Introduction
If you keep up with the casual horoscope crowd, you may have heard recently that your zodiac sign has changed thanks to the constellations moving over, or that there’s a 13th sign now. For those who didn’t know any better, the news about changing signs sounded like the foundations of astrology had been shaken. However, nothing has changed for the serious astrology community, because zodiac signs are not calculated from the constellations.
the culprit
This conversation was most recently brought up by a post on NASA’s Tumblr, which is based off a much older webpage on NASA’s Space Place. This older page briefly discusses the zodiac constellations as an introduction to NASA’s Star Finder tool. It explains that the constellations around the earth on the tool do not line up with your zodiac sign, even though they’re represented with the same symbols. The page then proceeds to speculate on ancient Babylonian cosmology and compare modern astrology to fairy tales.
Until January of 2016, there was also a link to find out what your new sign would be, presumably taken down following backlash from the astrology community (though personally I think this tool could be fun and informative if framed properly). The claims about people’s zodiac signs changing also been removed, but the site maintains its underlying misunderstanding of how signs are calculated. Though the zodiac signs share a name with constellations because of the historical relationship between astrology and astronomy, the Sun moves through the zodiac signs based off the seasons, and the stars don’t factor into this at all.
tropical and sidereal astrology
Writers of this page assume that modern Western astrology is a sidereal system, that is, that zodiac signs are calculated from the position of constellations in the sky. Western astrology is in fact calculated from a tropical system, that is, one based off of the seasons. Some kinds of astrology utilize the constellations, namely Jyotish, but the sidereal vs. tropical question is a nuanced topic that reaches far beyond the scope of this blog post.
the tropical system
In tropical systems of astrology, a persons Sun sign is calculated from the interaction of the ecliptic and the celestial equator. The ecliptic refers to the path the Sun appears to take around the Earth and forms an ellipse in the plane of the Earth’s orbit. The celestial equator is a giant circle formed when the Earth’s equator is projected out into space.
When the ecliptic meets the celestial equator we have an equinox. The equinoxes correspond to the beginning of spring in one hemisphere and fall in the other, as well as certain to zodiac signs. When Sun is furthest away from the celestial equator, we have a solstice, which corresponds to the beginning of winter or summer. The Sun enters a new sign during each solstice as well.
Because certain zodiac signs line up with the beginning of the four seasons, the tropical system is also called a seasonal system. The term tropical refers to the Sun’s interaction with certain latitudes on earth: during a solstice the Sun appears directly overhead at either the tropic of Capricorn (about 23.5° South) or at the tropic of Cancer (about 23.5° North).
The solstices and equinoxes divide ecliptic into four seasons, and astrologers further divide it into 12 equal signs. With this distinction the Sun passes through 3 signs per season, spending about 30 days in each sign. The signs that line up with a solstice or equinox are called cardinal signs because they guide our division of the ecliptic. These signs are Aries (corresponding to the spring equinox in the Northern hemisphere), Cancer (summer solstice), Libra (fall equinox), and Capricorn (winter solstice).
Not-Actually-fixed stars
As the NASA site will tell you, in ancient times the Sun would appear to rise in front of a particular constellation during a certain time of year. For example, the Sun once reached its zenith in front of the constellation Capricornus during the winter solstice, so for the first ~30 days of winter, the Sun is in Capricorn. However, the fixed stars change positions in the sky over thousands of years thanks to the tilt of the earth’s axis, a phenomenon called precession. This means that the constellations don’t line up with the seasons the way they did when the 12 signs were first determined, which can get a little confusing.
The writers at NASA do get close to realizing Western astrology is a tropical system, as they mention that the 12 zodiac signs lined up with the ancient Babylonian lunar calendar. However, they argue that this association is a scientific oversight on the part of the Babylonians and therefore all astrologers, not even mentioning that the practice has changed significantly over the last few thousand years. They also bring up the fact that the constellations don’t take up an even 30 degrees of the sky each and argue that Ophiuchus the Serpent-Bearer should be a part of the zodiac as well, since his feet rest in the line of the ecliptic.
In other words, these writers understand on some level that the Sun moves through the zodiac signs in accordance with the seasons. And they clearly understand that there’s a difference between astronomy and astrology, starting the page off with “astrology is not science. It’s something else.” But they’re stuck on the fact that the signs were named after constellations thousands of years ago and can’t accept that modern astrology is a seasonal system.
Conclusion
The writers of this NASA page simply don’t understand what a tropical system is. The modern Western zodiac is a calendar that lines up with the solstices and equinoxes. The months of this calendar have similar names to constellations because of the historical relationship between astronomy and astrology, but that’s where the relationship ends. One is a branch of physics, the other a kind of divination.
But the writers of this page don’t leave the distinction at that. They oversimplify astrology’s origins to portray it as nonsense, showing how little they care to do even basic research. I know that some of these simplifications are due to the fact that this website needs to be accessible to children and use simple language. Still, you can distinguish astronomy from astrology without calling one of them “make-believe” and “fantasy stories.”
At the end of the day, the writers of this NASA website grossly misrepresent astrology in their efforts to explain why your zodiac sign doesn’t line up with the constellations. They’re convinced that astrology should correspond exactly with the constellations, never stopping to ask if there’s a reason it aligns with the seasons instead. I’m not saying you have to believe in astrology, but if you’re not going to do basic research into an area outside your expertise, you shouldn’t talk about it in the first place.
Astrology by Johnny 