On a frosty northern winter evening, there is a jewel to be found in the evening sky: the brightest star in the night sky, Sirius, in the constellation of Canis Major, the great dog.
With a magnitude (the measure of a star’s apparent brightness in the night sky, the lower the brighter) of –1.46 , Sirius is roughly half the apparent brightness of Jupiter at present. But unlike Jupiter, which maintains a steady brilliance, Sirius twinkles because its light is deflected by the various layers and turbulence in our atmosphere. The same is true for all the stars, though only the brightest twinkle visibly. Planets remain steady because they are much closer and so actually appear as tiny discs to our eyes, which averages out the various effects produced by the atmosphere.
Canis Major itself was listed by Ptolemy in the 2nd century. Together with Canis Minor, it is said to be one of Orion’s hunting dogs. The chart shows the view looking south from London at 0000GMT on 26 December. Located below the celestial equator, in the southern celestial hemisphere, Canis Major never rises very high in the northern sky but the bright beacon of Sirius makes it unmistakable.
From the southern hemisphere, Canis Minor appears much higher in the sky, and is much easier to spot.
