Why would 18th century europeans have been interested in mapping the sky of the southern hemisphere?

Constellations: 

Throughout the centuries, people have looked to the stars to help them navigate across open oceans or featureless deserts, know when to plant and harvest, and preserve their myths and folklore. Ancient peoples used the appearance or disappearance of certain stars over the course of each year to mark the changing seasons. To make it easier to "read" this celestial calendar, they grouped the brighter stars into readily recognizable shapes, the constellations.