Constellations Explained

Many of us have heard of constellations. Often, one may be familiar with a few of them. But what are they and where did they come from? In general, a constellation is a group of stars that are said to resemble a person, animal, mythological being or object. The thing to remember here is that the patterns are a coincidence of where we are in the Milky way. If we looked at the same stars from a different part of the galaxy they would form different shapes. The International Astronomical Union (IAU) is the body that oversees the naming of objects in the sky. Officially they recognise 88 constellations, and this has been the case since 1922.

Some of the constellations of the northern hemisphere. Photo: Paul Smith

Constellations are not a new discovery. Constellations date back to pre history. Ancient people created constellations as a way to tell stories. They attributed various characters and mythological creatures in their stories to the patterns they seen in the stars. Although we have 88 modern constellations, this was not always the case. Throughout history many societies had their own constellations and some shared constellations but told different stories about them. Constellations also evolved and changed over time. You will notice that some constellations are very obscure and it takes a lot of imagination to see what they represent. 

Consider one of the more prominent constellations in the northern hemisphere, Cygnus. Cygnus is always high in the sky during the summer and into the autumn. In western astronomy, it represents a swan. You can imagine the long part of the cross is the swans neck and head while the wide part of the cross represents the wings. The name, Cygnus,  comes from the Greek word for swan.

Cygnus

There are many legends associated with the constellations. For example, one story in Greek mythology is associated with Phaeton. Legend says that Phaeton and Cygnus were racing across the sky when the came too close to the Sun. Their chariots burned up and when they fell to Earth, Cygnus found Phaeton’s body at the bottom of the Eridanus River. Unable to get his body from the river, Cygnus made an agreement with the god Zeus. He would be transformed into a swan and would only live the lifetime of a swan. Cygnus was then able to dive into the river, retrieve the body and give Phaeton a proper burial allowing his soul to travel to the afterlife. Zeus was so taken by the sacrifice Cygnus made he placed him in the stars. 

In contrast to this, the Chinese people called the constellation of Cygnus, Que Qiao and associated it with a magpie bridge. The story is that two lovers were separated by the goddess of heaven because one was a fairy and fairies and mortals couldn’t mix. When she found out they had secretly married she placed a barrier between which is represented by the Milky way. Eventually the Goddess relented and allowed them to see each other once per year. So one day each year all of the magpies of the world form a bridge to allow the two to cross the Milky Way and meet. 

In modern times, constellations are mostly used to break up the sky into smaller areas, making it easier to locate things. For example, Mars is currently visible in the constellation of Pisces. Provided one knows where Pisces is, one straight away knows the general area of sky that is being referenced. Some stars have names but typically also have a name in relation to the constellation they are in. Usually we use the Greek alphabet and the name of the constellation. Alpha being the brightest star in the constellation, Beta being second brightest and so on. For example, the brightest start in Cygnus is called Deneb or Alpha Cygni.

There are also patterns in the sky called asterisms. These are patterns in the sky that are commonly known but are not official constellations. Usually they form part of a constellation. An example of an asterism is the Plough or Big Dipper. This is a saucepan shaped asterism that is widely know but is not a constellation in its own right. The full constellation is Ursa Major which represents the great bear. Constellations are a good way to start learning the sky. You can get a sky chart for the current month from our blog post  each month which will show you where to start.

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