"Sing heavenly muse"

The muses were spirits in Greek mythology that were said to inspire creativity and wisdom. Here, Milton refers to Urania, the muse of astronomy. This is just one of many allusions to classical mythology in Paradise Lost. 

There are nine muses and each is associated with her own art form:  

Calliope - Epic poetry

Clio - History

Erato - Lyric poetry

Euterpe - Music

Melpomene - Tragedy

Polyhymnia - Choral poetry

Terpsichore - Dance

Thalia - Comedy

Urania - Astronomy