Constellations and the Calendar
Did you recently hear that NASA changed the zodiac signs? Nope, we definitely didn’t…
…Here at NASA, we study astronomy, not astrology. We didn’t change any zodiac signs, we just did the math. Here are the details:
Astronomy is the scientific study of everything in outer space. Astronomers and other scientists know that stars many light years away have no effect on the ordinary activities of humans on Earth.
Astrology is something else. It’s not science. No one has shown that astrology can be used to predict the future or describe what people are like based on their birth dates.
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, we didn’t change any zodiac signs…we just did the math.
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20 Sep, 2016
Constellations and the Calendar
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