Our view of constellations has changed since they were first mapped

JUANA SUMMERS, HOST:

OK. We are not about to start forecasting your love life or your future, but we are turning to something that humans have observed for thousands of years. I'm talking about zodiac signs. As part of our Weekly Dose of Wonder series, NPR's Regina Barber is going to tell us about the real astronomy related to your horoscope.

REGINA BARBER, BYLINE: As an astronomer, not an astrologer - important distinction - I have to walk a fine line here. This topic doesn't win me a bunch of fans in my line of work, but the horoscope can be a gateway to the graceful movements within the night sky. Looking up, you may be used to seeing the same constellations during the same times of year. And that regular cadence is the basis of the zodiac. Speaking of which...

I have to ask that age-old question. What's your sign?

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MELISSA RICE: No bias here. It's the best sign.

BARBER: It is the best sign.

RICE: We are both fiery Leos.

BARBER: We are.

(LAUGHTER)

BARBER: That's my good friend and planetary scientist Melissa Rice. She's a NASA team member that has worked on all the Mars rovers except for the first. Here's how she defines the zodiac.

RICE: 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.

BARBER: And when you're born, whatever constellation is behind the sun during Earth's yearlong orbit is your sign.

RICE: And so if you are a Taurus, then the sun and the Earth would form a straight line pointing at the constellation Taurus.

BARBER: Over 2,000 years ago, the Babylonians mapped these constellations, and later the Greeks built on that and created the zodiac we have today. But here's the thing that might surprise you, and it delights me as an astrophysicist. The positions of the stars have changed since then, not because the stars themselves have moved but because the Earth's view of them has changed. Picture a spinning top. As it slows down, it starts to tilt, so it's no longer pointing straight up. Now the top is pointing at an angle, and that angle is tracing out a circle.

This is essentially what's happening to Earth. It's called precession. And all this happens very slowly over thousands of years, which means over time we are seeing a shifted night sky marching forward from our ancestors. Their spring constellations in May would appear in a different place overhead. And this is 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 towards at that time. Two thousand years ago, soon after the zodiac was created, we were in the age of Pisces.

RICE: And since then, everything has shifted forward one sign - so one age. And guess what age we're going to enter next?

BARBER: I don't know. What age?

RICE: The age of Aquarius.

BARBER: (Laughter).

RICE: It's true. It's true.

BARBER: So your zodiac sign has actually changed over the last 2,000 years. If you were born today back in ancient Greece, the sun was actually in front of Taurus during the day. But now, because of precession, this week the sun is actually in front of the constellation Aries. Sorry, folks. If your birthday is this week, your sign is really Aries. In fact, the North Star, or Polaris, won't be our North Star forever. In 12,000 years it will be another star, Vega, which is all to say our reality can change when perceptions shift. The stars aren't moving in the sky. It's the Earth's view of them that has changed. Regina Barber, NPR News.

(SOUNDBITE OF MUSIC) Transcript provided by NPR, Copyright NPR.