June 2013 – Ursa Major & The Plough

Welcome to the first edition of Heavens Above!, a regular astronomy blog of the Sci@StAnd. Throughout the year, this blog will introduce the night’s sky to you bit by bit. Each month, a new constellation or small region of sky will be explored, with details about how and where to find it, as well as the mythology surrounding its name. On occasion, a few interesting facts about the region of the sky that the constellation encompasses will be included. This will always be of particular interest to the month ahead.

 

Constellation I – The Great Bear (Ursa Major)

Ursa Major (the Great Bear)

Figure 1: The constellation of Ursa Major (the Great Bear).

For this month, the constellation of focus is the Great Bear, or Ursa Major (see Figure 1), which is a useful starting point as it can be used to help navigate around the sky in later editions. This constellation is visible all year round in Scotland, due to its proximity to Polaris, the pole star, which sits directly above the north pole.

Due to the levels of light pollution in most towns and cities, the full constellation is very rarely seen, except in exceptional weather conditions. Instead, the brightest stars within Ursa Major are more often recognised as the “Plough” or “Big Dipper” (see  Figure 2). This group of stars is not a constellation in its own right. Instead, it is known as an asterism; a group of stars producing a commonly recognised shape within an officially recognised constellation.

The Plough

Figure 2: The more familiar asterism of the Plough.

 

Locating Ursa Major and the Plough

During the months surrounding the summer solstice (May-August), the Plough is found directly overhead. The ‘handle’ of the Plough will point towards the East and forms the tail of the Great Bear. If the sky is clear, you may be able to make out the full Ursa Major constellation. The main body of the bear is located further towards the west and its legs extend down into the southern part of the sky.

History and Mythology

The constellation of Ursa Major owes its name to Greek mythology. As the story goes, Zeus, the king of the gods, had many mistresses. One of these, Callisto, was a beautiful princess who bore Zeus a son by the name of Arcus. As is understandable, Hera, Zeus’ wife, was not best pleased when she found out about her husband’s illegitimate son. In response, she turned Callisto into a bear. This is where the story would have presumably ended were it not for young Arcus.

While his mother was reduced to roaming the forest in the form of a bear, Arcus grew into a fine young man, learning how to fight and to hunt. On one hunt, in pursuit of a dear in the forest near his home, Arcus stumbled into the path of a bear. In fear, Arcus drew his sword from its sheath and struck out at the bear. At that moment, Zeus appeared and ripped the sword from Arcus’ grasp to defend his dear Callisto. Fearing that she was no longer safe in the forest, Zeus placed Callisto in the sky and there she remains as the constellation Ursa Major.

This story was told to generations of Greeks and was of particular importance to the archers in their armies. In order to test their eyesight, prospective archers were asked how many stars made up the asterism of the Plough. If they answered “7”, they were rejected but if they answered “8”, instead, they were deemed to have good enough eyesight to continue their training. If we look at Figure 2, we can see the ‘seven’ stars labelled as the Plough. However, on a clear night, if you look at the star marked ‘Mizar‘, you may be able to see two stars instead of one. This is not a binary star, but a superposition of two stars situated many millions of miles apart.

 

 In the next edition (July) we will be focussing on Ursa Minor and the pole star.

Happy Stargazing!

 

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