Eighty-eight constellations cover the sky. No gaps exist between them, and there's no overlap. It's a logical system where every object — star, planet, or galaxy — resides within one constellation's boundaries. This arrangement resembles a map of the United States. Everything in the country belongs to one state (or the District of Columbia). A few quirks exist. For example, you can find a small Kansas City in Kansas and a huge Kansas City in Missouri. Don't ask.
The constellation layouts also contain an irregularity: Serpens, which represents a serpent, apparently has a split personality, with Ophiuchus the Serpent-bearer in between. The western section, Serpens Caput (the head of Serpens) stretches farther north than the bright star Arcturus in Boötes. Serpens Cauda (the tail of Serpens), lies east of Ophiuchus and butts up against Aquila, Scutum, and Sagittarius.
But why 88 constellations? Why not 75 or 100? Today's constellations evolved from a progression of lists, maps, and globes created by Greek cartographers. The earliest surviving list dates from roughly 270 b.c. The Macedonian poet Aratos included one in his Phainomena. Even Aratos had roots, however. He took 44 of his constellations from a globe created by Eudoxus of Knidos, who lived a century earlier.
In the first century, the list that would become "official" for more than 1,500 years appeared in Almagest, a work of the scholar Ptolemy (73–151). Of his constellations, only two no longer exist. The Pleiades (known as the Clusterers in ancient times) has been downgraded to a star cluster, and correctly so. And astronomers subdivided Argo Navis — the great ship Jason and his Argonauts used to sail for the Golden Fleece — into three smaller, more manageable, constellations: Carina the Keel, Puppis the Deck, and Vela the Sails.
A new-constellation outburst occurred between the beginning of the 16th and the middle of the 18th centuries. Of the more than 40 constellations created during this period, three-quarters linked Southern Hemisphere stars into patterns. But even into the 20th century, no "correct" number of constellations existed. The list depended on which map you chose to use.
Enter bureaucracy. In 1922, the International Astronomical Union appeared. This group quickly set about formalizing constellation boundaries, and, in 1928, a committee under the direction of Belgian astronomer Eugene Delporte produced the final constellation list with official borders.
As a budding amateur astronomer, you might be thinking, "88 constellations? How will I ever remember them all?" Well, a map would help, but the real answer is that you don't have to memorize every star figure in the sky. In fact, if you live in northern latitudes, you can eliminate more than a third of the constellations because they'll never rise above your horizon. Why learn Apus, Mensa, or Octans if you'll never see them? This one step reduces the list of 88 down to 50 or so. But even that number is too large, so keep reading my posts, I'll lay out a plan of attack to learn the constellations at your own pace