June 21, 2024

BBesides the traditional constellations, asterisms can provide a useful alternative way to find your way through the night sky. This week we look at one that spans four northern spring constellations.

The chart shows the view looking south-west from London at around 23.00 BST this week.

The Great Diamond is composed of four stars: Arcturus in Boötes, the herdsman; Cor Caroli in Canes Venatici, the hunting dogs; Denebola in Leo, the lion; and Spica in Virgo, the virgin.

All these stars are the brightest in their respective constellations except Denebola, which is radiated by Regulus in Leo. Arcturus and Spica are so-called first magnitude stars, meaning they are among the brightest in the entire night sky. For example, Arcturus is the fourth brightest star, and Spica is 15th on the list.

There is also a beautiful color difference between these two: Arcturus is a deep orange while Spica is a brilliant blue-white. Continue to follow the asterism counter-clockwise and the stars gradually decrease in brightness, with Denebola and then Cor Caroli both being second magnitude objects.

The asterism is also easily visible from the southern hemisphere, spanning the northern to northeastern quadrant of the sky.

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