Simplify the following text for a Grade 15 native English-speaking person.
Original: At this time the king also created the position of Astronomer Royal ( initially filled by John Flamsteed ) , to serve as the director of the observatory and to `` apply himself with the most exact care and diligence to the rectifying of the tables of the motions of the heavens , and the places of the fixed stars , so as to find out the so much desired longitude of places for the perfecting of the art of navigation . ''
Simplified: He was to `` apply himself with the most exact care and diligence to the rectifying of the tables of the motions of the heavens , and the places of the fixed stars , so as to find out the so much desired longitude of places for the perfecting of the art of navigation '' .