Constellation Family

It's often useful to arrange constellations into extended groupings, to help with navigation around the sky or as an aid to remembering the relations between different stars around the sky. 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 also 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 primarily aquatic or nautical themes, notably 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. The Zodiac family forms a band around the sky, a band that 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.