Skip to main content

Constellations of the western zodiac

This NASA graphic offers an introduction to the constellations visible in the Northern Hemisphere.
This NASA graphic offers an introduction to the constellations visible in the Northern Hemisphere. (Image credit: NASA)

The constellations in the night sky are connected to myth and legend, as well as the unscientific concepts of astrology. But they also have held importance and usefulness to science and exploration throughout history and still today.

Stars in the universe are scattered across a vast, three-dimensional space. When we look up from the surface of our planet, however, humans have historically observed constellations: two-dimensional pictures and shapes connecting the stars in the sky to stories from cultures around the globe. 

In addition to their cultural significance, constellations have also been important instruments that once marked the passage of time and the seasons. Today, constellations continue to be valuable tools to orient astronomers and stargazers in the night sky. 

One constellation tradition is the western zodiac, which is made up of 12 constellations: Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpio, Sagittarius, Capricorn, Aquarius and Pisces.

Constellations of the Night Sky: Famous Star Patterns Explained (Images)

This NASA graphic offers an introduction to the constellations visible in the Northern Hemisphere.  (Image credit: NASA)

The zodiac constellations

As Earth rotates (opens in new tab), the sun, moon and planets seem to move generally along a set path through the sky, called the ecliptic (opens in new tab)

The 13 constellations in the path of the ecliptic are:

Astrologers use 12 of these constellations to roughly correspond with the signs of the zodiac to make predictions. (The 13th, Ophiuchus (opens in new tab), it omitted because when the signs were first described, the stars were not in exactly the same position as they are today.) 

Today, the astrological signs differ from these constellations, bearing only a loose reference (opens in new tab) to one another. The sign of Pisces, for instance, currently corresponds to the rise of the constellation of Aquarius. 

Read more: How the constellations got their names (opens in new tab)

What is a constellation?

A visual mosaic of the 12 classic constellations in the western zodiac, photographed in the night sky, marked and outlined. (Image credit: Till Credner)
(opens in new tab)

From Earth, stars appear to move across the sky on a regular schedule. That appearance of star movement actually has more to do with the rotation of Earth (opens in new tab) than the movements of stars themselves. 

Similar to how our sun (opens in new tab) looks like it's moving across the sky when it's really our planet rotating as it orbits the sun, constellations appear to be rotating across the sky when that movement is really due to our changing viewpoint. 

Read more: Space junk is blocking our view of the stars (opens in new tab)

Although we're looking at stars across vast expanses of three-dimensional space, the arrangement we see looks two-dimensional. But stars that seem to be right next to each other in a constellation may in fact be hundreds of light-years apart (opens in new tab)

For example, in the well-known constellation Orion (opens in new tab), the nearest star to us is Bellatrix, a bit more than 200 light-years from Earth. But the farthest star in that constellation is Alnilam, which is about 1,300 light-years away. But from our viewpoint on Earth, they look like nearby neighbors.

Orion isn't one of the zodiac constellations, though. Those constellations are defined mostly by their position on the ecliptic (opens in new tab), an imaginary or projected line in the sky that marks the perceived path of the sun (as well as the rough paths of the planets and the moon, which are all more or less on the same plane) over the course of a year on Earth. 

The path of the ecliptic passes through 13 of the 88 constellations officially recognized by the International Astronomical Union (opens in new tab) (IAU) since 1929. Just 12 of those constellations make up the western zodiac.

History of the western zodiac

A 16th century engraving of Claudius Ptolemy, mathematician and astronomer who created a geocentric model of the solar system and wrote the Almagest, one of the most influential scientific texts in history. (Image credit: Theodor de Bry)

Most of the constellations in the Northern Hemisphere's skies bear Greek and Roman names. According to Encyclopedia Britannica (opens in new tab), that is thanks in part to the Almagest, an influential catalog of stars and constellations created by the ancient mathematician Claudius Ptolemy in the second century. 

However, people were mapping the sky long before these names took hold. Ancient China, Sumeria in the Middle East and Egypt each had their own star maps, for example. According to a 2017 paper from the journal of the American Philosophical Society (opens in new tab), many of the Greek constellations that we consider to be the Western zodiac today were adopted from Babylonian astronomers. They referred to "The Twins," "The Lion" and "The Crab" thousands of years ago, which are the same constellations we (and the ancient Greeks) know as Gemini, Leo and Cancer, respectively. 

Astrology was once considered part of the science of astronomy. In fact, the famous astronomer Johannes Kepler (opens in new tab) was considered an astrologer during his life in the 1600s, according to Time magazine (opens in new tab)

Now, however, astrology is commonly considered a pseudoscience. No evidence has been found for the predictive power of astrological predictions based on the zodiac, as demonstrated in this 1985 paper from the journal Nature (opens in new tab).

How are the zodiac constellations used today?

NASA astronaut Alexander Gerst learns how to use a sextant, a traditional navigation tool that could be of vital importance to future astronauts.  (Image credit: NASA)

In science:

The IAU's 88 constellations, which include all 12 of the zodiac constellations, are still relevant to researchers today. Astronomers can use those markers to explain what parts of the sky are part of their work, or orient themselves to find objects they have seen in space. 

The Morehead Planetarium (opens in new tab) also recommends using constellations as a reference point to spot faster-moving planets. For casual skywatchers, the zodiac constellations are recognizable because they have a story and shape associated with them. When a planet appears as a bright object in a constellation, watchers can spot that one of the "stars" is out of place and might actually be Mars, Jupiter or Venus instead. 

Similarly, constellations can be used to describe where a meteor shower might appear in the sky, or where a comet (opens in new tab) might come into view. 

Related: Try "star hopping" to use constellations as guides to find celestial events (opens in new tab)

In navigation:

Astronauts still learn star navigation, according to NASA's Space Place (opens in new tab). It's a backup to computerized navigation systems, but it's a good backup: Seafarers and explorers used stars and constellations to navigate for thousands of years. 

Even autonomous spacecraft, sent to explore far from Earth, can use constellations in their star maps to navigate. 

As inspiration: 

Throughout history, humans around the planet found the stars useful for timing harvests and plantings, but they also used constellations to tell stories and find meaning (opens in new tab)

Now, planetariums and skywatching programs retell those stories to capture attention, spark curiosity and keep the ancient history of space exploration alive. 

Additional resources

Join our Space Forums to keep talking space on the latest missions, night sky and more! And if you have a news tip, correction or comment, let us know at: community@space.com.

Vicky Stein is a science writer based in California. She has a bachelor's degree in ecology and evolutionary biology from Dartmouth College and a graduate certificate in science writing from the University of California, Santa Cruz (2018). Afterwards, she worked as a news assistant for PBS NewsHour, and now works as a freelancer covering anything from asteroids to zebras. Follow her most recent work (and most recent pictures of nudibranchs) on Twitter.