Starwatch: Sirius bright and beautiful in the pre-dawn sky

Starwatch chart Mon 1 October 2018

Early risers this week will be rewarded with a stunning sight in the south-east. Sirius, the brightest star in the sky, will be rising in the hours before dawn. The chart shows the view to the south-east at 04:00BST. Overall, Sirius is the fifth brightest object in the sky. It is outshone only by the Sun, the Moon, and the planets Venus and Jupiter. This week, Sirius should be unmistakable because of its brightness, but if a further signpost is needed, then look for the three stars in Orion’s belt. They sit half way between the red star Betelgeuse and the blue star Rigel, and they point in a straight line diagonally down to Sirius. Another characteristic of Sirius is that it twinkles. This rapid flashing of colour and brightness is caused by the star’s light being refracted in the layers of Earth’s atmosphere. It is most pronounced when Sirius is low and so its light is travelling through more atmosphere to reach our eyes. As it rises higher into the sky, the twinkling diminishes. All stars twinkle, it is just more noticeable with Sirius because it is so bright.