Simplify the following text for a Grade 10 native English-speaking person.
Original: Copernicus ' major theory was published in De revolutionibus orbium coelestium ( On the Revolutions of the Celestial Spheres ) , in the year of his death , 1543 , though he had formulated the theory several decades earlier .
Simplified: This was published in his book , De revolutionibus orbium coelestium ( On the revolutions of the heavenly spheres ) .