On northern winter nights, it is so easy to be beguiled by the gloriously bright constellations of Orion, the hunter, and Taurus, the bull, that one can overlook the fainter constellations.
So this week, find the three stars of Orion’s belt, follow them down to Sirius, the brightest star in the night sky, and then look eastward until you find the faint ring of stars that makes up the head of Hydra, the water snake. The chart shows the view looking south-east from London at 8pm GMT on Monday, but the view will be similar every night this week.
Hydra is the largest of the 88 modern constellations covering an area of 1,303 square degrees. To compare, nearby Orion only covers 594 square degrees. Hydra accounts for most of its area by its length, crossing more than 100 degrees of the sky (the full moon spans half a degree).
As evening becomes night and into the early hours, the rotation of Earth causes Hydra to slither its way across the southern meridian until dawn washes it from the sky. From the southern hemisphere, the constellation is easily visible in the eastern sky by mid-evening.
