We are as clouds that veil the midnight moon;
how restlessly they speed, and gleam, and quiver,
streaking the darkness radiantly! —yet soon
night closes round, and they are lost for ever:

by contrasting "darkness” with "radiantly,” shelley :

-emphasizes that although life is fleeting, it is exciting.
-compares the spark of creativity to the enduring night sky.
-introduces the idea that people are destined to live life in darkness.
-explains that people are permanent, bright fixtures like the heavens.