As an astronomer — not an astrologer — I have to tread carefully when it comes to zodiac signs.
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 a source of age-old wonder
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 yearlong 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 this week, I'm sorry to say — you are not actually a Taurus!
The sun wasn't in your constellation when you were born. Thanks to precession, it was really in Aries. 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?"
Yes, we're talking about the Age of Aquarius.
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 April 24 to May 14, 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 when you pass someone on the highway.
"You're both driving forwards. You're both going at pretty high speeds. But if you're passing them, you're going at a slightly higher speed, and it looks like they're going backwards," she says. "It's like both planets, the Earth and Mercury, are still moving forward in their orbits around the sun, but the Earth just overtakes Mercury for a short time."
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.
