Ancient guiding lights: what are constellations and how did they form?

Typically, these patterns represent animals, religious or mythological figures, or an inanimate object, perhaps with cultural significance. 

Constellations themselves are not uniformly defined across cultural groups, and since the positions of stars change over time, the shape of constellations does as well, so the constellations that our ancient ancestors saw in the night sky were likely different in appearance from the ones we know today.

History of constellations

The history of constellations extends far back into prehistoric times, with the earliest possible evidence of constellations showing up in the famous cave art in Lascaux, France about 17,000 years ago.

There are also some who believe that the temple site at Gobleki Tepe in Turkey, built around 12,000 years ago, may have been used as an ancient observatory and that the carvings adorning some of the pillars in the temple site depict ancient constellations, although there is no real evidence for this.

Once we get into recorded history, the earliest constellations may have been recorded by ancient Babylonian astronomers around the second millennium BCE, although it has also been argued that the Babylonians themselves were influenced by older Sumerian traditions. There are also records of astronomy from China dating to around 3000 BCE, but the first Chinese star catalogs were created in the 4th century BC. These star catalogs may have been influenced by the Babylonians, perhaps via India.

The modern constellations as we know them in the West are derived from ancient Greek sources, working off the older Babylonian star charts. The two major Greek works on the constellations, that we know of, were Aratus's Phenomena and Ptolemy's Almagest, which list the 48 traditional constellations of the northern sky.

The major southern constellations were established from the 15th century on, once European explorers began charting the southern sky. However, the ancient Polynesians may have been using sophisticated celestial navigation techniques, based on the movement of particular stars across the night sky, to plot their way across the ocean as early as 1200 BCE.

The formal list of 88 modern constellations was codified in 1922 by the International Astronomical Union and their "domain" in the sky was established in 1928.

The purpose of these official designations was to carve up the celestial sphere into identifiable regions that would make astronomical observation and identification easier, which is why you'll often see the positions of celestial objects indicated by what constellation they are "in."

Sagittarius A*, for example, is the supermassive black hole in the center of the Milky Way galaxy, and its position in the night sky can be found by looking for the constellation Sagittarius and refining the observation from there.

How many constellations are there?

Currently, there are 88 formally recognized constellations as established in 1922 by the International Astronomical Union. They are:

• Andromeda
• Antlia
• Apus
• Aquarius
• Aquila
• Ara
• Aries
• Auriga
• Boötes
• Caelum
• Camelopardalis
• Cancer
• Canes Venatici
• Canis Major
• Canis Minor
• Capricornus
• Carina
• Cassiopeia
• Centaurus
• Cepheus
• Cetus
• Chamaeleon
• Circinus
• Columba
• Coma Berenices
• Corona Australis
• Corona Borealis
• Corvus
• Crater
• Crux
• Cygnus
• Delphinus
• Dorado
• Draco
• Equuleus
• Eridanus
• Fornax
• Gemini
• Grus
• Hercules
• Horologium
• Hydra
• Hydrus
• Indus
• Lacerta
• Leo
• Leo Minor
• Lepus
• Libra
• Lupus
• Lynx
• Lyra
• Mensa
• Microscopium
• Monoceros
• Musca
• Norma
• Octans
• Ophiuchus
• Orion
• Pavo
• Pegasus
• Perseus
• Pheonix
• Pictor
• Pisces
• Piscis Austrinus
• Puppis
• Pyxis
• Reticulum
• Sagitta
• Sagittarius
• Scorpius
• Sculptor
• Scutum
• Serpens
• Sextans
• Taurus
• Telescopium
• Triangulum
• Triangulum Australe
• Tucana
• Ursa Major
• Ursa Minor
• Vela
• Virgo
• Volans
• Vulpecula

In addition to these 88 formal constellations, there are many informal but recognizable star patterns used across various cultures and known as asterisms. These can be part of one or more constellations. For example, the stars of the Plough or the Big Dipper are asterisms that are located within the constellation of Ursa Major, and the stars that are often thought of as 'Orion' are actually an asterism within the constellation.

What are the 12 most important constellations?

While the importance of any constellation is highly dependent on what you might need it for (i.e., navigation), there are about a dozen major constellations that appear along the ecliptic of the solar system, an imaginary line along the perceived path of the sun (as well as the rough paths of the planets and the moon) over the course of a year on Earth. 

These constellations are low enough on the horizon that the planets, the Moon, and the Sun all appear to move "through" them, and so these constellations, known as the western zodiac, have taken on special significance.

In ancient times, these constellations were believed by some cultures to govern the lives of everyone on earth and could be used for divining the future or to read omens sent from the heavens about current or future events, particularly calamitous events like famines, wars, or the deaths of rulers.

In the ancient Near East and West, these constellations roughly correspond to the following modern constellations:

• Aries
• Taurus
• Gemini
• Cancer
• Leo
• Virgo
• Libra
• Scorpio
• Sagittarius
• Capricorn
• Aquarius
• Pisces

What is the Zodiac?

The western zodiac is an important part of ancient astronomy and modern astrology. The zodiac is a region of the sky that hugs the ecliptic and which follows the apparent path of the Sun across the sky over the course of the year.

This belt extends about eight degrees north and south of the ecliptic, and the paths of the Sun, the Moon, and the visible planets are all contained within the zodiac.

The zodiac is divided into 12 parts, called signs, that each takes up around 30 degrees of the night sky, roughly corresponding to the 12 constellations listed above.

The zodiac is also used as a celestial coordinate system, using the ecliptic as the latitudinal origin and the position of the Sun during the vernal equinox as the longitudinal origin.

The exact origin of the western zodiac isn't clear, though it was largely defined by ancient Babylonian astronomers in the middle of the first millennium BCE and later adopted by early Greek astronomers through cultural exchange.

The western zodiac also spread to ancient India, and though the names for the signs in the Indian zodiac are different, they are closely analogous to the ancient Greek names.  

How does NASA use the constellations?

While constellations are not nearly as important as they used to be in terms of navigation, that doesn't mean that they have been forgotten or cast aside by science.

We've seen how the major constellations are used to divide up the night sky to make it easier for astronomers to identify and catalog objects in space, but the constellations still serve as important navigation aids.

Astronauts and even modern navies still learn celestial navigation to supplement modern computerized navigation systems.

Computers and other instruments can break or otherwise become unreliable, but the constellations remain more-or-less fixed for many centuries or even longer, so they will always be a handy backup for navigation purposes. Constellations are even used by robotic spacecraft to help navigate for the same reason.

It makes a lot of sense since a major key to navigation is following fixed points of reference. The constellations have been an important point of reference for humanity for time immemorial, and as we move closer to the stars in the decades and centuries ahead, they will almost certainly continue to do so.