The Thirteenth Zodiac Sign: Ophiuchus

Ophiuchus  has sometimes been used in sidereal astrology as a thirteenth sign in addition to the twelve signs of the tropical Zodiac, because the eponymous constellation Ophiuchus (Greek: “Serpent-bearer”) as defined by the 1930 IAU constellation boundaries is situated behind the sun from November 30 to December 18

 

The constellation is described in the astrological poem of Marcus Manilius: the Astronomica, which is dated to around 10 AD. The poem describes how:  Ophiuchus holds apart the serpent which with its mighty spirals and twisted body encircles his own, so that he may untie its knots and back that winds in loops. But, bending its supple neck, the serpent looks back and returns: and the other’s hands slide over the loosened coils. The struggle will last forever, since they wage it on level terms with equal powers”

 

18th century star map illustrating how the feet of Ophiuchus cross the ecliptic

 

Some curious symbols ring the outside of the Star Finder. These symbols stand for some of the constellations in the zodiac. What is the zodiac and what is special about these constellations?

Imagine a straight line drawn from Earth though the sun and out into space way beyond our solar system where the stars are. Then, picture Earth following its orbit around the sun. This imaginary line would rotate, pointing to different stars throughout one complete trip around the sun – or, one year. All the stars that lie close to the imaginary flat disk swept out by this imaginary line are said to be in the zodiac.

The constellations in the zodiac are simply the constellations that this imaginary straight line points to in its year-long journey.

What are Constellations?

A constellation is group of stars like a dot-to-dot puzzle. If you join the dots—stars, that is—and use lots of imagination, the picture would look like an object, animal, or person. For example, Orion is a group of stars that the Greeks thought looked like a giant hunter with a sword attached to his belt. Other than making a pattern in Earth’s sky, these stars may not be related at all.

Even the closest star is almost unimaginably far away. Because they are so far away, the shapes and positions of the constellations in Earth’s sky change very, very slowly. During one human lifetime, they change hardly at all.

A Long History of Looking to the Stars

The Babylonians lived over 3,000 years ago. They divided the zodiac into 12 equal parts – like cutting a pizza into 12 equal slices. They picked 12 constellations in the zodiac, one for each of the 12 “slices.” So, as Earth orbits the sun, the sun would appear to pass through each of the 12 parts of the zodiac. Since the Babylonians already had a 12-month calendar (based on the phases of the moon), each month got a slice of the zodiac all to itself.

But even according to the Babylonians’ own ancient stories, there were 13 constellations in the zodiac. So they picked one, Ophiuchus, to leave out. Even then, some of the chosen 12 didn’t fit neatly into their assigned slice of the pie and crossed over into the next one.

When the Babylonians first invented the 12 signs of zodiac, a birthday between about July 23 and August 22 meant being born under the constellation Leo. Now, 3,000 years later, the sky has shifted because Earth’s axis (North Pole) doesn’t point in quite the same direction.

The constellations are different sizes and shapes, so the sun spends different lengths of time lined up with each one. The line from Earth through the sun points to Virgo for 45 days, but it points to Scorpius for only 7 days. To make a tidy match with their 12-month calendar, the Babylonians ignored the fact that the sun actually moves through 13 constellations, not 12. Then they assigned each of those 12 constellations equal amounts of time.

So, NASA didn’t change any zodiac signs…they just did the math.