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MAKE a fist with your hand, then hold it out to the sky at arm’s length. The width covered by your fist will be around 10 degrees. Now imagine 10 of these in a row. The biggest constellation in the sky, Hydra (pictured right), stretches across this huge expanse, covering 102 degrees.

Named after a mythical water snake, the constellation’s head sits in the northern celestial sphere while its tail stretches into the southern one. This means that the entire constellation can be seen from many places around the world, anywhere located between 54 degrees north and 83 degrees south. Latitudes north of this can glimpse part of the snake, but not the entire thing.

Because of its size, Hydra sits alongside 14 other constellations in the sky. The southern part of Hydra, its tail, borders the constellations Libra and Centaurus, while its northern head resides between the constellations Virgo and Cancer. Eighteen stars within Hydra have been found by astronomers to have exoplanets of their own.

Although it is large, it isn’t a bright constellation, so you will need dark skies to find it. The easiest way to locate Hydra is to start by looking for its tail.

To find Hydra’s tail, look for the bright star Spica, in the constellation Virgo. Start by finding the Plough (known as the Big Dipper in North America). Follow the arc of the handle until you reach a bright star called Arcturus, then keep this line going to reach Spica.

Keep following this imaginary line and you will see four stars in a roughly rectangular formation. This is the constellation Corvus, or the crow. Next to this is another faint constellation called Crater, or the cup. Both sit “on top” of Hydra.

As the Greek myth goes, the god Apollo asked a crow to bring him water. The crow took a cup to the river, but it got distracted by a fig tree and took much longer than expected. In order to explain the delay to Apollo, the crow plucked a snake out of the river and pretended it had been attacked. The god saw through this, however, and threw the crow, cup and snake into the sky.

Now we have to find the head of the snake, which sits near Procyon (a star in the constellation Canis Minor) and Betelgeuse (a star in Orion). These are both part of the winter hexagon, which we learned to spot earlier in this series.

Draw a line from Betelgeuse to Procyon and keep going. You will reach a bright star called Regulus, in the constellation Leo. Hydra’s head lies around halfway from Procyon to Regulus. Join head and tail and you should see a trail of stars making up Hydra. Alphard, Hydra’s brightest star, sits closer to the head of the snake.

If the sky is dark enough, you will see the Milky Way running almost parallel to Hydra. If the crow and cup are “on top” of the snake, the band of the Milky Way will be below them all.

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