Thread by @JeniMillard

OK, here's the answer to the question: did NASA just ruin the zodiac and invent a constellation?

Tldr: no, Ophiuchus has always been there, it was just ignored when the signs were drawn up 3000 years ago. And due to precession, it's all out by about one sign anyway...

1/n

To answer this, we have to go back 3000 years to those cool cats, the ancient Babylonians, the first astronomers and astrologers (because back then, the two were intertwined, which is not the case now).

These guys were smart. They had a 12 month calendar, and they had (2/n)

noticed that the Sun and planets always seem to move through the same constellations.

Today, we know that is this because all the planets in the solar system orbit in the same flat plane, kind of like a CD. The axis of the earth is tilted (23.5deg) and this is what makes (3/n)

the path of the planets and Sun look tilted in the sky.

Anyway, back to the Babylonians. They noted the regularity of the paths of the Sun and planets, and even if they didn't know why, they thought 'hey, this looks important! Let's make it significant' and so they started (4/n)

drawing up the Zodiac. Unfortunately for them, space doesn't play nice and the Sun and planets pass through not 12 constellations, which would be hecking convenient for them, but 13... and not all the constellations are the same size, so the Sun and planets spend different (5/n)

amounts of time in each constellation. This didn't really work for them, so they decided to ignore one constellation - Ophiuchus, because it is fainter than the rest. The remaining 12 constellations were then assigned equal time units, a star sign and boom - zodiac. (6/n)

So before we even get onto the tilt of the Earth, we can already see that 3000 years ago, the signs of the zodiac were not exactly precise.

The next issue we have is that the axis of the Earth precesses. If you've ever spun a spinning top, you'll have seen it wobble. (7/n)

This wobble is precession, where the axis around which you spin traces a small circle due to instabilities. For the Earth, the wobble is caused by the gravitational tug of the moon on the equatorial bulge (spoiler: Earth isn't perfectly round, it's fatter in the middle, (8/n)

just like me after lockdown). Earth's precession takes about 26000 years for one cycle. It's relevant to this because 3000 years on, we're about 1/10th of the way around this precession. This means the axis of Earth is not pointing in the same direction as it was for the (9/n)

Babylonians. 1/10th of the way around a circle is about 36 degrees - or roughly one zodiacal sign.

This is why NASA's new zodiac looks different. They haven't actually changed anything. They've just released a table which is more astronomically correct, to account for the (10/n)

different sized constellations, the fact that there are 13 constellations along the zodiac despite only 12 signs, and that we are pointing to a different patch of sky now because of precession.

Interesting side fact: precession means that Polaris will not always be the (11/n)

pole star. 3000 years ago, it was more like Thuban in Draco. In 14000 years, it'll be Vega.

Final word: sorry to disappoint, but you were never a Scorpio to begin with 🤔 (12/12)

P. S full disclosure, I have borrowed all these images from various wonderful people.