The largest constellation is also one of the most ill-defined. From mid-March to mid-April, Hydra the Water Snake slithers onto the scene, fully taking nine hours to completely appear above the UK’s southern horizon. Tracing its full length takes some doing, but there are guides.


Hydra starts with a small sideways teardrop-shaped pattern. This is located below the faint, inverted-Y shape of Cancer the Crab. If you’re struggling here, extend the side of the Plough’s blade nearest the handle, down with respect to the plough, to reach bright Regulus in Leo the Lion.

How can I spot Hydra in the night sky? © Pete Lawrence
© Pete Lawrence

Above Regulus, identify the backward question-mark pattern called the Sickle. Look one-and-a-half Sickle heights right from Regulus. Just below where you end up is Hydra’s sideways teardrop-shaped head.

From here, look down and left for a solitary, orange-hued star called Alphard. This is Hydra’s alpha star, its name meaning ‘the solitary one’. Hydra’s body meanders left and down from Alphard but is not easy to identify.

Along the Water Snake’s back sit three small constellations. Sextans the Sextant lies near Alphard but being made up of three barely visible stars, is also not an easy find!

Keep going along Hydra’s back and eventually, you’ll find the faint but distinctive outline of Crater the Cup. If you extend the curved handle of the Plough away from the blade this brings you to bright orange Arcturus in Boötes. Keep going and you’ll arrive at white Spica in Virgo. Crater hangs down from the midpoint of the line between Alphard and Spica.

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Left of Crater sits Corvus the Crow, a small but thankfully more distinctive pattern. The compact quadrilateral made from Corvus’s brighter stars forms an asterism – an unofficial pattern – known as the Sail, so-called because it looks like the sail of an ancient boat sailing along the horizon.

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Pete Lawrence is an experienced astronomer and astrophotographer, and a presenter on BBC's The Sky at Night.