It's often useful to arrange constellations into connected groups, to help with navigation around the sky or as an aid to remembering the relations between different groups of stars. There are various ways of doing this, for example by assembling large asterisms, such as the Summer Triangle or Winter Hexagon, that combine some of brighter stars visible at the same time of year. An alternative is to divide the sky into quadrants - four quadrants in the northern hemisphere, and four in the south - and treat the constellations in each quadrant as a group.
A further approach is to divide the sky into 'families' of constellations, connecting star groups by traditional associations, location or theme. While constellation families are not formally defined, those most commonly used originated with Donald H. Menzel's Field Guide to the Stars and Planets, in the chapter 'Order and System of the Constellations'. This system introduces a series of eight families that together encompass all eighty-eight of the sky's constellations.
Of the eight families of this system, two derive from traditional groups dating back to ancient times. The best known of these is the Zodiac, the sequence of twelve Ecliptic constellations that mark the annual path of the Sun through the sky (omitting Ophiuchus the Serpent Holder which now lies on the Ecliptic, but was not part of the original twelve Zodiacal constellations). The constellations of the Perseus family are also derived from ancient tales, mostly representing characters and beasts from the myth of Perseus and Andromeda.
For northern and equatorial skies, the remaining constellations are collected into three further families, each named for a significant constellation that lies among their members. These are the Ursa Major, Orion and Hercules families.
The official constellations of the southern skies were not named until relatively modern times, and most of these are divided between the Bayer and La Caille familes, named for astronomers prominent in the naming process. The final family combines a scatter of constellations with a primarily aquatic or nautical themes, especially the ship Argo and the river Eridanus, and this family is known as the Heavenly Waters.
Only four of the eight families are completely contiguous: the Bayer, Orion, Ursa Major and Zodiac groups. Because the Zodiac family forms a band around the sky, it divides two of the other families into two parts, specifically the Hercules and Perseus families. The remaining two families, Heavenly Waters and La Caille, are rather more disconnected, collecting together constellations ranged across the sky, primarily in the southern hemisphere, that match their themes.
2,841 sq deg
|The Bayer family forms a ring of constellations around the Southern Celestial Pole (though not including the polar constellations of Octans and Mensa). The member constellations of this family were popularised by Johann Bayer in his 1603 star atlas Uranometria, mainly describing birds and animals from around the world.
3,802 sq deg
|This is a relatively disparate group of constellations generally united by their connection to water. The main axis of the family connects the three constellations of the ship Argo Navis with the river Eridanus, but other disconnected water-related constellations are also included, such as Delphinus the Dolphin and Piscis Austrinus the Southern Fish. This family also includes two decidedly non-aquatic members, Equuleus the Foal and Columba the Dove.
9,029 sq deg
The Hercules family is a huge group of prominent and recognisable constellations from the northern and southern skies. As well as the hero Hercules himself, this family includes the distinctive cross shape of Cygnus the Swan, as well as Ophiuchus the Serpent Holder (and Serpens his Serpent). Crossing the ring of the Zodiac, the family also includes huge Hydra and Centaurus within the same large collection of constellations.
This is by far the most populous and extensive of the eight families, stretching in declination from more than +60° at the northern tip of Cygnus to a point southward beyond -70° at the southern extent of Triangulum Australe. The family is divided into two almost equal parts by the Zodiac group, and it may be convenient to consider it as two 'subfamilies': nine constellations in the north around Hercules itself, and ten in the south arranged around Centaurus and Hydra.
3,011 sq deg
|A family of southern constellations, most of which were devised by Nicolas-Louis de La Caille in the eighteenth century. The member constellations of this widely scattered family are connected to the sciences or arts, with most representing pieces of laboratory apparatus.
1,929 sq deg
|With just five member constellations, this is the smallest of the eight families, but it is nonetheless one of the most prominent. It not only includes Orion, which is probably the most recognisable constellation in the sky, but also Canis Major, home to Sirius, the brightest star in Earth's sky other than the Sun.
5,865 sq deg
|A mainly northern family of constellations related to the myth of Perseus and Andromeda and also incorporating, for example, Andromeda's father and mother, Cepheus and Cassiopeia, as well as Perseus' winged steed Pegasus. The group is largely contiguous, except for the monstrous Whale Cetus that lies southward of the Zodiac.
6,090 sq deg
|The Ursa Major family circles the Northern Celestial Pole, incorporating not only Ursa Major but also the polar constellation Ursa Minor and the Pole Star Polaris. It also encompasses Draco the Dragon and Boötes the Herdsman, as well as several large but sparsely starred regions such as Camelopardalis and Lynx.
8,684 sq deg
|This family corresponds to the twelve traditional zodiacal constellations that lie along the Ecliptic, the Sun's apparent annual path through the sky of Earth. This family thus forms a ring of connected constellations that encircles the entire sky. Many of its member constellations - though by no means all - represent animals, and the name Zodiac derives from the Greek for 'circle of animals'.