The Ecliptic Makes Some Constellations Special

In the last few posts, I've talked a lot about the constellations Scorpius and Sagittarius.  Many people have heard of these two constellations but for very different, non-astronomy related reasons.  

Most people associate Sagittarius and Scorpius with the Zodiac, which of course they are.  However, the belief system of astrology and the science of Astronomy have long since parted company.  But why do we still talk about the zodiac anyway?  The reason has to do with our solar system.

Our solar system is essentially a flat disk, rather like a dinner plate.  The planets orbit around the sun within a degree or two of the plane.  This means that all the planets as well as the sun and the moon can be found along the same line in the sky.  This line is the Ecliptic.  

Ancient observers knew the planets moved differently than other stars but not why.  The considered them the gods, and so the constellations they moved through became special; what we call the Zodiac.  

In our sky there are 13 (yes thirteen not twelve) constellations that lie along the ecliptic.  Based on the division of the sky into constellations by the International Astronomical union, the sun does not spend an equal amount of time within each constellation.   

The constellations of the Zodiac are not all as bright and easy to spot as Sagittarius and Scorpius, and many might be rendered insignificant were it not for the planets passing through them.  However, Sagittarius and Scorpius are very bright, and easy to find even under light polluted skies.  

So get out there with a star wheel or sky chart and see if you can find other constellations of the Zodiac.  They will be along a line that connects Sagittarius and Scorpius and will slowly wheel from east to west as the night passes and as summer turns to fall.

Created by the Author using Stellarium