were informally defined by the shapes created by their star patterns,
but as the pace of celestial discovery accelerated in the early 20th century,
astronomers decided it would be useful to have an official set of constellation limits.
One reason was to help name new variable stars,
which light up and fade rather than constantly shine.
These stars are named after the constellation in which they reside,
so it's important to agree on the end of one constellation and the start of the next.