As 2024 approaches, some people might be looking at their horoscopes to see what the future holds. As an astronomer — not an astrologer — I have to tread carefully when it comes to zodiac signs. I find them fun, but what I really love is using these constellations to share actual astronomy with the public.
So let's set aside questions about your future or love life and instead look at why the horoscope can be a gateway to the graceful movements in our night sky — and why those movements could mean your birth sign has shifted. Check out the video below for a visual explanation, and read on for more:
But first of all, what is the zodiac?
It's the collection of constellations that go along with our horoscopes, but it is also the physical path that the sun takes in the sky over the course of a year.
Here's how planetary scientist (and full disclosure, my good friend) Melissa Rice puts it: "The zodiac is the 12 constellations that the sun passes through in its motions across the sky. And because there are 12 of them and 12 months in the year, it's convenient for us to associate time with the position of the sun in these specific constellations."
And when you're born, whatever constellation is behind the sun during Earth's year long orbit is your sign. "So if you are a Taurus, then the sun and the Earth would form a straight line pointing at the constellation Taurus," says Rice, who's also a NASA Team member and has worked on all the Mars rovers except the first.
Over 2,000 years ago, the Babylonians mapped these constellations and later the Greeks built on that work to create the zodiac we have today.
But here's what might surprise you — and what delights me as an astrophysicist: The positions of the stars have changed since then, not because the stars themselves have moved around, but because Earth's view of them has changed.
A new night sky comes into view
Imagine a spinning top that starts to tilt and then points at an angle. That angle is also tracing out a circle. This is essentially what's happening to Earth, what's known as "precession."
In fact, our planet's tilt is making this circle over the course of 26,000 years, which is why the North Star, or Polaris, won't be our North Star forever. In 12,000 years, it will be another star, Vega.
All of this means that over time humans are seeing a shifted night sky, marching forward from our ancestors.
So if your birthday is, let's say, Dec 1st, I'm sorry to say — you are not actually a Sagittarius, astronomically speaking! Which is what most horoscope sources would say you are.
The sun wasn't in your constellation when you were born. Thanks to precession, it was really in Scorpio. This is related to why horoscopes talk about certain periods of time, called "ages." Each age is associated with a specific constellation that the Earth's tilt is pointing toward at that time. 2,000 years ago, soon after the zodiac was first created, we were in the Age of Pisces.
"Since then, everything has shifted forward one sign, so one age," says Rice. "Guess what age we're going to enter next?"
That would be the Age of Aquarius, the age that lent its name to the 1969 hit song by the band the 5th Dimension. (Though, depending on who you ask, we may have already entered it.)
Mercury in motion
Finally, let's talk about the ominous phrase, "Mercury in retrograde."
This is a real scientific thing and it's happening right now.
From December 13 2023 to January 1, 2024, Mercury will appear to move in one direction and then, after days of observation, it will seem to slow down and move in the other direction. Over the course of three weeks, it will trace out a loop shape in the sky.
Rice compares this retrograde motion to cars on a racetrack.
"You're both driving forwards. You're both going at pretty high speeds. But the car on the inside is moving faster, and when it makes a turn before you do, it looks like it's going in the opposite direction," she says. "It's like both planets, the Earth and Mercury, are still moving forward in their orbits around the sun, but Mercury just looks like it's going backwards for a short time."
It's not an exact analogy — from our perspective on Earth, Mercury appears to go in the opposite direction when both planets are on the same side of the metaphorical track — but it gives you a sense of why this illusion exists. Space geometry! Anyway, here's a helpful video that illustrates what's happening:
Which is all to say, our reality can change when perceptions shift. These horoscope constellations aren't really moving in the sky — it's the Earth's view of them that has changed. So like life, in space, change can sometimes be a matter of perspective.
Editor's note: The story was originally published on May 4, 2023 as part of the science desk's Weekly Dose of Wonder series.
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