Most of the official 88 constellations trace their roots to ancient Mesopotamia and Greece, where they served as seasonal markers and fanciful representations of mythological figures. For the latter purpose, their size was unimportant, so they spanned a wide range from the smallest to the largest.
Fast forward to the early 20th century, when astronomers repurposed constellations as a way to organize the sky into manageable regions, which meant they needed official boundaries. In carving out areas for each, it was decided that the largest constellation, Argo Navis (representing the Argos, the ship of Jason and the Argonauts), was impractically large, and subsequently subdivided it into Carina (the Keel), Puppis (the Stern), and Vela (the Sails).
The breakup of Argo Navis left Hydra, the Sea Serpent, as the sky’s largest constellation, spanning 3.16% of the celestial sphere. Hydra, currently visible low in the south after dark, sprawls across such a wide east-west span that it takes nine-and-a-half hours to fully rise, and begins to set just two hours later. Its brightest star, Alphard (similar in brightness as the stars of the Big Dipper), currently stands midway up from the south-southwest horizon at 10 p.m.
The second largest constellation, Virgo, the Maiden (representing Ceres, the goddess of agriculture), is only slightly smaller than Hydra and also shares the spring sky. Virgo’s brightest star, Spica (sixteenth brightest star in the sky), currently lies low in the southeast at 10 p.m. The nearly full moon sits above Spica on Sunday.
The sky’s third largest constellation, Ursa Major (home of the Big Dipper), reaches its apex high in the north in springtime.
Together, Hydra, Virgo, and Ursa Major comprise nearly 10% of the celestial sphere, all visible in northern hemisphere spring skies.
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Chris Anderson manages the College of Southern Idaho’s Centennial Observatory in Twin Falls. He can be reached at 732-6663 or email@example.com.