The Constellations – Introduction

Lesson 5

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A constellation is a grouping of stars forming an imaginary outline representing an animal, a mythological character, or, especially in the southern hemisphere, a navigational object.

The Latin word for star is stella, so an imaginary grouping of 'stellas' is called a constellation. Stars belonging to particular constellations are not necessarily neighbors in space. Most happen to be in our line of sight as they circle the Milky Way Galaxy.

All of the stars we can see without optical aid are near neighbors. The millions upon millions of stars farther away in the Milky Way band are not part of any constellation.

There are 88 constellations in all, the best known being the twelve astrological signs of the zodiac.

Our constellations are unique to the Solar System. You are already aware that our neighboring stars are so remote that their night sky positions usually take thousands of years to alter even slightly. Indeed, the patterns we see today are not much different from those witnessed during Sumerian times, some 6,000 years gone. Our Galaxy is indeed vast.

If we could journey on the Starship Enterprise to another part of the Galaxy, we would have to create an entirely different set of imaginary groupings of stars. Every location around the Galaxy has its own, unique line-of-sight snapshot of neighboring stars.

So, we get what we are given from our position in the Galaxy, and we have to make do with it to find our way around the night sky.

Due to the spherical nature of the Earth, we get to see different constellations as we change latitude. Indeed, the plain fact that some constellations are visible in the northern hemisphere and not the southern hemisphere, and vice-versa, was proof enough even to the ancient Greeks that the Earth is spherical.

The ancient Greeks told the story of Orion, the hunter. He leaped into the sea to escape a scorpion's sting, which links to why, as the constellation of Orion sets, the constellation of Scorpius rises.

In 1929 the International Astronomical Union (IAU) defined today's 88 constellations. Many are based on Greek mythological characters, while others come from the Egyptians and Babylonians (Chaldeans).

Navigators, until recently, used the constellations to sail across seas, and certain constellations marked the seasons of the year. Today, astronomers still use constellations as markers to indicate the general area of the sky where particular celestial objects reside or, in the case of Solar System objects, through which they are passing.

A Short Celestial Cartography History

Before trying to learn about the constellations in this course, it will be a good idea to be familiar with a bit of history about the origins of constellation names, especially those in the southern skies. We begin with Greece and Rome to find out who were behind the naming of the most familiar constellations.


Hipparchus of Nicaea c. 190 – 120 BC, compiled the first star catalog in the western world. He constructed a celestial globe depicting the constellations based on his observations:

Previously, Eudoxus of Cnidus (Knidos, Asia Minor) in the fourth century BC had described the constellations in two books: Phaenomena and Entropon. Aratus of Soloi, a Greek poet, wrote a poem called Phenomena (Arateia), meaning "Appearances," the first half of which contains a verse about a lost work by Eudoxus describing the constellations:

Hipparchus later wrote a commentary on the Arateia containing stellar positions and times for the rising, culmination, and setting of the constellations based on his measurements.


Claudius Ptolemy c. AD 100 – 170 included a star catalog in his Almagest is based on Hipparchus' work:

Books 7 and 8 contain a star catalog of 1022 stars, described by their constellation positions, together with ecliptic longitude and latitude. The constellations Aries through Virgo are in Book 7, while the rest are in Book 8.

Ptolemy marked stars of magnitude one as m=1 and the faintest visible as m=6. So, there you have it – the origin of today's magnitude scale.

The stellar positions are of Hipparchian origin, despite Ptolemy's claim to the contrary.

Ptolemy identified 48 constellations: 12 zodiacal ones, plus 21 to the north of the ecliptic, and 15 to the south: 


The Almagest was later updated by the Persian astronomer Abd al Rahman al-Sufi in 964:

The Almagest was updated further by Nicolas Copernicus in 1543:

In 1437, Ulugh Beg re-observed the Hipparchian stars visible from Uzbekistan:

Beg's catalog was superseded in the late 16th century by Tycho Brahe using more accurate instruments, which significantly improved knowledge of stellar positions before the telescope's invention:

The 48 traditional Western constellations are, therefore, Greek in origin.


Since the days of Tycho, the southern constellations have been mapped by mariners sailing the southern seas.

The southern constellations were mapped between the 14th to the16th centuries by navigators such as the first European to report the Southern Cross, Andrea Corsali, and Antonio Pigafetta, a survivor of the first circumnavigation of the world on Ferdinand Magellan's ship:

And Amerigo Vespucci, whose forename is the origin of the name 'America:'

In the late 16th century, Petrus Plancius developed celestial globes based on observations by the Dutch navigators Pieter Dirkszoon Keyser and Frederik de Houtman:

These globes became widely known through Johann Bayer's Uranometria star atlas of 1603:

Seventeen more constellations were added in 1763 by the French astronomer Nicolas Louis de Lacaille, which first appeared in his star catalog of 1756:

Since then, several constellations became quite common, like those proposed by the French astronomers Pierre Lemonnier and Jérôme Lalande:

Those have since been disused. For example, the northern constellation of Quadrans Muralis survived into the nineteenth century, hence the name for the Quadrantids meteor shower that is still used today. The old constellation is now divided between the constellations Boötes and Draco.

The names and boundaries of today's 88 constellations were arrived at by the International Astronomical Union way back in 1922.



DISTINCTION: 90-100%   GRADE 2: 75-89%   GRADE 3: 60-74%   GRADE 4: 40-59%

FAIL: 0-39%

Although these lessons are free, they require putting together and hosting on our web servers. So, any donations via PayPal to help this using the link below will be much appreciated.