Season 17, Episode 45: What’s Your Real Zodiac Sign? (According to Astronomy)

Hey StarTalkians! Episode 45 of season 17 was a “Things You Thought You Knew” edition, where Neil explains common astronomical misperceptions to a usually bewildered Chuck. The title-fact comes from this discussion at the end of the episode:

Things You Thought You Knew – Your Zodiac is Wrong - StarTalk Radio

(from 45:37)

Neil’s explanation was great, but as an astronomer, he – understandably – didn’t know exactly how this affected Chuck’s zodiac sign. So here’s your new sign, along with a brief look at the physics.

Mandatory Astrology Disclaimer

Suffice it to say that the miniscule gravitational interaction between a distant constellation and you at the moment of your birth has no bearing whatsoever on your personality or what happens in your life.

Astronomical Precession and the Shifting of Zodiac Signs

The discussion of precession in the podcast mainly focused on the changing “north star” over time, but it’s the same idea. If you imagine a very (very!) large physical pole sticking out of the pole of the Earth, the tip would trace out a circle over a period of around 25,800 years.

Since the Earth has an equatorial bulge, there’s slightly more mass at the equator, and the Sun’s (and Moon’s) gravitational influence isn’t evenly spread across the Earth. This creates an effect a little like the wobbling of a spinning top as it slows down.

This slowly changes the point in the orbit where we get solstices and equinoxes. The solstice happens when the axis of the Earth points towards the sun. But precession means that this happens slightly earlier in our orbit each year, since the axis itself is also moving. The season-based “tropical year” is about 20 minutes shorter than the “sidereal year,” measured relative to fixed star locations.

Astrologers over two millennia ago didn’t really take account for this – they effectively assumed a sidereal year, where zodiac signs would remain fixed over time. But they actually cycle around with precession.

Plus, even the Babylonians knew there were actually 13 signs, including Ophiuchus as mentioned on the show. But they stuck to 12 signs, and split the year equally between them.

Your Accurate Star Sign

So if you like your pseudoscience to be as accurate as possible, check the pictures. For Chuck, his July 10th birthday would mean that he’s a Gemini, not a Cancer! Also note that there are only 6 days you could really be a Scorpio.