What does do chase the ebbing Neptune and fly him mean?

"Do chase the ebbing Neptune and fly him" is a line from the poem "The Faerie Queene" by Edmund Spenser. The line is a metaphor for the pursuit of an unattainable goal, or for trying to control something that is beyond one's reach. Neptune is the Roman god of the sea, and the ebbing tide represents the ever-changing nature of the ocean. To "chase the ebbing Neptune" is to try to catch the tide, which is an impossible task. Similarly, to "fly him" is to try to control the sea, which is also impossible. The line thus suggests that the speaker is pursuing a goal that is ultimately unachievable.