Field guide · Reading the sky
AsterismAsterism
A pattern people recognise that is not one of the 88 official constellations — like the Big Dipper, which is only part of the Great Bear.
- Official?
- No — informal but widely used
- Best known
- The Big Dipper / Plough
- Used for
- Finding your way around
The shapes people actually find their way by are often not constellations at all. The Big Dipper is the brightest part of Ursa Major; the Summer Triangle takes one star each from three constellations; Orion’s Belt is three stars inside Orion.
Asterisms can be huge or tiny. The Winter Hexagon spans six constellations; the Coathanger is a little upside-down hanger you need binoculars for.
Because they are simple and bright, asterisms are the signposts of star-hopping: follow the Dipper’s pointers to Polaris, arc to Arcturus, speed on to Spica.
See it in the sky
Each one opens the sky map pointed at it, for tonight, from where you are.