Britannica Educational Publishing 
Astronomy [PDF ebook] 

Fathoming the boundlessness of space and the universe, we are immediately filled with curiosity about our own origins and wonder about the objects, life forms, and matter that populate the cosmos. Through the observations and work of astronomers over time, we have slowly been able to reduce the number of unknowns and develop explanations or theories for some of the celestial objects and phenomena we see. This space-traveling survey recounts some of the major discovery milestones in the field of astronomy and examines the tools and techniques currently used by astronomers to study the universe.