For at least the second time in just a few years, astrology fans all over the internet are freaking out over a shift in the starry skies. A NASA blog post for children that explains constellations and the zodiac recently made the rounds, leading some believe that the space agency has thrown the astrological calendar completely out of whack. But there’s no reason to blame NASA if your zodiac sign has shifted—it's the nature of the stars themselves.
First things first: Unlike astronomy, astrology is not a science. It is a system of belief that a person’s character and future are based on the position of the stars. Most astronomers recognize some version of zodiacal constellations as the Babylonians originally drew them about 3,000 years ago. But like all constellations, they are essentially arbitrary designs picked out of the sky, Ben Guarino reports for Washington Post. These constellations appeared significant to ancient stargazers primarily because they could draw an imaginary straight line connecting them to the Earth through the position of the sun.
As NASA researchers wrote in a recent Tumblr post:
They divided the zodiac into 12 equal parts—like cutting a pizza into 12 equal slices. They picked 12 constellations in the zodiac, one for each of the 12 “slices.” So, as Earth orbits the sun, the sun would appear to pass through each of the 12 parts of the zodiac. Since the Babylonians already had a 12-month calendar (based on the phases of the moon), each month got a slice of the zodiac all to itself.
However, the Babylonians didn’t realize that the Earth’s rotation was a bit wobbly, which would throw their drawing off after a few thousand years of spinning through space. As a result, the position of these stars in relation to our planet has shifted somewhat over the millennia, meaning the star groups charted by these ancient people don’t quite appear at the same times anymore, NASA’s SpacePlace blog points out.
"We didn't change any zodiac signs, we did the math,” NASA researcher Laurie Cantillo tells the BBC. “NASA reported that because the Earth's axis has changed, the constellations are no longer in the same place they were thousands of years ago.”
Some astrology fans may be wringing their hands over this, but if they were to consider all constellations that could fit the definition of a zodiacal sign, things only get more complicated. According to Slate’s Phil Plait, there are 21 different constellations that fit this arbitrary definition—they have just long been left out of astrologers’ star charts.
This isn’t the first time astrologers have gotten worked up over shifting star charts—back in 2011, nearly the same news broke when an astronomer suggested that most fortune-tellers had been neglecting a 13th star symbol known as Ophiuchus. This constellation, which was first identified by the ancient Greeks and is also called the “Serpent-Bearer,” would fit neatly into a zodiacal slot between November 30 and December 17, as the Minnesota Star-Tribune wrote at the time.
Astrology may be a fun pastime for some, but when it comes to predicting the future, stars aren’t the best of judges.