What makes it the zodiac?

Traditionally, the zodiac consists of 12 constellations - Capricorn, Aquarius, Pisces, Aires, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpio and Sagittarius.

These constellations exist along the ecliptic (the path of the sun across the sky). Ancient astronomers and astrologers identified these signs and their dates based on how long the sun passed in front of each constellation. It takes about a month for the sun to shift from one constellation to the next. Their dates are as follows:

◊Capricorn (The Goat): December 22nd - January 19th

◊ Aquarius (The Water Bearer): January 20th - February 18th

◊ Pisces (The Fishes): February 19th - March 20th

◊ Aires (The Ram): March 21st - April 19th

◊ Taurus (The Bull): April 20th - May 20th

◊ Gemini (The Twins): May 21st - June 20th

◊ Cancer (The Crab): June 21st - July 22nd

◊ Leo (The Lion): July 23rd - August 22nd

◊ Virgo (The Virgin): August 22nd - September 22nd

◊ Libra (The Scales): September 23rd - October 22nd

◊ Scorpio (The Scorpion): October 23rd - November 21st

◊ Sagittarius (The Archer): November 22nd - December 21st

So whats new?

In recent astronomy,the constellation Ophiuchus has inched it's way onto the ecliptic, forcing its way between Scorpio and Sagittarius in the zodiac. This is due to the movement of objects, specifically stars, in our galaxy.

"[...] the stars, including the Sun, travel in their own separate orbits through the Milky Way galaxy. The stars move along with fantastic speeds, but they are so far away that it takes a long time for their motion to be visible to us."
- NASA

This shift in the sky has caused there to speculation of a "new" zodiac that includes Ophiuchus. However, since modern astronomy considers constellations to be not just the connect-the-dot figures in the sky, but also the space and additional stars around it, Sagittarius and Ophiuchus tend to overlap. This creates discrepancy in the dates associated with this new piece of the zodiac, but the general consensus seems to be that the sun is in front of Ophiuchus from November 30th to December 18th.

Did anything actually change?

According to NASA, no. Experts say that about 3,000 years ago, 13 constellations were identified to be part of the zodiac, but since the calendar the Babylonians were creating at the time only fit 12 ranges of dates, Ophiuchus was chosen to be omitted from the zodiac. NASA also made a statement that the traditional zodiac would not be altered, and that anything astrology did in terms of creating a "new" zodiac has nothing to do with astronomy.

On that note:

It is totally up to you what sign you choose to identify as! Choosing to include Ophiuchus is no more right or wrong than omitting it; there are valid points for both sides, which is good news (especially if you just got that tattoo!).