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The Plough as an Observing Aid

Wednesday, 1st February, 2012 12:30pm
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The Plough as an Observing Aid
The Plough as an Observing Aid

Only a handful of constellations are easily recognisable for anyone looking up at the night sky. Orion with its three stars lined up in an almost perfectly straight line marking its belt is one, as is the Pleiades (or Seven Sisters) as a tiny cluster above the head of Tauras and Cassiopeia with its unmistakable "W" shape. Apart from these, the most recognisably asterism in the night sky must be the Plough (or Big Dipper).

Part of the larger constellation of Ursa Major or Great Bear, it has been recognised by man since the earliest writings found on earth. Visible all year round, it changes its position through the seasons as it rotates around the Pole Star and is made of seven main stars - Alkaid, Mizar, Alioth, Megrez, Phecda, Merak and Dubhe. The closest of the Plough's stars, Mirak, lies 78 light years away from Earth while the furthest, Dubhe, is 124 light years away.

For centuries the Plough has been used as a guide to finding other celestrial delights and targets, and below you'll find how to do just that with a decent pair of 10x50 binoculars or larger, or even small amateur telescopes.

How to Find...

Polaris

Follow a straight line from Merak through Dubhe heading upwards and the next bright star you come to is Polaris. When you look at Polaris you are facing north. Polaris is the brightest star in the constellation of Ursa Minor. It lies 432 light years away and would be seven times the size of the Sun, making is a Supergiant, F-type star. It is in fact a multiple system with its companion, Polaris B, visible only in telescopes. A much fainter companion, Polaris Ab also exists.

Arcturus

Follow the curve of the Plough's handle from Megrez to Alkaid in an arc and the first bright star you come to is orange Arcturus. Arcturus is the brightest star in the constellation Bootes, the brightest in the northern hemisphere and the fourth brightest star in the sky. It shines as a bright, yellow star and has been confused by inexperienced observers as Mars. Relative to our Sun, Arcturus moves quite fast through space at 122km/sec towards the Sun. It will never collide with the Sun but will makes its closest approach in around 4,100 years time. It is only slightly larger than our Sun and lies around 36.7 light years away.

Spica

Continue the arced line mentioned above past Arcturus heading downwards and the next bright star you find is Spica, the brightest star of Virgo and the 15th brightest star overall. It is a blue giant that would consume ten Suns and lies 260 light years away.

Gemini

Follow a straight line from Megrez through Merak right across the sky and you will come to the next bright star of Pollux. To the right is his brother Castor: they are the heads of the twins of Gemini. One myth from Greek mythology portrays that when Castor died because he was mortal, Pollux begged his father Zeus to give Castor immortality. He did so by uniting them together in the heavens.

Cygnus

Follow a line drawn from Phecda through Megrez all the way across the sky and next bright star you come to is Albireo, the tip of the head of Cygnus, the Swan. Also known as the Northern Cross, Cygnus lies along the path of the Cygnus Arm of our Milky Way. You can see this as a background haze stretching from horizon to horizon with Cygnus along its path.

Regulus

Follow a line drawn from Megrez through Phecda down and the next bright star you come to is Regulus, the King Star of the constellation Leo and its brightest.

Cassiopeia

Follow a line from Mizar through Polaris and you will come to the constellation of Cassiopeia. One of the most easily recognised constellations, it simply looks like a large 'W' or 'M' depending on the time of year. Cassiopeia lies along the Cygnus arm of the Milky Way.

Further Help

Any good book store will be able to help you get star atlases to help you know your way around the sky. You can also download many free software programmes from the internet. One of the most recommended is Stellarium, which can be downloaded from www.stellarium.org. This programme, featured regularly at astronomy club meetings, provides many options for display and animation of the night sky on your computer.

To find out more information about what you can see in the night sky you can visit www.midlandsastronomy.com or www.irishastronomy.org, or you can come to any of the monthly public lecture nights (first Tuesday of each month) hosted by the Midlands Astronomy Club.

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