sign, and why the traditional Sun Sign dates given to you by astrologers are not actually wrong. 


A Sidereal calendar tracks how long it takes for the sun to travel from the beginning of one constellation all the way around the ecliptic to the same spot again. Sidereal (also called Eastern, Indian, or Vedic) astrology begins when the sun crosses in front of the constellation Aries, until it circles around to Aries again. According to the dates on that Time Magazine article, a modern sidereal calendar (with sections showing how much time the sun spends in front of each constellation) would look something like this:

A modern Sidereal astrological calendar (image made by the artist Kaleidoscope!)


On the other hand, a Tropical calendar measures the length of time between equinox and equinox. As you may remember from me talking about the Elements and Qualities, in Tropical (also called Western) astrology, Aries begins with the spring equinox, Cancer with the summer solstice, Libra with the fall equinox, and Capricorn with the winter solstice. Drawn as a circle, this divides the circle into 4, which is further divided into 12 equal parts, with each Sign being a section of the seasonal year. You can see this in the image below:

A Tropical astrological calendar (image also by the artist Kaleidoscope!)

A Tropical astrological calendar (image also by the artist Kaleidoscope!)

ptolemy's influence

See how there are two different calendar systems? One tied to the stars, and one tied to the seasons? All astrology began with a Sidereal calendar somewhere around 1800 BCE, when the stars on the ecliptic were first grouped into a sidereal calendar by the Babylonians. But Ptolemy switched to a Tropical calendar back in the first century CE, and all the West followed suit. As Nicolas Campion explains:

[Ptolemy’s] definition, whether by intent or through a misunderstanding of the zodiac as beginning with the sun’s location on the spring equinox, was to cause problems in the future because, by tying the zodiac to the seasons, he cut its ties to the stars, which gradually moved away from the signs. The Babylonian zodiac consisted of 12 equally sized sectors, but it was tied to the stars - it was sidereal. Eudoxus had located the vernal equinox at 8° Aries. In the first century Pliny had recorded that the sun on 21 March was at 8° Aries. However, according to Ptolemy’s calculations the gradual shift of the stars meant that the sun was now at 0° Aries on 21 March and his statement to this effect was taken by subsequent astrologers as an indication that the sun always occupied 0° Aries on the equinox regardless of the star’s movement. This phenomenon, known as the precession of the equinoxes, displaces the Babylonian/Persian sidereal zodiac at a rate of one sign every 2,156 years from the Ptolemaic, ‘tropical’ zodiac. The sidereal zodiac remained in use in India while the tropical zodiac was gradually established as the norm in the West, the consequence being that the two systems have diverged.
— A History of Western Astrology. Vol.I, by Nicholas Campion


Western astrologers use a Tropical calendar to practice astrology, and have for thousands of years. This means that each Sign is not a constellation, but a section of the seasonal year. Eastern astrologers use a Sidereal, star-based calendar to practice astrology; most of this group are found within Indian communities, or within the Hindu faith. These are two different systems based on two different calendars, and any arguments for or against astrology's scientific standing should be directed at the appropriate side. So if you have questions or arguments against astrology that have anything to do with the stars, just note that they will not be valid against the vast majority of astrologers in the West.