Hey baby, what's your sign?

I'm having a tough time trying to fall asleep tonight, so I figured I would write something here.

Recently, as I educate myself on all things astronomical, I came across an interesting bit of info on the Zodiac. As you probably know, the Zodiac was originally laid out 2,600 years ago with the twelve signs we're all familiar with. Each sign represents the constellation that the Sun travels through (more or less, it's measured in 30° increments) over the course of a year. So, for example, 2,600 years ago, the Sun was in the constellation of Aries from March 21 – April 20. Then, as the earth moves in its orbit, the Sun would appear to be in the constellation of Taurus, and so on.

But over the 2,600 years, things have changed astronomically speaking. Due to a phenomena of orbital mechanics called Precession, Earth's view of the Sun has changed slightly, but just enough to add a 13th constellation to the original Zodiac, and this is the constellation of Ophiuchus. Ophiuchus now sits between Scorpio and Sagittarius, and shifts the dates of all other Zodiac signs accordingly. If the Zodiac were revised to fit today's sky, the chart would be like so:

Capricorn – Jan 20 – Feb 16
Aquarius – Fb 16 – Mar 11
Pisces – Mar 11 – Apr 18
Aries – Apr 18 – May 13
Taurus – May 13 – June 21
Gemini – June 21 – July 20
Cancer – July 20 – Aug 10
Leo – Aug 10 – Sept 16
Virgo – Sept 16 – Oct 30
Libra – Oct 30 – Nov 23
Scorpio – Nov 23 – Nov 29
Ophiuchus – Nov 29 – Dec 17
Sagittarius – Dec 17 – Jan 20

Interesting, eh? So if you were born on December 8th, you're no longer a Sagittarius. You're now a Ophiuchus. Of course, modern astrology sticks to the original Zodiac.

So what's Precession? Precession was first discovered by the classical astronomer Hipparchus in the second century BC by comparing star charts he made to those made hundreds of years before him and noting how the sky "shifted". Earth is not a perfect sphere. Because it rotates, the centrifugal force makes Earth bulge at the equator (Earth's equatorial diameter is 13 miles larger than its polar diameter.) The Moon, Sun, and to a much lesser extent, other planets, pull on this bulge. This gravitational tug of war causes Earth to precess (wobble) on its rotating axis (much like how a gyroscope wobbles but doesn't fall over). Over the course of 25,800 years, the wobbling makes Earth's polar (rotating) axis trace a complete circle. This shifts the regular position of the stars we see, and so we now see the Sun go through an additional constellation.

Keep in mind that all other stars we see shift in the same manner, in the same direction and at the same rate. Polaris, our "north star", is currently about 3/4° from Earth's northern celestial pole. 600 years ago, it was 3.5° away from the NCP. 2000 years ago it was 12° away from the NCP and couldn't really be considered the north star. In fact, back in 2,700 BC, the north star was actually the star Thuban, which as a result held a great importance in ancient Egyptian theology. Today, Thuban is about 25° from the NCP.

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