Humanity has always looked up and found stories in the sky, connecting the dots of light to make pictures with a bit of imagination. Religion, tradition, superstition, and even practical calendar-making and navigation techniques influenced how those pictures took shape, starting many thousands of years ago with cuneiform texts written in western Asia. Although both ancient and modern influences shape how we stargaze, the shapes and stories we see in the sky come primarily from Greek and Roman traditions.
Greek and Roman Origins
You've probably heard of the Greek writer Homer, who wrote the most famous ancient Greek texts students around the world still read in school: the Iliad and the Odyssey. One of the very oldest references we have to Greek constellations actually comes from the Iliad. A key plot point features Hephaistos, the craftsman god, creating a shield for Achilles to use in the Trojan War; the design is said to include the sun and the moon as well as the constellations Orion and "the Bear," Ursa Major.
With the exception of Orion, most of the constellations didn't have particular heroes or gods associated with them when Homer wrote his epic. Often, they were merely named for the things they represented, such as the bear, the lyre, or the ram. But in the 5th century B.C., the Catasterismi of Eratosthenes changed that, associating the shapes people saw in the sky with specific images from mythology. In this era, the Greeks took things one step further: The stars didn't just depict stories and figures; they were the stories and figures. Orion literally circled the heavens with his hounds. The Bear was a literal bear. And Pegasus really soared high above. Catasterismi described and named 43 constellations.
By the 2nd century A.D., the Romans led the western world, and it was a Roman who provided the next great contribution to astronomy. Ptolemy of Alexandria documented 1,022 stars, and he arranged them into 48 constellations. The Romans adopted lots of Greek mythology, and the constellations came along with that.
It's important to remember that constellations aren't just pretty pictures based on old stories. They have practical applications for astronomers, helping them to create star maps that locate and chart particular celestial bodies and simplifying the process of finding and describing the positions of things in the sky. By breaking the sky into sections based around commonly recognized constellations, Ptolemy essentially created an organizational system modern that astronomers still use in their work today.
The International Astronomical Union has a canon of 88 official, recognized constellations they use in their work, which is built directly on the shoulders of Ptolemy's 48 constellations. Although many of the constellations were first described by the Greeks, because Ptolemy is the primary reference for modern astronomers, we typically refer to these constellations by their Latin names.
Greek and Roman mythology helped name more than just constellations, though. The planets are also named after mythological figures perceived to have similar characteristics. When the Greeks saw one planet that appeared to move faster than the others, they named it for Mercury, the fleet-footed messenger god. The blood red planet obviously belonged to the god of war, Mars, and the biggest planet was named for the greatest of the gods, Jupiter. Even the Galilean moons around Jupiter, discovered long after the Greek and Roman empires collapsed, were named according to the mythology surrounding Jupiter and his violent conquests.
Today, we still draw inspiration from the sky, whether we're trying to figure out the science of the universe, trying to figure out each other's personalities and fortunes from the zodiac, or coming up with creative Halloween costumes that glow in the dark. No matter what you're searching for, the answers could be found in the stars.