On a beautiful cloudless night, what’s more magnificent than the myriad of stars populating the celestial sphere. Constellations are the easiest approach to finding your way around these thousands of stars.
What is a constellation?
When we look at the sky, we can imagine characters, animals or objects by drawing fictitious lines between stars. All civilizations have pictured the sky that way, often integrating myths that were part of their belief system. Although most of the groupings differ from one culture to another, some of them, like Ursa Major (the Great Bear) or Orion, are common to many.
In the early 20th century, the International Astronomical Union officially recognized 88 constellations and defined their boundaries. Those in the northern hemisphere and on the celestial equator are the best known and come to us from Ancient Greece and Rome. Southern-hemisphere constellations primarily represent discoveries made by European explorers in those regions beginning in the 17th century.
The constellations of the zodiac
The Earth turns around the Sun in one year. We don’t feel that motion, but we can perceive it by observing that the Sun moves about among certain constellations in the course of the year. The constellations that the Sun passes through constitute the zodiac: Aries, Taurus, Scorpio, and so on. These are certainly the constellations that the public is most familiar with. But when we look at them more closely, we see that there are not 12, but actually 13 constellations in the zodiac. The Sun also passes through the constellation of Ophiuchus (the Serpent-Bearer) from November 29 to December 18.
The circumpolar constellations
Just like the Sun, the constellations rise in the east and set in the west. Only the pole star seems fixed in the northern hemisphere, because it’s located in the projection of the Earth’s rotational axis.
Nevertheless, there are constellations that are visible at all times. These are called circumpolar constellations. The closer you are to one of the poles, the more of them you can observe. If you’re right at the poles, all the constellations you see in the sky will always be visible. Conversely, if you find yourself at the Equator, no constellation will be circumpolar.
In Québec, the circumpolar constellations are Ursa Major, Ursa Minor, Draco (the Dragon), Camelopardalis (the Giraffe), Cassiopeia and Cepheus.
The distance of the stars in constellations
Stars making up a constellation are not grouped together in space. It’s extremely rare for them to be physically close, except for star clusters found in certain constellations: for instance, the Pleaides in Taurus. The stars forming the outlines of constellations are simply our interpretation of two-dimensional figures on the celestial sphere. In reality, the constellations are made up of stars of varying brightness and at different distances from one another.
Discovering the sky with constellations
Learning to recognize constellations allows you to feel at home in the starry sky and find your way around it. Sky maps always indicate the principal constellations. Start with a simple constellation like Ursa Major or Orion in winter. Armed with a good sky map like the Planétarium’s Star Wheel, on the basis of these reliable landmarks you’ll be able to find the other constellations. All that’s needed then is a little patience and some practice.
And all that’s left is for me to wish you dazzling starry nights so that you can set off on a discovery of the beauties of the sky.
To learn everything the sky has in store for you this month, consult the Monthly sky.