What Are Constellations?

Contents

Looking at constellations

Where did the constellations come from?

Looking at the Constellations

The constellations are totally imaginary thingsthat poets, farmers and astronomers have made up over the past 6,000years (and probably even more!). The real purpose for theconstellations is to help us tell which stars are which, nothing more.On a really dark night, you can see about 1000 to 1500 stars. Tryingto tell which is which is hard. The constellations help by breakingup the sky into more managable bits. They are used asmnemonics, or memory aids. For example, if you spotthree bright stars in a row in the winter evening, you might realize,"Oh! That's part of Orion!" Suddenly, the rest of the constellationfalls into place and you can declare: "There's Betelgeuse in Orion'sleft shoulder and Rigel is his foot." And once you recognize Orion,you can remember that Orion's Hunting Dogs are always nearby. Thenyou might recognize the two bright stars in the upper and lower leftof the photograph as Procyon in Canis Minor and Sirius in Canis Major,respectively.



When you look in a sky atlas, you might see diagrams like this:

Obviously, this is very different from the photoabove. This type of schematic draws the stars as different sizes torepresent different brightnesses. In addition, there is a standardway to connect the stars that allow astronomers and others who usecharts like this to quickly tell what they are looking at. In almostevery star atlas, you will see Orion drawn with these same lines.

You might also notice that every star on the chart is labeled (sorrythat it came out a little blurry). This chart is useful because itaccurately shows the relative positions of the stars in this smallregion of the sky. In addition, other things besides stars are alsolabeled on the chart. For example, Barnard's Loop on the left and M42in the bottom middle are pointed out. Barnard's Loop is a cloud offaintly glowing gas, which can't be seen without a telescope. M42 isthe Great Orion Nebula and it is the red splotch in Orion's Sword inthe photo above.


 

Where did the constellations come from?

Yes and no. Around the world, farmers know that for most crops, youplant in the spring and harvest in the fall. But in some regions,there is not much differentiation between the seasons. Sincedifferent constellations are visible at different times of the year,you can use them to tell what month it is. For example, Scorpius isonly visible in the northern hemisphere's evening sky in the summer.Some historians suspect that many of the myths associated with theconstellations were invented to help the farmers remember them. Whenthey saw certain constellations, they would know it was time to beginthe planting or the reaping.

This dependence on the sky became a strong part of many cultures.Perhaps there is something about the mystery of the night sky thatmakes people want to tell stories about the constellations. Thepicture at the left is an ornate star chart printed in 1835. Like theothers, it shows the great hunter Orion. In this one, he is holding alion's head instead of his traditional bow or shield. He has an eagerlook in his eye as he stalks Taurus, the Bull. Behind him, hisfaithful dog, Canis Major, is chasing Lepus, the Hare. Compare thispicture to the photo near the top of the page. They are at about thesame scale and they show the same stars.

The constellations have changed over time. In our modern world, manyof the constellations have been redefined so now every star in the skyis in exactly one constellation. In 1929, the InternationalAstronomical Union (IAU) adopted official constellation boundariesthat defined the 88 official constellations that exist today.

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