The longest constellation is Hydra, the water snake. It stretches across over 1000 square degrees of the sky, making it the largest constellation by area.

However, it's important to note that "longest" can be interpreted in different ways.

* Longest in angular distance: Hydra has the most angular distance on the celestial sphere.

* Longest in time: Hydra is also one of the constellations that is visible for the longest time throughout the year.

While the term "longest" may be used to describe Hydra's angular distance or visibility time, it's not the only way to define a constellation's length.