Welcome to the October night sky. This month shows that the long hot days of summer are over, and the cool nights of autumn are here.
The gas giant planet Jupiter has just slipped below the western horizon as darkness falls, leaving only planets Saturn and Mars visible in the southwest during October evenings. The Moon will be near to Saturn on the 14th and near to Mars on the 18th. Although Saturn is much larger than Mars, it is now 15 times further away, so it is only about half the brightness.
As soon as the stars become visible in October, the constellation of Cygnus the Swan can be seen overhead and, looking over towards the northeast horizon when it is fully dark, the star cluster called the Pleiades or Seven-Sisters can be seen rising. This is a sure sign that winter is on the way in the northern hemisphere.
Left over from summer, we can see three bright stars of the summer triangle. One of these stars is Deneb, the brightest star in Cygnus, and it is a super-giant star over 2,000 light years away. The lower star of the summer triangle is Altair in the constellation of Aquila, and it is only 16 light years away. The third star in the triangle is Vega in the constellation of Lyra and this star is 25 light years away.
All these three stars appear in the night sky as approximately the same brightness, but they are totally different stars. Deneb is much further away than the others and over 200,000 times the mass of our Sun, making it a super-giant star. The other two stars, Vega and Altair, are respectively 40 times the mass of the Sun and Altair twice the mass of the Sun.
The Moon is at last quarter on the 2nd, new on the 9th, first quarter on the 16th, full on the 24th and last quarter again on October 31.
By Clive Jackson
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Clive Jackson is the Director of the Camera Obscura (next to the Castle in Tavira), specialising in education and public outreach.
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To see the October Sky Map click on the pdf link below https://www.portugalresident.com/sites/default/files/ar_-_algarve_skies_2018-10_october.pdf